Solution
The PXIe-7856 features a user-programmable FPGA for high-performance onboard processing and direct control over I/O signals to ensure complete flexibility of system timing and synchronization. Among the I/O signals this device includes, there are eight Analog Outputs (AO) that can be programmed to output ± 10 V.
The behavior exposed in this article where the board outputs a different voltage level than the expected one is often cause by the way the device is programmed. There are two key aspects to consider to ensure the device is being programmed as intended. First of all, the AO range on the PXIe‑7856 is fixed at ±10 V. Therefore, users don't have any setting or configuration option where the output range can be changed to a different output range that limits the voltage in the AO.
Then, it is key to highlight that the AO data type used by the FPGA I/O Node can be either calibrated fixed‑point (FXP) in engineering units (volts) or raw integer (I16), depending on the I/O configuration in the LabVIEW Project. If you switch to Raw, the AO node expects I16 codes (not volts). To output +10 V, you must drive the correct numeric value at the AO node:
- If FXP (volts): write +10.0 to the AO I/O Node. FXP is already scaled, so the value is translated as is to output voltage.
- If Raw (I16): write approximately +32767 (0x7FFF). With a 16‑bit DAC over ±10 V, the Less Significant Bit (LSB) weight ≈ 20 V / 65,536 ≈ 0.305 mV/LSB. Therefore, 32767 corresponds to about +10 V (within device accuracy).
To ensure you are configuring the board with the intended data type, check the section Changing FPGA I/O Configuration From the LabVIEW Project Explorer from the following article, Working With Calibrated and Uncalibrated Data on NI R Series.