PXI-4132 Auto Zero Effects and Timing

Enabling auto zero on the PXI-4132 corrects for offsets in the measurement path. Auto zero increases measurement accuracy but increases the time necessary to make measurements.

Only the PXI-4132 supports auto zero.

When auto zero is enabled on the PXI-4132, the instrument switches the measurement path from sampling between the HI and LO terminals (signal conversion) and samples an internal short (zero conversion). The PXI-4132 then subtracts the value of the zero conversion from the signal conversion sample to correct for offset in the measurement path.

Note While in the zero conversion state, the output of the PXI-4132 continues to operate with normal functionality.

Use the Configure Auto Zero function to set the auto zero mode for the PXI-4132. You can choose among the following settings to suit your application.

Auto Zero Off

Turning auto zero off allows for faster acquisition rates but does not correct for any drift in zero measurement over time.

When auto zero is off, a zero conversion is not performed; instead, the measurement subtracts the last zero that was acquired (or the zero that is automatically retrieved upon entering the Committed state for the first time in the session) to correct for offsets in the measurement path.

Figure 1 illustrates a measurement in which N samples are averaged and where auto zero is off.

Figure 2.

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Auto Zero On

Using auto zero produces measurements with the best accuracy for the PXI-4132, but adds settling time to the measurement of each sample.

When auto zero is on, a zero conversion is performed after every sample conversion and subtracted to correct for measurement path offsets and drift.

With auto zero is on, the PXI-4132 operates as follows for each sample of a measurement:

  1. The PXI-4132 digitizes and measures the signal
  2. The measurement path is switched to an internal short
  3. Settling time of ≈300 µs elapses, which allows the analog circuitry to settle
  4. The zero conversion is taken using the same aperture time as the signal measurement
  5. The switch is changed back to sample the signal and the ≈300 µs settling time elapses again

Figure 1 illustrates a measurement in which N samples are averaged and where auto zero is on.

Figure 3.

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Auto Zero Once

Setting auto zero to Once allows the PXI-4132 to acquire measurements faster than when auto zero is on but does not correct for any drift during the measurements.

When auto zero is set to Once, zero conversions are performed for the first measurement after a channel enters the Running state. Measurements of subsequent samples then use the zero conversions from the first sample for their offset correction. Settling time occurs after all measurements are performed and then again after all zero conversions are applied, for a total of ≈600 µs settling time for the entire measurement.

Figure 1 illustrates a measurement in which N samples are averaged and where auto zero is set to Once.

Figure 4.

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Timing for subsequent measurements is identical to having auto zero off.