What is the effective ADC resolution of the Usb6361 board

[Denver_Al]

I have been using a USB-6361 board with 16-bit ADC. I went to simulate a lower bit count ADC implementation and I noticed that the data sampled from the 6361 does not appear to have 16-bit resolution. Here's an example of some of the waveform samples (of type Double), using 0 to 5V range:

1.0727841521285737 V

1.09002147857432474 V

With a 5V range and a 16-bit ADC the LS bits should quantize to 5V/2^16 = 5V/65536

0.0000762939453125 V

neither of these samples can be accurately represented in 16 bits; there are fractional bits remaining.

example: 1.09002147857432474/5V * 2^16 = 14287.12952396...

if it was 16-bit quantized and the range was unipolar 5V I would expect an integer result.

Now, it's fairly easy to requantify the data to be 16-bit resolved but what gives with the raw data from the USB-6361 ?

Why is it not 16-bit quantized for the range ?

The data does not appear to be 16-bit quantized for 0-10V or for +/-10V either.

Any clues here ?

[BenRobinson]

Hi Denver_AI,

The resolution for each DAQ device is stated in the series manual. The X series manual, in this case, contains the expected resolution of each device based on the range. The ranges included in the manual are the only range value choices, so any other range will be coerced to one of those ranges. So, the expected resolution of the 6361 at +/- 5 V is 160 microVolts according to page 4-3 of the X Series User Manual linked below.

https://www.ni.com/docs/en-US/bundle/pcie-pxie-usb-63xx-features/resource/370784k.pdf

[stilly32 · Accepted solution]

Ben covered that X Series has bipolar ranges but there are other factors that lead to what you are seeing:

X Series applies calibration in software and this affects the HW and the data returned. For one, X Series has 5% overrange, details covered in the AI section of the manual Ben linked. Also, when applying calibration the USB-6361 actually uses a third order polynomial for scaling - so the scaled voltage you read doesn't map directly to code widths. You can do an unscaled DAQmx read and get the direct bits from the ADC. You can get the scaling coefficients with the AI Device Scaling Coefficients Property. Hopefully by looking at these you can get what you need from the device.

Thanks,

Andrew S.   

[Denver_Al]

Thanks for your informative answers. It turns out the USB-6361 uses an ADC chip ADS8412 with 14 effective bits (ENOB) so the 3rd order poly was probably applied to improve the data and make it more like 16-bit resolution. I also found this link helpful
Is DAQmx Raw Data Calibrated and/or Scaled?