How the output voltage is coded on 16 bits DAQmx devices?

In our laboratory, we have two devices DAQmx, the NOR-PCIe-6363 and the NOR-PCI-6733. Both have 16-bit for bipolar analog output precision. I understand that the small voltage difference that can be made is 2 * Vref/2 ^ 16, where Vref is the reference AO voltage (10 Volts or externally provided for 6733, 10 or 5 Volts or externally supplied for the 6363).

I wanted to know how the output voltage is coded. DAQmx functions take 64 bit floats as input, and at some point, they must have their reduced accuracy. How is this done rounding, is a floating point around the nearest possible tension, or is always rounded down or always rounded upward?

What is all the possible output voltages? Some diagrams in NOR-DAQmx help/measurement Fundamentals, signals, Analog, sampling considerations seem like they could involve the maximum voltage + Vref is not achievable, so I think that is all of the possible tensions - Vref + 2 * Vref * n/2 ^ 16, with n ranging from 0 to 2 ^ 16-1 included. This includes - Vref and does not zero but + Vref.

Could I get confirmation on this point, or be corrected if it is wrong?

Hi Chris,

The scale of writing DAQmx version performs double floating precision scaling and then he made a turn, the closest to convert the resulting code of the DAC to double int16_t (or uint16_t for unipolar devices). Floating point scale includes the custom scale AO if you have configured one, the conversion of volts or AMPS to the codes of the DAC and for some devices, the calibration scale.

You can check the coefficients of scaling using the AO. Property of DevScalingCoeff. It takes V / A-> CAD codes and scaling into account calibration, but not the scales to customized AO.

The PCIe-6363 X series devices preset scaling in software. The internal reference of the AO is slightly higher than 10V, to correct the errors of gain and offset does not limit the output range. It also means that you are not limited to 9.9997 V on this device when you are using an internal reference.

The PCI-6733 uses calibration DAC instead of software scaling. RAW - 32768 means - 10 V, 0 corresponds to 0 V, 32768 is impossible because of two of the 16 bits of the add-in and 32767 translates 9.9997 V. When you continue 10 V to write DAQmx with this device, DAQmx he forced into 9.9997 V.

Note that for these two devices, the absolute accuracy full scale includes over 305 uV of error. Look at the tables of absolute precision AO in the specs of the device for the full story.

Brad

Tags: NI Hardware

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