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I am using a high-common mode differential amplifier (AD629) to measure several DC 10 to 15 voltages stacked together. Since the AD629 is not the cheapest amplifier in the world (though not as expensive as an instrumentation amplifier), I want to multiplex one to measure every signal and save power, probably with a solid-state relay (SSR) since they have a relatively long lifetime (number of cycles) relative to electro-mechanical relays. Any mismatch in the SSRs will reduce the common-mode rejection ratio. Since this is unavoidable, so I would like to calibrate the unrejected portion out, as well as any other bia voltages that may enter the signal later down the circuit path before it is sampled.

What is the best way to do this? My guess is to use a high precision voltage reference on the input. If the reference is not connected to the common-mode voltage, it only calibrates to some of the biases, but not anything due to the common-mode voltage. Would it be possible to connect the reference circuit's ground to the common-mode voltage as a virtual ground?

Then a new problem is introduced, which is how to power the reference using the common-mode voltage as a virtual ground? I could use a voltage regulator with the common-mode voltage as ground and the top of the signal as power, but I do not want an input capacitance to the regulator impacting the signal. Besides, isn't drawing unnecessary power from the signal of interest frowned upon, even if it is essentially a DC supply voltage?

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SSRs can have a diode drop accross them and may not work at all below some current level.

Step back and explain what you are really trying to do. I'm guessing you have a stack of 12V batteries and want to find the voltage of each one. What about a small micro on each battery that reports the voltage occasionally via UART thru a opto coupler?

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  • \$\begingroup\$ Right now it is for a battery bank, but it will probably be applied to small solar cells and fuel cells. I am trying to accurately measure the voltage across each battery in a battery string, run some calculations, and log the data to later analyze the batteries' internal parameters. Because of the wide range of possible applications, I want to power the microcontroller from an external source and minimize the power draw from the measurement circuit. Cost is a major problem and scalability is a limited requirement. \$\endgroup\$
    – Joshua
    Nov 16, 2011 at 17:35
  • \$\begingroup\$ Just checked some spare signal SSRs we used in another design. No problems when the same input resistance for the differential amplifier was used. The voltage difference between the input and output could easily be attributed to noise. \$\endgroup\$
    – Joshua
    Nov 17, 2011 at 16:53

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