Getting 1 megohm input impedance for a piezo pickup.

[abbey road d enfer]

https://sound-au.com/project202.htm

Which one? There are 5 different circuits there.

 

[deedee]

Sorry, fig5.

Charge amp, with eq.

 

[KIDVOX]

Ive seen simple opamp circuits using 100%nfb these claim to give input z in the order of megs and a very low output z

 

[abbey road d enfer]

Ive seen simple opamp circuits using 100%nfb these claim to give input z in the order of megs and a very low output z

Almost any opamp does that. However, JFET opamps are better suited to high impedance sources, in terms of noise.

 

[murrayatuptown]

I don't fully understand enough to explain or argue points, but charge amp for these and other sensors that are hi-z, I'm told is low-input-z. Maybe because it's I-V converter...not sure. Read a bunch on them some years ago during some piezo projects and while repairing an accelerometer signal conditioner, which is common for measuring vibration profile for one form of stress screening, particularly in manufacturing of automotive & aviation electronics.

A current source is often utilized to power JFET charge amps.

 

[abbey road d enfer]

I don't fully understand enough to explain or argue points, but charge amp for these and other sensors that are hi-z, I'm told is low-input-z.

A charge amp uses current feedback , which results in very low input impedance, but also in very high sensitivity to input current. Basically it forces the transducer to operate as a current generator.

A current source is often utilized to power JFET charge amps.

For reasons that are decorrelated. It usually is done to eliminate the problems due to long cables. Current drive makes the influence of wire resistance negligible. Many measurement systems use current loops.

 

[murrayatuptown]

Thank you

 

[Eef]

High-Z Input Preamps for nice explanation of charge amplifier.