I have designed an ADC board and code for sampling and am guaging interest before I get the PCB's made.
My code uses direct GPIO access to talk to the ADC rather than the standard SPI interface. In tests, I have been able to take one million samples in 11.5 seconds which is almost 87k samples per second. My test code took 1000000 samples then displayed 200 rows of the data from 1-200, 1000-1200, 10000-10200 and 100000-100200 so I could make sure that I had a sufficiently large delay to ensure that the data was accurate. My Pi is not overclocked and I haven't tested for faster sampling rates using overclocking.
The pcb will be about 62mm X31mm and will have an 8 pin socket to connect it to the underside of the Pi via P5.
the board can take MCP3008 (10 bit) MCP3208 (12 bit) and MCP3304 (13 bit) ADC
Estimated cost is £15.00 (including 12 bit ADC) and If I have sufficient interest, they can be available mid September.
Please contact me if you are interested. email@example.com
I should add that the board is not a kit but that the Pi's 8 pin P5 header needs to be soldered. There is conductive glue available on eBay for the fainthearted but I have no idea as to how good it is.