NI USB 6008 Voltage Offset using RSE and Diff. Measurement

[SpeerResi]

Hello everyone,

I'm currently trying out the NI USB 6008 and I'm getting problems when reading an analog voltage using RSE or Differential.

So basically all I want to measure is a PWM signal (0 to 12V), which is divided by a voltage divider (by two). But instead of measuring 0V and 6V

I am measuring a constant 0.8V and about 3V.

On the digital side of the DAQ I'm giving out the pulses for the SSR... and this works just fine.

I'm connecting it this way: http://digital.ni.com/public.nsf/allkb/95CC0CB11D7DF3D18625712E000C4ABD?OpenDocument

Would apreciate any help

Best Regards

EDIT: Attached are acquired graphs

[johnsold · Accepted solution]

What is the impedance of your voltage divider?  The input impedance of the USB-6008 is not very high. If the impedance of the divider is large, it could cause the effect you are seeing.

Lynn

[SpeerResi]

Hey Lynn,

both are about 23 MOhm.

I'm going to change them and reply afterwards.

Thanks for your reply.

Regards

[johnsold]

The input impedance of the USB-6008 is 144 kohm, so your high resistance divider is definitely a problem.

Lynn

[SpeerResi]

Hey Lynn,

sry I was wrong, those resistors only had like 500 kOhm.

But still, I changed them and now its working.

Thanks a lot.

[johnsold]

Even at 500 kohm the low input impedance of the USB-6008 would have created a significant error.

Lynn

[SpeerResi]

I'm now using 10k. Works great!

[SnowMule]

Another option is to buffer the signal before going into the 6008 - use an op-amp voltage follower and feed that output into the DAQ.

Input-Z of op-amps is huge, so it doesn't load the signal.