Dear TI Technical Support,
I am currently working on a measuring circuit that measure the capacitance in a DC circuit. I have posted my Schematics in this post.
The output readout at the oscilloscope screen is not as we expected. As the voltage across the two capacitors is 6V, we expected to see the voltage across the larger capacitor to be 1.1V with RG = 50kOhm, and 0.55 with RG = infinity (not connected).
1. However, while the VCC and -VCC are held stable at 15V and -15V, there are occasions that the oscilloscope still read a few volts even when the battery to the two capacitors are disconnected.
2. The output of the oscilloscope seems to be influenced by electromagnetic interference. When I curbed my hand and periodically move my hand closer to or away from the two Vin+ and Vin- wires, there was oscillation at the readout.
3. When i switched the order of the two capacitors but still measuring across the larger capacitor, the voltage readout of the oscilloscope was not the same.
4. Sometimes, I see the voltage readout is reversed, sometimes it was upright.
I did connect guard- to guard- and guard+ to guard+ pins. I also connected two pairs of capacitor and resistor to ground like the figure 3 page 8 of the datasheet, but the behavior persists. I also made sure that the common-mode voltage of the measured capacitor is small (it was about 250mV).
I understand that some of these issues are very simple to fix, but this is my first time working with op amps in general. So I really appreciate your support.
University of Colorado Boulder.