Electrical engineers!!!

[alphatarget1]

This is not a homework question, actually quite practical.

I?m using a SRS 830 lock in amplifier to try to detect phase difference between two signals (load versus displacement, to be exact). Basically it compares a reference signal (TTL, converted from sine) with a signal of interest (a sine wave). The sine wave from my experiment has a DC offset, and we?re trying to use a blocking capacitor to block the DC component. I?m not sure if we selected the right size (capacitance)?. Some of the experiments? signals have really low frequencies. (sweeps are 0.02, 0.1,1,10,25 hz).

Let's say the signal is 400 mV (at the sweep frequencies shown above) pk-pk with a 300mV offset. The problematic thing is that at different experiments, the magnitude would also change.

Any help is greatly appreciated!!

 

[alphatarget1]

anyone?

 

[PottedMeat]

Use a difference amplifier to remove the offset in the input signal?

http://en.wikipedia.org/wiki/O...s#Difference_amplifier

If the signal is different with each experiment you will have to put a gain or attenuating stage after the difference amplifier and adjust each stage for each experiment - maybe using potentiometers for convenience. ...assuming I understood what you're trying to do.

 

[blahblah99]

If you use a dc blocking capacitor, make sure the resulting circuit's high pass filter value is at least a decade in frequency above the freq. of interest, otherwise you'll introduce additional phase shift from the capacitor.

The best thing to do is pass it through an opamp with an opposite offset to remove the dc offset.

 

[alphatarget1]

Thanks guys. What I'm trying to do is: I have specimens that needed to be tested in compression. A hydraulic actuator compresses the specimen and that is accomplished by a function generator generating sine waves to drive the actuator. The load (force on the specimen) and displacement (deformation of specimens) are recorded as sinusodial signals. I'm using load as a reference point and I need to know the phase difference between load and displacement (think shock absorbers, sort of).

I do a haversine wave type loading, which is all compression. The minimum load is, say, 500 pounds (the bottom peak of the sine signal from the load). The load signal is actually converted to a TTL signal so I don't care about that. What I do need to remove is the offset in the displacement signal, which will have an offset due to initial deformation (from the minimum 500 pound load) and will have a sine wave shape.