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Author Topic: High Resolution ADC for Noisy Textile-Based Sensors in Variable Conditions  (Read 1172 times)
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Intro

In response to this question: http://electronics.stackexchange.com/questions/11577/simple-adaptive-amplifier-options-for-sensor-interfaces 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.

Overview

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, 10kΩ-100kΩ 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 100Ω to 300Ω.

Signal Path

[textile] -> [Wheatstone bridge] -> [lowpass] -> [instrumentation amp] -> [ADC] -> [AVR]

Requirements

So, I'm looking for an ADC that will meet my requirements. The ADC should be:

16bits+
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...

Research

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):
http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7783/products/product.html
http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7715/products/product.html
http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7709/products/product.html
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)

Voltage Reference?

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 and I've read that a stable voltage supply helps a lot to minimize noise.

Conclusion

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.
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I think you're headed in the right direction. No red flags I can see.

Yes, a voltage reference is recommended and I think it's necessary. If you need 16 bits of A/D resolution you don't want to waste those bits on an imprecise reference voltage.

You seem to be suffering from too many choices! I'd suggest cobbling a schematic together based on part selections from your list, narrowed down by price and availability (and solderability if you're going to solder them yourself) then post here and let us have a look. Personally I find it a lot easier to make comments or suggestions on a "straw man" design than to suggest an entire design.

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Ok, I think I'm going to try the following to start:

textile -> instrumentation amp (AD623) -> LTC2418 (24bit, 8 differential channel ADC) + Analog Devices REF19x Vref -> ATmega328

I suppose I should use the precision Vref with the instrumentation amp as well as the ADC? For the sensor too?
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oops! LTC2418 is too slow. I'll be trying the LTC2449 instead. The LTC2449 also has a nice feature called "MUX Out, ADC In" which allows you to create a kind of "loopback" so that you can pass all the inputs through an amplifier before the ADC stage.
« Last Edit: March 27, 2011, 03:44:12 pm by msutherl » Logged

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