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phase extractor question

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pwlps:


--- Quote from: Achu on January 02, 2020, 04:22:14 am ---I used an SPST switch ti mix non shifted input and shifted reference and sampling gave same results I think, is that

wrong?

--- End quote ---
I don't use arduino so I'm not aware of its possibilities and constraints but ok if you say you can shift the ADC reference to measure negative voltages.

--- Quote ---Also if I were to remove the coupling transformer, how should I modify it?
And could I use an op amp current source to drive the impedance?

--- End quote ---
Yes, as I was saying in a previous post using a transimpedance (as a current source) is another possibility and could be simpler to implement. 
But now I'm on vacations and can't do any schematics, try to do it yourself  :)


--- Quote ---One more question, don't we need two readings for current and voltage?

--- End quote ---
You wanted to measure the phase shift between the voltage and current, the circuit does it measuring the voltage components in and out of phase with the current  (we could do it the opposite way, measuring the current components in and out of phase with the voltage). Make sure you understand well the math I explained and the principle of this measurement.
Now, if you want to measure the module |Z| as well you will need another input to separately measure the current amplitude (here sensed by R1 or, if a current source is used, from the voltage driving the current source).

Achu:
Hi

I made this design. Is it good?
I used a transimpedance amplifier to measure current.

unitedatoms:
@Achu. There are few improvements you can do.

1. Instead of commutating voltage, commutate the current. This way: Follower opamp loaded with R->switch->Transimpedance amplifier. This will maintain switch close to zero volts, where it is most linear.
2. Use much higher current to commutate, instead 10k, go to reasonably low R, 350Ohm, if op amp can stay stable and linear with that currents. This will allow to mask switch imperfect conductance injected charge better.
3. Instead of commutating opposite polarities through same switch/low pass filter, like positive vs inverted signal, commutate zero vs signal. This will make each feeding opamp not trying competitively regulate same filter aggregating results of 2 opamps. I found this being a precision problem. When you let two loop compete instead of precision of single loop you will get may be still fairly accurate value, but it will never settle in deterministic manner. It looks like chaotic wandering value with never disappearing small value. I did modelling last month and it took a lot for me to solve the cause of puzzling low frequency noise even in perfectly noiseless LTSpice.
4. Use a dedicated part for positive/inverted pair of signals. The phase will match perfectly. The inverter in your schematic is inevitable lag, making shift slightly more than expected 180 degrees.

pwlps:

--- Quote from: Achu on January 04, 2020, 04:29:07 pm ---Hi

I made this design. Is it good?
I used a transimpedance amplifier to measure current.

--- End quote ---

Just one comment (on your version, I did not study yet the modifications suggested by unitedatoms): in your design Vrefc has some arbitrary additional phase shift with respect to the V signal, depending on C1,R6,R2.  If we call this phase shift theta, and with definitions Z=|Z|exp(i*phi), V=V0cos(wt), I=I0cos(wt-phi), I0=V0/|Z| then your measured voltages are (within a factor 2/pi or something like this):
Vcosphi=V0 cos(theta)
Vsinphi=V0sin(theta)
Icosphi=I0 cos(theta-phi)
Isinphi=I0sin(theta-phi)

You might avoid some redundancy designing the quadrature generator so that Vrefc is in phase with V and theta=0 (e.g. using an active differentiator), then you could get rid of the Vsinphi channel.

Achu:
I actually simply modified the previous design you provided earlier to remove the transformer.
So does your earlier design also create an arbitrary phase shift to the voltage?

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