Analog Output

The term pseudodifferential refers to the 50 Ω resistance between the AO- terminal and chassis ground.

If the DUT inputs are floating, use either the pseudodifferential or differential configuration.

If the DUT inputs are grounded or ground referenced, use the differential configuration. Using the pseudodifferential output configuration on a grounded DUT creates more than one ground-reference point. This condition may allow ground loop currents which can introduce errors or noise into the measurement. The 50 Ω resistor between the negative input and ground is usually sufficient to reduce these errors to negligible levels, but results can vary depending on your system setup.

Configure the channels based on the signal source reference or DUT configuration. Refer to the following table to determine how to configure the channel.

The PXIe-4468 is automatically configured for differential mode when powered on or powered off. This configuration protects the 50 Ω resistor on the negative pin.

You can minimize output distortion by connecting the outputs to external devices with a high input impedance.

Each output channel is rated to drive a minimum load of 600 Ω. However, you can achieve optimal performance with larger load resistances such as 10 kΩ or 100 kΩ. Refer to the PXIe-4468 Specifications for more information.

The differential output impedance between positive and negative signal legs is approximately 40 Ω. For high impedance, it is greater than 1 MΩ.

The delta-sigma DAC on the PXIe-4468 functions in a way analogous to a delta-sigma ADC. The digital data first passes through a digital interpolation filter, then to the DAC resampling filter, and finally to the delta-sigma modulator.

In the DAC, the delta-sigma modulator converts high-resolution digital data to high-rate, 1-bit digital data. As in the ADC, the modulator frequency shapes the quantization noise so that almost all of the quantization noise energy is above the Nyquist frequency.

The digital 1-bit data is then passed to an inherently linear 1-bit DAC. The output of the DAC includes quantization noise at higher frequencies, and some images still remain near multiples of eight times the effective sample rate.

Output filter delay—the time required for digital data to propagate through the DAC and interpolation digital filters—varies depending on the update rate for DACs. This delay is an important factor for stimulus-response measurements, control applications, or any application where loop time is critical. You often might want to maximize the sample rate to minimize the time required for a specific number of update clock cycles to elapse, since it varies with frequency.

Refer to the PXIe-4468 Specifications for more information on the AO filter delay.