Pseudodifferential Connections for Ground-Referenced Signal Sources

NI 6115/6120 only

The following figures show how to connect a ground-referenced signal source to a channel on the NI 6115 and NI 6120, respectively.

NI 6115

NI 6120

With this type of connection, the instrumentation amplifier rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground, shown as V_cm in these figures.

Common-Mode Signal Rejection Considerations

The instrumentation amplifier can reject any voltage caused by ground potential differences between the signal source and the device. In addition, the instrumentation amplifier can reject common-mode noise pickup in the leads connecting the signal sources to the device. The instrumentation amplifier can reject common-mode signals as long as V+_in and V–_in (input signals) are both within the working voltage range of the device.

Like any amplifier, the common-mode rejection ratio (CMRR) of the PGIA is limited at high frequency. A common-mode choke on each channel of the NI 6115/6120 compensates for this limitation.

NI 6115 only

The purpose of the 10 nF capacitance on the AI <0..3> – connection of the NI 6115 is to provide an impedance for this choke to work against at high frequency, which improves the high-frequency CMRR. Depending on your application and the type of common noise at your source, it is possible to gain further common-noise rejection by placing a 0.1 µF ceramic bypass capacitor between AI – and AI 0 GND.