DIff Amp

So I have a unipolar differential input ADC here ad need to feed some unipolar single ended signals into it (gyros, thermopile horizon sensors, etc). THe proble is all these outputs are single-ended and feeding them directly into the ADC would cause me to lose half my dynamic range (the negative ADC codes would be unused since the output signal could never go below the reference signal, which for single ended signals is ground).

Also, seeing as how a lot of these sensors (like gyros) have bipolar physical values (ie. +/- rotation) it would be kind of nice to get rid of their single-ended DC-bias so the ADC could read 0V differential as 0 degrees/sec). It would seem I need a bunch of differential output amplifiers to solve this problem and I seem to have having some trouble finding ICs with a large amount of diff amps packed into them (ie. the THS4524 is not available at either Digikey or Mouser). THere's also the problem that these differential output amps have a very low input resistance (ie. the THS4524 has an input impedance of 100K). It's not that big a problem for other sensors, but for thermopiles with an output impedance of 60K it is.

Is there no way around this than to find higher input impedance differential output amps? Or to use a single-ended buffer between the thermopiles and the diff amp?

Don't know of any high impedance differential output amps so you may have to use a high impedance buffer amp in front of the differential amp inputs where needed.

As to needing several amps, do you have one ADC and switching the inputs, or using one ADC per signal? If you are using one ADC, then you might switch the signals at the input to the diff amp so you would only need one.

Oh, guess I should have said. It's a simultaneous sampling ADC- not muxed inputs. Muxed inputs wouldn't be such a big problem since that ADC has an output for the mux and an input for the ADC so I could just use one fancy signal conditioning assembly.

EDIT: Oh, I think I might have a solution...

Since a differential ADC has a dynamic range twice as large as it's absolute input range, I can use a voltage reference half as large with a single-ended input to make use of all ADC codes. I still connecting the output voltage of the sensor to the positive ADC input, but instead of connecting the sensor ground for the negative ADC input, I use the bias voltage of the sensor or my desired common mode voltage.

So a gyroscope with a 0-5V output and a 2.5V bias for 0deg/sec would have the ADC measure the differential voltage between it's output and 2.5V bias. If a reference voltage larger than 2.5V was used then you start losing dynamic range since the gyro output never strays more than 2.5V away from the 2.5V bias. So instead of using a 5V reference (which would be ideal for a sensor 5V differential output), I would use a 2.5V reference to measure single ended signals. In actuality I'll probably use 2.048V reference and give up a bit of dynamic range. Since the ADC has an analog 5V power source it can still tolerate any input signals whose absolute values are between 0-5V even when it's reference voltage is less due to the magic of differential inputs.

Of course, with a 2.048V reference, if I measured the differential voltage between a 0-5V signal and ground I would get clipping past the reference voltage, and lose half the negative portion of the dynamic range. But most analog sensor signals are bi-directional so they have some bias that the output voltage can be measured with respect to.

Differential inputs are cool...if not requiring lots of parts.