[quote author="rsdio"]I'm sure Atmel has a few options, but they don't seem to have the same convenient parameterized search page. With Microchip, it's easy to check USB, check SPI, and see the price-sorted list of processors that have both. I know that there are AVR chips with USB, but I don't know how to find the ones with both USB and SPI that support high clock speeds.[/quote] Yeah, they 'improved' the parametric product table at least two times, so now it contains even less info than ever ;).
USB ATmega's are either AT90USB* or ATmega*U*, and I think all ATmega's support SPI. Clock speed is an issue, standard ATmega's are 16MHz/20MHz (MIPS) max, so SPI speed is limited to half that. Xmega goes to 32MHz, SPI clock max. 16MHz.
Finding parts supporting 24MHz SPI clock is actually harder than I expected. The STM32/LPC17xx ARM Cortex M3 parts only go up to 12.5MHz. From a quick glance at the PIC32 datasheet, it appears to be able to do 25MHz under some conditions (peripheral clock = 50MHz, which I believe requires a 50MHz system clock). 25MHz would be too fast for the LPD6803, so you're limited to 20MHz with a peripheral bus clock of 40MHz, i.e. 40MHz or 80MHz system clock.
But maybe something like 8MHz SPI clock is plenty, in that case almost any MCU with USB should be fine.
[quote author="robots"] There is a statement in official forum (official statement). In short it means, that we own the copyright to the library, and take no responsibility for it, and you are allowed to use it modify it freely. I don't think that there are any restrictions like in the Mchip library. (I have already posted the link to the forum in here some time ago, and i am too lazy to look for it) [/quote] That is a license (MIT-style), even though they explicitly deny the existence. I would be careful (why not put that statement on the download page) and request the statement in writing from someone with authority before using it in commercial works or spending lots of time on it, it seems strange to state that there's no license and include a license in a forum. I would not normally consider a forum statement to override the download page, and I doubt a judge would. The forum might even contain a disclaimer to that effect.
My $0.02: How about having some actual content before starting a new forum? Most of the forums on this site don't exactly suffer from high traffic, the Arduino only has a few topic I believe. I'm sure no one would object to MSP430 topics in the general discussion forum. As soon as they start to make up a significant part of the general forum, they can be moved to a separate forum. The lack of information may just be because the smaller user base, not sure how a new forum will magically change that.
But it's not like an extra forum really hurts, so if Ian feels like it, why not .
[quote author="IPenguin"] The examples are all based on uIP and the release notes of the Ethernet Library package (and the USB OTG Library) read:
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The enclosed firmware and all the related documentation are not covered by a License Agreement, if you need such License you can contact your local STMicroelectronics office.
... whatever that means. [/quote] No license / copyright statement = all rights reserved (not sure if you're even allowed to use it, let alone distribute/modify) in all countries that signed the Berne convention as far as I know (not a lawyer).
I don't see the point of JPEG for screenshots and graphs, it was designed for photos, and produces ugly artifacts with smooth surfaces and sharp lines. PNG should be fine, and also includes indexed formats. TIFF is rarely used outside the graphics industry, I don't think most browsers can render TIFF by default.
I don't think converting designs like this to DAC input is very risky, the pots are just voltage sources.
Hmm, that's not how I understood the E3620 circuit... It seemed rather like a resistive divider used for feedback? (The voltage control pot is between +5V(reg) and -Output) Short of using a digi-pot I don't see other easy conversions. Anyway.[/quote] True, I oversimplified, but that's still basically level translation, and can be easily solved with a differential op-amp powered from said rails (which I believe is what you've done), or even with a passive resistor network (if output impedance isn't an issue). I don't see any unsolvable problems there, and I'm convinced we can get it to work. Not so sure about the other issues.
[quote author="fenugrec"] We'll get there yet ! To be honest I don't have any religious attachment to this or that layout... If we need bipolar supplies, so be it. I just find that a totally single-ended circuit would be really elegant, and maybe easier to float. But in any case, it's definitely not time for a PCB yet. [/quote] I don't have anything against a single-ended supply, as long as we're convinced it will be stable. I agree it would be a really neat solution, because I've never seen it done before. But that makes me more careful at the same time, since it's possible that it's never been done because it's a bad idea. And bipolar would add some cost/complexity (two switchers?) with a single DC input.
A 'large scale' production should have more margins than a one-off, minor component variations shouldn't make the board unstable. It would suck to have complaints about oscillating supplies, and have to tell people to add a cap somewhere to make it stable.
[quote author="fenugrec"] ADCs : We want to monitor at least output V and I, but it would be vary nice to also take a reading of the input voltage, to compute a maximum power dissipation: if the user plugs in 25VDC, the output current should be capped in relation to the max transistor dissipation... Something like Imax = Pdis / (Vin - Vset) , not counting the sense resistor. Without this, the regulator can hardly be considered "short-circuit proof" - we can count on people not paying attention and asking too much of the reg. Or just assuming it's "smart enough" to not self-destruct. [/quote] That might be a neat idea, as long as we have ADC channels to spare. Adding a temperature sensor to the heat sink would accomplish the same in a more robust manner (ambient temp, air flow), but assembly might be expensive (I tend to glue these on with heat conducting glue, we would need something in a TO-220-like package). I was mainly talking about the external measurement channels that were in the original proposal, the other ones are easier since it could be fixed range (as long as we have enough resolution) and we won't have any unexpected voltages either.
[quote author="fenugrec"] Re ADC range selection : what about a resistive divider with MOSFET-selectable legs ? Top resistor = 10k, a few 10k bottom resistors which can be switched in through software control. Add diodes between the ADC pin and VDD, VSS to clip the range to safe levels... I figure the added Rds resistance should be negligible with 1% resistors , two or three magnitudes higher in value. [/quote] I would prefer a scheme where the input impedance remains constant, especially since our input impedance is unlikely to be so high that it doesn't matter at all (>=10Mohm). A switchable resistor divider would work too, and allow us to skip the PGA, either way would be fine for me. Whatever proves the easiest/cheapest/best (accuracy of at least 1% would probably be a requirement, even a crap DMM or panel meter does <=0.5%).
[quote author="tayken"] Yes, ADC inputs are probably the next problem but if we dig too deep, it will be really expensive and hard to design. If we use the plan I suggested (using internal ADC of PIC, direct connection for regulator voltage and current readings, connection through an analog mux for measurement inputs) we can save the ADC module and the damage will be contained at the analog mux. We can add a protection for a certain voltage value (like 12 V DC max) so that people will not probe mains with this (Ian doesn't like people messing with mains seriously, there might be liability issues) if there is the need and demand, we can design an interface board which will expand the voltage range. Or this might be left as a hack. I can work on this part of the problem if no one wants otherwise. [/quote] I didn't imply designing it to measure mains or documenting it, but that doesn't mean it should go up in a puff of smoke and fry everything close to it if it's connected to mains. An upper limit of something like +/- 15V for measuring is fine in my opinion (allowing it to measure at least all power supply outputs in parallel would be nice, since you might want to measure something connect to it). Mistakes do happen, and it would be nice to at least limit the damage. But I agree that it should not add lots of costs and complexity. Can't imagine how designing it to be less likely to kill the user can be bad for liability, but I'm not a lawyer.
My idea would be something like differential input -15V to +15V or so -> precision fixed voltage divider (bring it within range of the amplifier, plus limit currents in case of overload) -> differential amplifier (eg. op-amp) to convert it to single-ended, also shift it to positive only between 0-5V or so for the PGA -> PGA (for ranging) -> mux/ADC. If you have a simpler idea, that would be great, because it sounds more complex than I'd like. The fixed divider might make it more sensitive to noise and offsets on low ranges, but would make the input more robust without much complexity compared to switchable resistors.
[quote author="tayken"] For PWm generation, I was thinking of using a 24F series PIC and as far as I can remember they have multiple CCP modules (correct me if I'm wrong). [/quote] How much would this add in costs compared to a cheaper PIC with external DAC? Or do we need/want a PIC 24F either way? I'm not very familiar with the PIC models (more of an AVR person).
[quote author="tayken"] Actually as this is a prototype board, we put in both options and test their performances. [/quote] This is what I meant. Would it be an issue if we build a prototype with a PIC with enough PWM channels, but switch to a cheaper PIC if we decide to go with the external DAC? Would the firmware require significant changes when moved from PIC 24F to for example a PIC 18 something?
[quote author="tayken"] I think the only thing left is to finalize the linear regulator part. [/quote] And the ADC inputs, which have been mostly ignored. What's the current plan regarding grounding, differential inputs, ranging and negative voltages? The ideal DMM would have all inputs fully floating and protected, but that would require multiple isolated ADC's, which is probably way too expensive. There has been the idea to use the PGA, but it's not exactly clear to me how to use it. A quick glance at the datasheet suggests that the Microchip PGA's don't have differential inputs, so either one of the inputs is connected to the power supply output, or we have to isolate it (and the ADC). Some sort of protection on the inputs so it at least has a chance of surviving mains may not be a bad idea either (eg. series resistors and TVS diodes), or at least contain the damage.
I expect me and fenugrec to converge on a design soon, no major disagreements as far as I see. I would prefer to breadboard this supply (analog part, can use lab supply instead of DAC and DMM instead of ADC) in both Spice and real hardware and run some tests (eg. the National app note) before committing to PCB. I probably have most of the parts in stock, unless we decide on more exotic pass transistors or op-amps.
[quote author="fenugrec"] All excellent points, sorry I can't answer most of them. One thing I might say is that the E3620 was designed for potentiometer panel controls. I saw no other simple way to convert it to DC control... Last time I checked HP service manuals, I didn't find a linear lab supply that could be digitally controlled with i.e. GPIB. The digitally-controlled units I found were SMPS designs. [/quote] I think there are plenty, but finding one with full schematics might be a problem, since their latest designs don't tend to have schematics in the service manuals. I downloaded a few in the past for my own project, one example is the manual for the Agilent 6621A/6622A/6623A/6624A/6627A, but it looks pretty complex and uses a custom hybrid for actual regulation. The HP 6632A/6633A/6634A service manual looks simpler, but still much more complex than the current design (probably because it can both sink and source). Other similar designs would be the ELV PPS-5330 (German electronic hobbyist magazine know for high quality designs, just look at the pretty pictures) design, which uses PWM and a discrete ADC for some reason (educational value?). The ELV design without the DAC/ADC (plain old potmeter version) is pretty well known in local hobbyist circles, but I would trust Agilent over a magazine.
I don't think converting designs like this to DAC input is very risky, the pots are just voltage sources. I don't really have an issue with these modifications in your design (although I admit I haven't looked at them very much). We can dampen the heck out of the DAC outputs if we need to, it's not like we're going to convert this into an 1MS/s arbitrary waveform generator, we can consider them almost DC if that's necessary to get it stable. We can't do the same with the regulation loop, since it needs to be both stable and have good transient response, and will see a much more variable load (the load might modulate the output, the DAC just drives an op-amp).
My issue is mainly with the regulation to 0V without negative voltage, I can't quite put my finger on it, but my gut tells me there's a problem with it. Every linear lab supply I've seen uses negative potentials, and I can't imagine that all designers had shares in transformer manufacturers. I'm afraid it will misbehave close to ground (high output impedance, bad regulation, whatever). But I don't have any facts to back that up, so maybe I can be proven wrong. The only concrete example I have is the emitter resistor for the transistor sinking from the pass transistor base (Q3 from memory, but I don't have the schematic handy). Adding an emitter resistor that would drop say .5V to prevent tiny Vbe variations from making everything unstable would kill 0V operation. And the op-amps can't sink very well at <0.6V either (which is why you added Q3 in the first place).
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I would ask what you would consider to be a more conservative design ? Some problems I see to using i.e. the E3620 (or any other) circuit is : component availability, unkown design decisions and different specifications.
Sure, there is risk in every design unless you use the exact same PCB and components. But the fewer changes, the lower the risk in my opinion.
What I would consider more conservative is the design I've seen many times: most of the circuit is fed from a symmetrical supply around the output voltage (like I sketched out last week, although don't mind skipping the voltage doubler if we have a better solution). A constant current source is connected to the base of the pass transistors, the op-amps sink to below Vout, even at 0V output (no issues with running into rails or saturation, and no need to invert their output). If we go with the DC input, we would need an SMPS for these voltages, not sure what this will do for noise. The main change compared to most designs would be the DAC, but if it's just scaling the 0-5V or whatever to some other range, it should be pretty safe.
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They do make center-tapped AC wall warts. I'm not sure how internet-available they would be however (I bought a bunch at a local electronics dollarstore)
That would eliminate the voltage doubler that you dislike (unless we want to use it for voltage selection, double the voltage at 110V, the fact that most 110V places are 60Hz helps with ripple), but would keep the symmetrical (not two floating) outputs. I'm not a big fan of making two output circuits with inverted polarity either (mostly the extra effort and BOM), although I don't see any major issues. No idea about availability of these wall warts.
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Ok, as I said I'm all for it - but the guy making the boards gets to decide, in the end.
My proposal is to keep it as a requirement for now, and drop it if it turns out to be a pain to design/source.
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Indeed. FT232 is ok, but I'm partial for a USB PIC speaking to the output channel(s) through an opto-isolated bus.
Sure, as long as we have a working USB stack, that would be fine, too. Especially if we have multiple isolated outputs. But we might use a small PIC on the isolated side(s), too, since SPI requires tons of opto's (at least one multi-channel DAC and ADC per output) and I2C is a pain to isolate.
My preferences for DAC would be a real DAC, but if this turns out to make it too expensive and PWM performs good enough, we might use that. I don't see the point of using a more expensive PIC so we can skip a DAC, these things are <$1, even in single quantities at the likes of Digikey.
The advantage of the AT90USB for this purpose is that there's an open source USB stack (LUFA). I believe this is an issue with using the USB PICs for open source projects, but I admit I haven't been paying close attention.
[quote author="fenugrec"] Thanks for looking at the design in detail. I was afraid nobody would go so far! Indeed, I didn't "select" the pass transistor, I just took one for which I had a SPICE model (I forgot HP used a darlington). You're right, it probably only has a hFE of 15-20 at Ic=1A. You're also right that the bias resistor is probably inadequate in a large number of conditions. [/quote] Nothing wrong with the pass transistor, but it would require a second transistor to make it a Darlington. I also wouldn't trust the spice model. If the hFE is so inaccurate, what else did they get wrong?
About the R2, I think a transistor/JFET constant current source would be better, since HP used a resistor to a constant voltage above Ve(Q1). No need for great accuracy, it just should be better than between 14V/R2 (almost zero output voltage) and 2V/R2 or so (max output voltage). Something like -50%/+100% should be fine.
[quote author="fenugrec"] Just to clarify some of the features of the linear stage I posted some time ago ( http://dangerousprototypes.com/forum/in ... 87#msg9187 ) : 1- it will regulate down to 0 volts, without requiring a negative supply, if the op-amps are well chosen (LM358 should work, I can't say how well though) 2- op-amps U1, U2, and their compensation network (absent on the SPICE schem I drew), are critical. To repeat myself once more : it's farily easy to mess up the component choice, and have a supply that sorta-works but will be unstable. Unstable to the point that you can get substantial oscillation on the output. Or just terrible transient load change response. It's a great topology, and very flexible, but more delicate than a single LM317. [/quote]
I have some doubts about your design, even though you appear to know more about this: 1. How's the stability of Q3 without emitter resistor? Isn't it very sensitive to tiny Vbe and temperature variations, making it hard to keep stable? 2. The HP design of Q1/Q6 appears be temperature compensated, since the Vb(Q6) is at about the same level as Ve(Q1). Your design isn't, which may make the job of the op-amps harder. 3. Why isn't this topology used in other lab supplies? All commercial designs I'm aware of use a negative and positive voltage relative to the output voltage. Even in a commercial design, this has to add some extra costs, why do they use it if they could easily omit it? There has to be some downside, it's not like an LM324 is a recent invention. 4. In general, the major changes compared to the original design makes me somewhat uncomfortable without doing extensive analysis and testing. Hence my preference for something closer to the HP design if we can (doesn't add much costs with the symmetrical design).
If you're convinced that it will work fine, I'm willing to believe that. But if I would have to build it, I would go for a more conservative design.
[quote author="fenugrec"] - having two symmetrical regulators is nice but a bit restrictive for those times when you would really need a 12V supply and a 5V supply. Sure you can set one supply to 7V and the other to -5V, but it's still not great. The best is two floating regulator outputs. [/quote] Agreed, but not possible without two floating inputs (two wall warts).
[quote author="fenugrec"] - for an initial release, I think it should be most basic. Straight DC-input with polarity protection is most important, and the .3 or 0.4V drop caused by a series diode is insignificant. Or use a "crowbar" type circuit with an input fuse (useful in any case) with a diode to ground. - I don't like AC-input with voltage-doublers: how do you accomodate the unknown range of voltages that a user might use ? Supplying 1A (or 1A + 1A) will put a lot of strain on capacitors (high ripple current). Also, if you don't want huge 60Hz ripple you want large capacitorvalues, which are expensive and physically large. [/quote] I agree with the downsides of AC input. Input voltage could be compensated by two wall warts (110V and 230V version). About 4700uF/25V or so per side was required according to my spice simulations. Not enormous, but not small or very cheap either. It is the only way I see to offer symmetrical output with a linear supply and one wall wart, however, which is nice. Another issue is that I wouldn't trust all cheap DC wall warts to be stable and safe for delicate electronics (like the leakage issue and wall wart suicide issue I referred to earlier). Designing/manufacturing an AC wall wart is much harder to screw up.
[quote author="fenugrec"] -whether to float the output needs to be decided. One thing is for sure : the very desirable flexibility automatically means it will be more complex & more expensive. Is it worth it ? In my experience, it's a nice feature, certainly worth a few extra $. I'll also vaguely quote someone on this thread (I think it was rsdio) concerning the basic utility of a lab supply :
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If I need to use a lab supply, it's either because A) I don't have a supply built yet (unknown requirements) or B) I'm troubleshooting and suspecting the regulator
[/quote] I don't think extra parts will be too bad, some opto's and an extra PIC/FT232. Supply for the grounded part can be USB.
[quote author="fenugrec"] -I really see no need for a dsPIC. I think almost everyone now agrees that we're not going to kludge up a software feedback loop, therefore the requirements for the regulating uC would be: *a bunch of ADC channels for metering the output (10-bit is plenty enough, this is not a 5-digit calibrated multimeter); *either a couple DAC channels or PWM outputs that are capacitor-filtered to give variable DC. I would tend to prefer a DAC solution here since filtered PWM inherently has ripple, and possibly a slow response time. [/quote] Agreed.
[quote author="fenugrec"] -if a DAC chip is used in addition to the USB-speaking chip (PIC or FT232), it becomes easy(er) to add opto-isolation and have fully floatable outputs. But then you want to meter the output, so you also need ADCs on the isolated side ! At this point it would then sound justified to add a small, very basic PIC on the isolated side, which would serve only to monitor and steer the regulator. I like this idea, but one thing to consider is how much cost overhead it adds if we need to program 2 or 3 MCUs. Ian ? [/quote] The reason for considering an FT232 was that we don't really need more than an USB interface on the grounded side. We could isolate the UART connection and connect the PIC to lab supply ground. I've used the small MCU per output on the isolated side in the past, because an SPI/I2C ADC is not much cheaper, and the MCU can also control the DAC's and switch some other stuff (I had relays for tap switching). With multiple channels, we would need more intelligence on the grounded side, since we would need to control two outputs with one UART. Using an FTDI device with dual UART is ugly in my opinion, and probably even more expensive.
[quote author="fenugrec"] To summarize : floating outputs are nice to have, but significantly more complex and expensive. I would opt for a single-channel, single-output version 01 with modest, but solid features. I'm attaching a symbolic diagram of what I have in mind. [/quote] Not sure if the extra complexity for a single channel is that bad. What's wrong with a single FT232 (or USB PIC doing something similar) and two opto's? Another advantage of an isolated power supply is that you get protection of your USB port from connecting the output to something like mains for free. The choice of single vs. dual output depends on our choice of input. If we want two fully floating outputs, we would need two wall warts, so it would basically be a copy/paste job. Only the grounded side would be more complex (USB pic or FT232 + PIC with two UART/SPI outputs).
[quote author="tayken"] - For the initial release we might go for PWM + caps instead of a seperate ADC IC. Lets see its performance, if it is really poor, then we can go for the ADC IC in the real design. [/quote] I would offer both options in that case, so we can compare (populate one board with DAC and one with PWM, or switch with a jumper).
[quote author="arhi"] I would like to see 3-5A more then 1-2A .. but .. maybe it would be cool to make the project out of 2 segments where one segment does the read/write and the other segment does the "power" stuff so one can easily change the power segment to get 5A or 10A if needed [/quote] That's way outside the scope of this project. The design is scalable, but you would need to replace the current shunt, add extra pass transistors with emitter resistors, and probably add an extra transistor stage to provide enough pass transistor base current. You would also need much more cooling. Also much fatter traces for the high-current part. Not sure if it's worth it, it would probably be better to make your own PCB, possibly based on this schematic. I would also seriously consider SMPS for 10A, especially if low noise is not a requirement (eg. motors).
[quote author="rsdio"] I agree with "arhi." Any design based on ADC should have a voltage reference, not just a mere voltage regulator. Some ADC chips have a built-in voltage reference, others require an external one. [/quote] Can't remember anyone disagreeing with that, as long as the improvement over the internal reference (not much point in replacing a 0.5% internal reference by a 0.1% reference if you use 1% resistors as divider). I only disagreed with adding extra references for calibration purposes, I would rather use that money to buy a better reference for the ADC (or better ADC/PGA/whatever). Plenty of better references if you want to spend money, all the way up to the LTZ1000 which probably costs more than the total BOM of this project and wouldn't make sense at all.
I don't really understand the point arhi's latest post. Apart for correcting me on the SPI bus (thanks for that, I must have been confused with other PGA's), I don't see what he's responding to. I don't think anyone suggested using a 780x for ADC reference, and I see no need to shout about the PGA. I thought both I and tayken responded pretty positive.
By the way, don't the ADC channels need some sort of differential (ideally floating) inputs? Apologies if this was already decided, I haven't payed much attention to the ADC inputs yet. What's the current idea about that? I assume the power supply for the PIC/ADC is connected to the common output of the lab supply, since we don't want yet another wall wart. We want to measure the voltage between any two points (eg. over a high-side shunt), so connecting all negative sides together wouldn't work. Can the PGA's accept differential inputs? We have symmetrical supplies for the lab supply (if ADC ground is connected to lab supply ground), so that would give us a common mode range of -15V to +15V or so, which together with a fixed divider should do, if offset is low enough that it won't compromise accuracy at low ranges.
What about the isolation, how do we do that? A separate PIC/FT232 fed from USB power (will be turned off when USB is not plugged in) and isolated UART/SPI to the dsPIC? Or do we isolate the dsPIC from the lab power supply and ADC inputs somehow? Where would the dsPIC get its power from? USB? Requiring it to be connected to USB may be a fair trade-off, since you can't monitor or control it any other way.
I have some experience with both gEDA and KiCAD, but haven't done a major design in either yet. My impression was that gEDA was more mature and had more features, but KiCAD was more integrated, had a nicer interface and the library appeared better (though smaller).
With gEDA I had crap like the polarity of the gschem diode symbol and PCB diode footprint not matching (which would have meant reversed silk screen and some debugging if I hadn't caught it in time). They had two (Schottky) diode symbols, one with pin 1 anode, the other with pin 1 cathode. I found a complaint about the same topic on the gEDA mailing list from a few years ago, the response was basically that the designer should just check it all (I thought we integrated schematic capture and PCB design for exactly this reason). Library does seem a weak spot of both packages, both in consistency and selection. In gEDA, I had to draw basic footprints like a 5x20mm fuse holder. This will improve if they get more popular (many people are using Eagle libraries from places like Sparkfun). gEDA schematics look pretty crude to me because of strange proportions.
In KiCAD, I had the issue of not being able to connect anything not in the schematic without completely switching DRC off. Plus I had to search for obscure libraries for (in my mind) basic footprints like BNC connectors.
I think the Windows support of gEDA is currently pretty lacking (is there even a regular build?), and the learning curve seems steeper. I expect Kicad to get more popular in hobbyist circles for these reasons, although gEDA may be more suited for professional work.
Compared to Eagle, these packages do pretty well in my opinion, but I've never considered Eagle professional software. They probably do pretty poor compared to commercial solutions like Orcad (isn't that discontinued in favor of Allegro?), Allegro, PADS and Altium. I dislike Eagle because they believe they should use DRM to use their customers data to prevent piracy (copy an object from a file saved in a cracked version, and be locked out of all your files using this object in future Eagle versions).
[quote author="tayken"] This might be used for calibration purposes. A few jumpers can put the unit to calibration mode. Or we can build some test points for a calibration card which can be used to calibrate the unit from time to time. Just a thought. [/quote] I think the money and effort is better spent on improving intrinsic accuracy (use more accurate reference for ADC, more accurate dividers/PGA, better filtering for signal / power supply, better ADC). If we have more accurate/stable parts, use these in the main circuit instead of using them once a year for calibration. People can probably use the lab power supply and a DMM for calibration: connect power supply to ADC input and DMM, adjust until DMM reads exactly 1.000V, click calibrate, repeat for 10.00V, 100.0mV and 10.00mV. DMM's are pretty accurate and ubiquitous. Use the same voltage for 100% of lower range and 10% of next range to save time. Calibration constants can be stored in EEPROM.
[quote author="tayken"] Yes, symmetric power supply for op-amps if we have use a DC input. I, too, prefer something with a lower parts count but only solution if we use DC input. It is probably not necessary for AC input the way I see from your post. [/quote] Agreed, we can't do the peak voltage trick with DC. But we would need an inverting SMPS anyway for the negative input voltage, unless we use the dreaded two wall warts or one wall wart with symmetrical output (if such a beast is cheap and readily available). Can't do voltage doubler from DC, and I don't know any other way of splitting the voltage without dissipating lots of power or building an SMPS. If we use an SMPS for the negative voltage, we might evaluate if we skip the linear post-regulator all together (depending on parts count and noise). My current idea pretty much requires an AC output (transformer-only) wall wart with enough power (rough guess: 18Vrms/5A or so if we want 2x15V, 2x1.5A output?). Output voltage and current upper limit can be lowered as far as I'm concerned.
[quote author="rsdio"] Equivalent time sampling looks like a good feature to plan for the DSO wing. I think it probably requires help from special FPGA programming. See How oscilloscopes work as linked on the fpga4fun site page mentioned above. [/quote] Note that you do need a very accurate timing and stable trigger (which used to require custom hybrids, not sure what the modern way is for low-end scopes) to make equivalent time sampling even remotely useful. Not much point in equivalent time sampling if trigger noise causes the trigger point to fluctuate by more than the equivalent sampling rate.
Equivalent time sampling also has more pitfalls for the user (you will see interesting things if the signal is not perfectly periodic), and is generally just a lame way to get more bandwidth without adequate sampling rate (except the real sampling scopes that sample at 10kS/s or so and have bandwidth >>1GHz, these actually provide bandwidth that's hard/impossible to get any other way). But if the hardware makes a reasonable implementation possible, and it's just a matter of writing some HDL, sure, why not, it gives a higher effective sample rate for free under some conditions (which is why commercial manufacturers implement it, so they can advertise 50GS/s equivalent sample rate in big letters).
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seriously, there might be liability issues) if there is the need and demand, we can design an interface board which will expand the voltage range. Or this might be left as a hack. I can work on this part of the problem if no one wants otherwise.