Hello all,

I'm building a lock-in amplifier circuit based on the AD630 chip for an optical measurement system (circuit is similiar to this one: http://blockyourid.com/~gbpprorg/mil/cavity/lock/SimpleLockInAmplifier.pdf)

Anyways, I've noticed that the output is negative, so I need to modify the phase of the reference signal. I've found a simple phase shifter circuit (attached), but I can't find a lot of info on the circuit itself. After building it (with R = 10k and C = 23nF) I get a 90degree phase change of a square wave input at each extreme of the potentiometer (100k pot), but in between these two extremes the waveform changes significantly from a square wave profile. I believe the choice of components are designed for a 5kHz, can anyone suggest suitable values for a 1kHz wave (I'm assuming that that is the reason the output isn't square-wave like.

Also, do I need to use an op-amp with both negative and postive supply rails? (I'm just using a single supply op-amp (TLC271)).

Any info (links or otherwise) would be much appreciated.

Thanks!

 

Hello all,

I'm building a lock-in amplifier circuit based on the AD630 chip for an optical measurement system (circuit is similiar to this one: http://blockyourid.com/~gbpprorg/mil/cavity/lock/SimpleLockInAmplifier.pdf)

Anyways, I've noticed that the output is negative, so I need to modify the phase of the reference signal. I've found a simple phase shifter circuit (attached), but I can't find a lot of info on the circuit itself. After building it (with R = 10k and C = 23nF) I get a 90degree phase change of a square wave input at each extreme of the potentiometer (100k pot), but in between these two extremes the waveform changes significantly from a square wave profile. I believe the choice of components are designed for a 5kHz, can anyone suggest suitable values for a 1kHz wave (I'm assuming that that is the reason the output isn't square-wave like.

Also, do I need to use an op-amp with both negative and postive supply rails? (I'm just using a single supply op-amp (TLC271)).

Any info (links or otherwise) would be much appreciated.

Thanks!

To change the phase of the reference signal by 180 degrees, simply swap the connections to pins 9 and 10 of the AD630.

 

If your output is negative use a Inverting amplifier. http://en.wikipedia.org/wiki/Operational_amplifier

Also, do I need to use an op-amp with both negative and postive supply rails? (I'm just using a single supply op-amp (TLC271)).

Yes indeed. What is your supply voltage at the moment? I am not sure that the TLC271 is the best selection for your project. Do you use a sine wave or a square wave as source.
Also strictly speaking your setup is not a lock-in amplifier. As it have no (phase-locked loop) PLL thats lock on to some external reference signal. Your setup is basically a synchronous demodulator followed by a low-pass filter. The heart in every lock-in amplifier

 

The AD630 has differential inputs for the reference signal, so you just reverse the input signal connection at pins 9 and 10 to change the phase by 180 degrees and thus the polarity of the output signal.

 

Thanks all, much appreciated. And saved me messing around with a phase shifter

I've got a few other questions on the AD630, but I'll start another thread for them

 

Thanks all, much appreciated. And saved me messing around with a phase shifter

I've got a few other questions on the AD630, but I'll start another thread for them

NO stay in this thread. I think it will provide a better background. I have used the AD630 in some AC skin conductance measurement equipment. Very like your application. A nice chip, but somewhat expensive