In Bus Pirate prototype “Ultra” v1b we added analog voltage measurement to all the IO pins using a 74HCT4051 8:1 analog multiplexer and an op-amp. In the next revision we’re moving control of as many peripherals as possible into the FPGA. The FPGA doesn’t have an Analog to Digital Converter feature, so we need to add an external ADC chip.
We chose ADS7041/ADS7042 10/12 bit ADCs capable of 1MSPS with an SPI interface. The 12 bit version is $1.75 in 100s, the 10 bit version is slightly cheaper ($1.06). The 10 and 12 bit versions are pin-compatible. We’ll try both and decide later what works best.
There are much cheaper SPI ADCs, but for around a dollar this chip does 1 MSPS with a simple 3 wire interface. That’s the same top speed as the DSO Nano v3, so we can record samples in the two SRAMs and have a very minimal oscilloscope function on any IO pin.
The interface is read-only and doesn’t have any registers to configure, that’ll keep it simple to work with from the FPGA. Each conversion begins with two clock ticks, then the 10 or 12 bit reading follows. The maximum clock speed is 14MHz to achieve 1MSPS, easy to do with the FPGA.
This post is just a quick followup to yesterday’s look at three Digital to Analog Converter chips.