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Would DC in signal effect measured impedance??

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zoulzubazz

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hello,

this is another elementary question, assume a complex impedance Z, to evaluate Z i measure the voltage across it while passing a known sinusoidal current, sin(2*pi*f*t). My question is will the value of Z remain the same if evaluated with a DC component; dc added to the signal (dc + sin(2*pi*f*t))??

thank you very much.
 

will the value of Z remain the same if evaluated with a DC component
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Yes, if
- the vectorial voltage measurement isn't affected by DC (usually it shouldn't)
- the measured device is linear
 
thanks FvM, but in point no.2 do you mean the measurement device??

also could you please elaborate slightly more about the term 'vectorial voltage'?? thanks. cheers zoul.
 

zoulzubazz,

Z is defined by the circuit components, not by what currents or voltage are present. Voltage is not a spacial vector. It has magnitude, but no direction. AC voltage can be defined as a phasor with respect to another voltage or current, however.

Ratch
 
hey ratch, thanks, but now I have another question, lets say I choose to pump the DC-Offset-sinusoid (Idcs) through the impedance (Z).Both both (Idcs) and voltage (Vdcs) across the impedance is recorded. After recording and storing for say 1sec worth (assume f is a few kHz) the DC component is filtered and only the sinusoidal components of current and voltage (Is ,Vs) are used to evaluate Z (Vs/Is). Will the estimate of Z be right in this case or will it come out wrong since DC-offset-sinisoid was used to excite Z but later filtered out during evaluation of Z ?? thanks
 

zoulzubazz,

The DC component is part of the output, so if you don't have it, your value for impedance is going to be different. By the way, finding the impedance with a sin wave only gives you the impedance at one particular frequency. I hope you realize that.

Ratch
 
hello,

one last query, if the idea is to measure impedance using a sinusoid (of say 5kHz) but the circuitry adds a DC offset noise and assuming i am being forced to use this noisy signal, would filtering the DC from recorded I&V and then evaluating impedance give me the same impedance as it would if there were no noise added to the 5Khz sinusoid. thank you very much. you guys are stars.
 

zoulzubazz,

Since noise is a spurious signal that does not relate the input signal signal to the output signal by RCL components, getting rid of it by any means should improve your measurement.

Ratch
 

one last query, if the idea is to measure impedance using a sinusoid (of say 5kHz) but the circuitry adds a DC offset noise and assuming i am being forced to use this noisy signal, would filtering the DC from recorded I&V and then evaluating impedance give me the same impedance as it would if there were no noise added to the 5Khz sinusoid.
Click to expand...
State-of-the-art method for vectorial measurement (e.g. used by commercial LCR meters) is synchronous demodulation, it removes DC as well as AC signals of other frequency. With synchronous demodulation, wide band noise will increase measurement uncertainty but shouldn't cause systematical errors.

And an open point from a previous post:
but in point no.2 do you mean the measurement device??
Click to expand...
I meant the "measured device" respectively device under test. If it's not linear, superimposed DC as well as different AC amplitudes are expected to change the measured impedance Z(ω).
 
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