Analog input issue on 6225

[zimmic]

I'm using the 6225 to read several analog input signals. It seems to be working for some signals, but in three cases it is not. I am trying to measure a 0 - 10 volt analog signal, but the signal is being pulled low (around 10 mV) when the measurement device signal line is connected to the DAQ card. When I disconnect the signal line from the DAQ card, the measurement device outputs the correct voltage range.

Any ideas why this would be happening? I don't think it is an input impedance problem, since the DAQ card has over a gig ohm input impedance for the analog input channels. I'm not using a signal conditioner before the DAQ card, but I wouldn't think it is necessary for the 0 - 10 volt signal.

Thanks. 

[AnalogKid2DigitalMan]

Can you provide more information on the cases where it works and the 3 where it does not? Such as the devices used, analog input configuration, etc.

Is the polarity correct to the DAQ card? If they are ground referenced signals from the measurement device and you connect it backwards to the DAQ card, it may be effectively shorting the ouput of the measurement device to ground.

[mallorim]

Hi Zimmic,

Welcome to the forums! I too have a couple of questions for you before we can figure out what the issue is.

1)     Are you using a PCI card or a USB-6225 with screw terminals?

2)     When you say that your analog input does not work in three cases, do you mean with 3 types/specific signals, or 3 specific channels of your card? For instance, when you connect one of the problematic signals to a new channel, is it still pulled low, or does the issue remain with the channel when a previously working signal is connected? If the issue does not follow the signal, but is channel specific, you may need to contact National Instruments to have the board repaired.

3)     Can I ask how you are connecting the signal to the 6225 card- differential, non-referenced single-ended (NRSE), or referenced single-ended (RSE)? Are you using any biased resistors? An good explanation of these terms can be found in the Field Wiring and Noise Consideration for Analog Signals Developer Zone Article. AnalogKid makes a good point about possibly shorting to ground.

4)     What is providing the analog signals you are measuring? You said that when you disconnect the signals, the device is output the right voltage. How are you measuring this correct voltage, a desktop scope?

Regards, Mallori M.