How do you connect a low pass filter to the SCB-68 for a differential input?

[Control Phreak]

Hi, I am trying to measure the output voltage of a load cell in the 5mV to 30mV range.  The load cell gets it's excitation voltage from the existing scale and I am connecting in parallel (with the scale) to the output of the load cell.  There doesn't appear to be any loading effect as the voltage reads the same whether it is connected to the SCB-68 or not.  I am using a 6220 M series DAQ card as well.  I have the Ai(0) and Ai(8) configured as a differential input.  I am using shielded wiring but the voltage is spiking erratically.  The average of the voltage is correct but there is a lot of noise that is erroneously triggering my calculations.  How do you connect a low pass filter to an input that is configured as a differential?  Thanks!

Kevin

[Ryan_V[DE]]

Kevin,

Do these spikes occur when the load cell is not connected to the SCB-68? Is the excitation voltage ground-referenced? If it is ground referenced , I would construct a single ended lowpass filter on each leg, connecting them together with a capacitor value twice that of the single ended design. If the power supply is floating, I would use identical lowpass filters on each leg with equivalent values to the single ended design, both connected to the AI GND on the SCB-68. Please let us know if you have any additional questions.

Hope this helps,
Ryan

[Control Phreak]

Hi Ryan, thanks for the info, the scale gets it's power from a plug in transformer so I assume the excitation voltage is floating.  I will try external lowpass filters on both legs first with the -ve of the capacitor connected to Ai(gnd) before soldering to the SCB-68, hope it works.

Kevin

[Control Phreak]

I will check if the spikes are still present with the load cell disconnected.  I do know that if I short the Ai(0) to Ai(8) with a jumper wire they are not.

Kevin

[AnalogKid2DigitalMan]

Kevin:

Have you tried anotehr power supply for the scale? Some plug in wall transormers are not of the best quality in terms or regulation and noise. Ideally you could run it from a battery source temporarily as a sanity check.

[Control Phreak]

I will hook up my scope and see if the noise is coming from the power supply, thanks for the idea!

Kevin