In response to this question about adaptive amplifiers, It was recommended that in order to deal with variable conditions, it may be more economical to simply use an ADC with higher resolution so that I don't need to worry about amplification and I can do scaling in software.
I'm trying to design a data acquisition circuit for body mounted textile-based stretch sensors. The textile varies resistance as it's stretched (about 1 order of magnitude, 10k\$\Omega\$-100k\$\Omega\$ with 30% stretch). The exact ranges will change depending on how the textile is cut, whether it's soaked with sweat, the temperature, how old the material is, how it's mounted, etc. The entire thing needs to be as small as possible because it's mounted on the hand, so minimizing the number of components is a big plus.
Moreover, I'd like the circuit to be reusable for other applications that may have worse performance. For instance, if I use a cheaper version of the textile, my resistance range may be as bad as 100\$\Omega\$ to 300\$\Omega\$.
[textile] -> [Wheatstone bridge] -> [lowpass] -> [instrumentation amp] -> [ADC] -> [AVR]
So, I'm looking for an ADC that will meet my requirements. The ADC should be:
- As easy to use as possible: much better if there is interface code already written for AVR/Arduino...
- ...yet at the same time as comprehensive as possible: I've seen some ADC's with lowpass filters and PGA's built in – all the better as long as it doesn't make configuration a pain
- 8+ channels, or if it's easy enough to implement, 2x 4+ channels. EDIT: If I'm using a Wheatstone bridge, perhaps I want 8 differential input channels (so 16 channels)...
- I don't think operation voltage matters... (best if not above 5V)
- Surface mount
- Doesn't need to be cheap (it's a one-off)
- SPI vs. I2C doesn't matter I think...
- 100+ Hz
So far through Googling, I've found the following chips:
- Linear devices offer various 16-24bit delta sigma ADCs, some of which I've seen recommended: http://parametric.linear.com/html/no_latency_delta_sigma_adcs?p=5312974
- Microchip has a range of options, some of which I've seen recommended: http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=11022&mid=10&lang=en&pageId=79
- Analog devices have a number of comprehensive data acquisition chips with amplifiers and filters (no need for external signal processing stuff):
- I haven't looked at the TI chips yet...
and the following tutorials:
- http://arduino.cc/blog/2010/11/29/tired-of-a-10-bit-res-hook-up-a-better-analog-to-digital-converter/ (LTC2400)
- http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1275676171 (TI ADS8341)
- http://forums.adafruit.com/viewtopic.php?f=31&t=12269 (MCP3424)
- http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1248751435 (LTC2410)
Finally, some people have recommended a precision voltage reference, such as the Analog Devices REF19x series. Do you think this is necessary? Resolution is definitely important for me.
Let me know if you have any recommendations! I'm also not sure exactly what I'm looking for, so tips on how to decide are also appreciated.