Input signal is clipped to +/-0.5V when connected to an analog input

[JoelN]

Hello,

I am having a difficulty connecting an analog source to the analog inputs of my DAQ (USB-6215). The analog signal comes from an op-amp's output thru a 10k resistor. I scope the signal at the op-amps output and it is 10V peak, I then move the probe to the other side of the 10k (the analog input terminal) and the signal is clipped at +/-0.5V. If I reduce the source signals amplitude to less than 0.5V peak there is no clipping.

Perhaps the analog input range set to +/-0.5V which is causing this as a form of protection ? I do not have LabView to try changing the input range since I am just doing the wiring.

The analog source is connected to AI 0 and the analog source ground is connected to AI GND.

Thanks in advance.

J

[Henrik_Volkers]

50 Ohm input impedance?

[JoelN]

Hi,

I checked the specifications of the USB-6215 and it doesn't have 50 ohms input impedance. The input impedance is 10GOhm when device on and 1200 ohm when device off. Perhaps this low input impedance when the device is off is the reason ? I was testing this with the device off but also tried plugging the USB cable in to power it up but with the same results.

But if it is simply scaled because of a voltage divider due to low input impedance, I should expect a scaled down (but otherwise identical) signal, but instead I see clipping.

J.

[GarPar]

Hi Joel-

     I am a little confused - are you getting the clipped readings on a probe/hand-held DMM?  Or are you reading this signal through the device?  You said you don't have LabVIEW, so how are you measuring this signal?

     Does it appear to be an issue with the 6215 or with your signal?

[JoelN]

Hi GParente,

I am not reading this signal thru the device, I am using an oscilloscope to view the signal at the input of the DAQ. Like I said earlier, before I connect the analog source to the DAQ, there is no clipping. After connecting the analog source to the DAQ the signal is modified (clipped). So it is the actual signal that is being modified and it is not just an issue with how it is viewed thru the DAQ/software. It appears to be an issue with the 6215, specifically the input circuit or its configuration. I dont think that there is a problem with my signal, it seems that it is being loaded down or shorted thru some protection diodes once the DAQ is connected. Perhaps I have not configured the analog inputs of the DAQ properly, in fact I haven't configured them at all, the device is simply off. I would have expected that the DAQs input impedance is so high that it should have no effect on what it is connected to. 

Thx,

Joel

[Henrik_Volkers]

If you connect the turned off device, and it probably has clamping (protection) diodes to +- Vcc  that would be a good explanation.

Since +-Vcc is all Zero the result will be ta clipped signal. 

[GarPar · Accepted solution]

Joel-

     Have you tried reading the data through the DAQ device?  If that is what you are eventually trying to do, I am curious what we are reading.

     If you have Measurement and Automation Explorer, go ahead and open a test panel for the device and see what your voltages are.

     Let us know how this goes.

[JoelN]

GParente,

Sorry for the delay, but I was busy acquiring a PC with NI software to operate the DAQ. I then used the Measurement and Automation Explorer to opened a test panel like you recommended, with the input voltage range set to +/-10V there was no clipping ! Problem solved.

Thanks for the help

Joel