The NI programmable gain instrumentation amplifier (NI-PGIA) operates normally by amplifying signals of interest while rejecting common-mode signals under certain conditions.

  • Common-mode voltage (Vcm)—The voltage of measurement reference potential versus AI GND. It depends on the configuration of the input terminal.
  • Signal voltage (Vs)—The value you are trying to measure. It depends on the configuration of the input terminal.
  • Total working voltage of the positive input—Equivalent to (Vcm + Vs), or subtracting AI GND from AI <0..x>+. Must be less than the maximum working range specified in the USB-6421 Specifications.

    • If any of these conditions are exceeded, the input voltage is clamped until the fault condition is removed.

    Table 15. Common-Mode Voltage, Vcm
    Input Terminal Configuration Condition
    Differential Mode Vcm is equal to the voltage on AI<x>- vs. AI GND, where AI<x>- is the negative half of the differential pair. Refer to the Differential Connections for Ground-Referenced Signal Sources section.
    Referenced Single-Ended (RSE) Mode Vcm is equal to 0 V since AI GND is the reference for the measurement. Refer to the Non-Referenced Single-Ended (NRSE) Connections for Ground-Referenced Signal Sources section.
    Non-referenced Single-Ended (NRSE) Mode Vcm is equal to the voltage on the NR pin vs. AI GND.
    Table 16. Signal Voltage, Vs
    Input Terminal Configuration Condition
    Differential Mode Vs is equal to AI<x>+ minus AI<x>-, where AI<x>- is the negative half of the differential pair.
    RSE Mode Vs is equal to AI<x> minus AI GND.
    NRSE Mode Vs is equal to AI<x> minus the voltage on the NR pin.