Low Pass Filter Help?

Question

I am using labview to record pressures measured from transducers. I hooked an op amp up to the transducer output because it maxed out at 24mV and I needed it to range between 1 and 10 Volts.

Now the op amp is amplifying the very slight fluctuations in the (DC) voltage. What once was barely noticeable fluctuations (+- 0.3mV) is now being amplified roughly 210 times, and is on the order of +-10-20mV!

I have tried using a low pass filter to get rid of some of the fluctuations, but it either does nothing or stops the voltage from changing with pressure. Any advice?

Answer 1

The LP filter will only further distort your data; LabView can do signal processing on the raw data if you need further low pass filtering.  You need some serious help on the electronics end to get the raw signal to the card input...

Most NI cards have programmable input ranges.  Find out how to set yours to 1 V (or lower) max instead of 10 V max.  This will allow you to change your op amp to a gain of less than 50 instead of more than 200.  The local rep or the phone support can help you with this.

Seriously consider a two stage amp -- a gain of 10 followed by a gain of 5.  Two low gains are much easier to stabilize than a single gain of 50.  Op amps and resistors are cheap...

Use the vendor recommended power bypass caps on the op amp(s).  Use low inductance power wiring (i.e. coax braid or a PCB plane) to the op amp(s).

Use coax to send the signals to your NI card; possibly using two coaxes - one for the signal and the other for the return on the center conductors.  Consider a video buffer (50 or 75 ohm drive capability) for your final op amp stage and drive matching coax with the proper termination resistor at the card input.

Use a voltage regulator with appropriate bypass capacitors on your op amp card to eliminate power supply fluctuations.   I have seen some analog designers use triple-regulation to completely eliminate any power supply problems.  Something like 15V from supply 12V out of reg 1, 9 V out of reg 2, and 5 V out of reg 3 (your voltage requirements may vary).  All with the appropriate bypass caps and bleeder resistors for correct loading/discharging.  Lots of parts but zero DC movement (due to supply variation) after reg 3.

Shield the op amp(s) and/or sensor in a metal box, if possible, to eliminate any EMI issues.

Let me know where I can send my bill...   :^)