I'm new to circuit design and am unable to figure out why I'm getting inconsistent voltage measurements under certain circumstances at the inputs of an op amp in a very simple non-inverting DC amplifier, when the input voltage is from a TMP36 temperature sensor.
First, a circuit that uses a voltage divider as the input and that gives the expected results:
In particular, the voltages at both inputs are very close to equal (sometimes they differ by a millivolt).
Now I replace the voltage divider with a TMP36 sensor, which outputs a voltage proportional to temperature. According to the the datasheet, it's able to source up to 50 uA from the output.
Now I see an 8 mV discrepancy between the inputs when they're measured relative to ground, but if I measure directly across the inputs, I measure a 0 mV difference.
Why the inconsistency? My multimeter has 10M impedance and so shouldn't be drawing more than 0.07 uA, which is far, far below the published 50 uA limit. The TMP36 datasheet doesn't give any hint that the output voltage should be influenced at all by the output current within the specified range. A 1% reduction in Vout seems too large, but maybe my expectations are too high.