Dear all ,


Current is been injected into Human body through pair of outer (Ag-Cl) electrodes - 2mA (p-p) of 70kHz frequency .

Whereas the Voltage difference is measured across the inner pair of electrodes , now if I feed this into a Instrumentation INA111 amplifier (gain 10), the output is not accurate , the gain is more or less than expected .
Signal 1 : 2.3Vpp , 68kHz
Signal 2 : 2.1Vpp , 68kHz with phase shift;

difference - 200 mVpp to be amplified

Possibility of Phase shift induction might cause this variation , now since the signal is not generated ,
What is the possible way to phase synchronize these two signals before feeding into the Instrumentation Amplifier ?

Is there a way possible with analog signal processing like PLL(Phase locked loop ) or Do we really have to digitize the signals and use the signal processing methods like Cross-correlation or so ? Find attached schematic.

Thanks in advance .

 

This sounds like a good job for a lock-in amplifier. http://www.thinksrs.com/products/SR810830.htm DSP lock-ins are fantastic pieces of equipment. Analog lock-in's exist. I was using chopped light, so the waveform was trapezoidal, not square. there's a fudge factor that you need to apply to get the amplitude.

There is probably lots of interference, so you need a frequency selective detector.

 

I second the Lock-In

 

Instead of Lock-in amplifier , can I use
PXle6163 NI data acquisition device and use the cross-correlation method to correct the phase shift , could occupy less space.

Is there anyway to use the analog circuitry for this phase correction without digitizing the signals immediately.

 

There might be some useful info here: http://www.analog.com/en/design-cen...circuits-from-the-lab/cn0363.html#rd-overview

Synchronous detection is another term.