![]() I would like the Vref to the ADC to be 4.096 V to make the math simple so the maximum voltage at ∺DC (+) needs to be less than 4.096 V. The DUTs (samples) will generally range from about 10 to 200 Ω. ![]() This is where I am stuck I have no idea how to spec out which MOSFET would be suitable. The figure shows an n-type bipolar transistor but the text suggests using a MOSFET for better accuracy (I assume this is because the base current is much less than a transistor?). MAX6126) and an n-type MOSFET (see Figure 2 on page 15). I came across what looks to me to be a very simple two-device circuit to generate a 5 mA source. So now I am looking for a circuit that will produce a nice steady and repeatable 10 mA of current that I can send to each sample via some DPDT relays. ![]() I figure with a good steady 10 mA current source and a 16-bit differential input ADC I should be able to easily detect aging of at least 1% in the resistance. I am currently using the Propeller to automate the thermal cycling of these samples and now I want to add the ability to automatically measure their resistance values periodically (ranging from once to several times each day). I need to measure the resistance aging of several samples over a long period of time (ranging from a few hours to several months).
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