Electrical – Building a VCCS with AD8220 Instrumentation Amplifier

1. You can use any amplifier you like. You will get better performance if you use a modern $5 dual op-amp than a 2-cent/amplifier LM324. Sub-zero means nothing to me. If you want to the amplifier to have guaranteed characteristics below 0°C, then you should buy one that is guaranteed for that temperature range. Below -55°C you may have to qualify them yourselves. For temperatures down to -20°C it's unlikely that anything untoward will occur other than possibly slightly worse performance for a chip guaranteed over the commercial temperature range. A single 5V supply will impose significant limitations on the input common mode voltage. You can add pullup resistors to the output of an LM324 to get a bit more range (you probably do not want to do this on the output amplifier). Note that the left two amplifiers can saturate at either rail, so even if the output should be within range that doesn't mean that the amplifier will work properly. I presume your common mode input voltage will be close to 2.5V so this may not be a problem. LM158 and LM124 ancient designs are particularly crappy amplifiers for low level DC- large offset voltage and large offset drift with temperature.

2. See above. Offset voltage, offset voltage drift. Probably useful range of input. Closed loop gain accuracy will likely be lower too, because of op-amp gain.

3. If you could use an analog switch with your micro to auto-zero the amplifier you could improve performance. A SPDT switch is sufficient (eg. 1/2 74HC4066). Just transfer one input to be shorted to the other at ~2.5V and measure the amplified offset voltage. Average over many measurements and subtract from the measured signal.

The circuit looks OK as far as it goes, but I have two observations.

a) Is the material capable of polarisation? If so, then uni-directional current will steadily change the apparent conductivity over reading time. Examples would be damp earth for instance, or skin under gel electrodes.

A mitigation for this would be either to make your current source switchable polarity, or make the analogue multiplexers switchable between ground and current source. While full AC would work, it would complicate detection by your DAQ, and short periods of DC excitation, reversed between readings to provide zero average current, may work well enough.

b) If your multiplexers are switchable between GND and the current source, and you use a bipolar current source, then you need only 1:8 multiplexers, not 1:16, which may be a worthwhile hardware or tracking saving. Connect the outputs of the two multiplexers alternately to the electrodes, and you still have full control over which pair of electrodes receives which polarity of excitation.