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I am measuring the impedance response of a piezoelectric component. It has a unique impedance response having a resonance portion (low impedance ~10-30ohmns) and large impedance (100k-200kohmns) within a 2kHz bandwidth. See the webpage for a figure showing the impedance behavior. The frequency is around 20kHz.

I am driving the piezo component using a high power amplifier and a function generator. I am measuring the voltage and the current to determine the impedance.

I am using a unique drive control: constant vibration level. I am monitoring the vibration level and adjusting the voltage to keep it constant. Therefore, we have large voltage (50VRMS) and low current (1mARMS) in antiresonance, and low voltage (0.2VRMS) and high current (50mARMS) in resonance

The problem is that I want to use one setup to measure both the high and low impedance cases (so I can do a computer controlled frequency sweep while measuring). In the high impedance case, there is a voltage drop across the probe, which causes error, therefore I use the voltage monitor from the amplifier. But, in the low voltage resonance case, the voltage monitor has a lot of noise and is jumpy due to the low voltage level, therefore more error.

For current, I decided to use a textronix current probe (TCP305Amplifer w/ TCPA300 probe) and wrapping the wire 10x around the probe to measure the low anti-resonance currents. I also have used a shunt resistor with a voltage probe for current.

What are your suggestions on how I can measure voltage and current accurately? I am most concerned about the current probe working at such low currents ( I do get a somewhat coherent signal) in the antiresonance and finding a voltage probe I can use for high impedance measurments.

The phase is important for me because I want to calculate power. I have found that using a probe in antriresonance increases the power significantly (~20%), even with a 100x probe with ~3pF capacitiance.

Any suggestions/help/comments are greatly appreciated.

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# Measure high and low impedance

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