Jump to content
  • 0

PCIe-DAS1602/16 giving odd voltage readings, and also calibration confusion



Hello, we are currently using a PCIe-DAS1602/16, along with the API it came with for a project. There are some peculiar behaviors involving the DAQ that I wanted to ask about and see if anyone knows the fix.

From what I understand, the DAQ was originally in its out of box state, just plugged into a PC. I know there is a calibration tool in InstaCal, but I will touch more on that later. For the API, we used the included C code, that was ported up to C++.

To begin, we had some issues reading voltages using the cbVIn() call. We did eventually get it to work, but only when configure to bipolar +/- 10 volts mode. We only need unipolar 5 volts, however that configuration never gave any readings. We did move the jumpers on the card, change the settings in InstaCal, and make the appropriate calls in software accordingly when we tried other modes. We did get some reading in other modes like bipolar 5 volts, but results were very odd, as it was appearing to distort waveform or have strange edge detection.

In bipolar 10 volts, we get voltage readings, but they float around a lot and are not accurate. E.g. 5 volts reads as 7.3 volts and if left being monitored it will skew up to 10 volts. Voltage and skew directions are sometimes random

When monitoring more than one channel, like two or 3, each channel seems to affect another, as they skew differently opposed to being monitored alone.

When monitoring 4 channels, they seem to stabilize each other, as the skewing becomes minimal and hovers around a value

I figured this could very well be due to the fact that the card is not calibrated. I attempted do some of the calibration options in InstaCal, but I didn’t have any luck there. In the A/D configuration, I couldn’t get the sliders to move with the potentiometers. On top of that, I the notifies you that if you reach the end of the adjustment range of a pot, close the app, go all the way back to the other limit and try again, but the pots I attempted to adjust appear to be infinitely free spinning with no limits in either direction. Is this normal? Also, due to the limited equipment we have in this nature, I attempted to use the output of the digital pins using cbDout() for the calibration, as well as the ground.

I mostly want to know what causes the floating and skewing voltage reading, and if there is a way I can fix that. Also, I want to know if the free spinning potentiometers is normal, or if there is something wrong with the card, as it makes calibrating the card confusing.  

Note: this was originally an email to Measurement Computing's support email. They replied telling me to post it here, so hopefully this is correct

Link to comment
Share on other sites

1 answer to this question

Recommended Posts

  • 0

Hello @DJQ

Answers to your questions:

23 hours ago, DJQ said:

I mostly want to know what causes the floating and skewing voltage reading, and if there is a way I can fix that.

A missing ground connection.  if you are using the PCIe-DAS1602/16 in differential mode, you still need to apply a ground.  For example, if you are using differential channel pair CH0 High (CH0H) and CH0 Low (CH0L), you will still need to attach a ground reference (LLGND).  When set to differential inputs, this device and most other A/D boards take 2 measurements; CH0H referenced to ground, and CH0L referenced to ground, then subtract the low from the high. To be clear, they are not true differential inputs i.e. not needing a ground.

If your signal source is single ended (2 wires), and you have the device configured for differential, you need to tie the CHx Low input to ground (LLGND).

If your signal source is differential (3 wires), as stated above, you need to connect all 3 wires: CHxH, CHxL and LLGND

If your signal source does not have a 3rd wire, the recommended practice is to install a 100K Ohm resistor between CHxL and LLGND.

23 hours ago, DJQ said:

I want to know if the free spinning potentiometers is normal, or if there is something wrong with the card

More often than not, and regardless of the manufacturer, PCB mounted trim pots do not have hard stops at the ends of their wiper/screw travel.  one benefit is it keeps them from being damaged by over zealous technicians trying to get 'just a little more range from the trimmer.'  I was a bench tech in the early 1980's where my job was to calibrate devices with multiple trim pots per unit.  Even then trim pots would allow themselves to be turned way past their physical travel.  But since they were larger than those made today, there was a faint click your fingers could feel when you reached either end of the travel.  Today's surface mount trim pots are much smaller and the wipers have less distance to travel so it is just about impossible to discern (or feel) the end of travel.  In short, this is normal behavior.



Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...