Other Parts Discussed in Thread: INA116
We're using the INA116 in a differential coulombmeter probe. On both sides of the pcb, guard tracks surround both electrode inputs. The guarding is remarkably accurate as shown by the extremely low DC drift at the output of the 116. Even with nothing connected to the inputs, the output has low DC offset and very low drift. We are able to connect capacitive sensors to the inputs and maintain this type of operation for many hours. However, we do observe small (50-100 microvolts pk) low frequency pulses (about 0.5 to 1 sec pw) appearing occasionally at the output (one every two or three minutes or longer). The larger the input sensor capacitance, the smaller and less frequent are the pulses. We want to use small-capacitance probes and continue operating without input bias resistors, but need an explanation for the source of the pulses. They appear regardless of the shielding or location of the device, so this does not seem to be an external E-M source. We also have voltage regulators (+and - 5V) near the IC, with a battery for pwr source. Any ideas on what causes the pulses?
