Help understanding charge amplifier gain ?

[eliw64]

I'm in the lab working with a Bruel & Kjaer Nexus charge amplifier, connected to a piezo element. Outputs are being fed to a PXIe-1075. I'm having trouble understanding the preamp's gain options - it gives me adjustable output sensitivity options in [V/Pa] and allows me to input the transducer sensitivity in [pC/Pa].

The Nexus manual says that gain is defined by output sensitivity / transducer sensitivity (which would be voltage sensitivity [V/Pa] * transducer capacitance [pF]), but I thought the whole point of a charge amplifier was that gain is independent of transducer capacitance?

Basically I just need to know how many dB of gain the preamp is putting out so that I can correct the measured voltage from LabVIEW. Really confused here, thanks for the help!

[a.cisneros]

Hi,

What instrumentation modules are you using in your PXIe-1075 chassis? 

Regards,

Angie Cisneros 

Applications Engineering 

National Instruments 

http://www.ni.com/support

[Henrik_Volkers]

The nexus support different piezoelectric devices, the V/Pa indicate a pressure sensor configuration 🙂

The traditional CA setting should result in a (m)V/pC output .

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@eliw64 wrote:

The Nexus manual says that gain is defined by output sensitivity / transducer sensitivity (which would be voltage sensitivity [V/Pa] * transducer capacitance [pF]), but I thought the whole point of a charge amplifier was that gain is independent of transducer capacitance?

Some (pre)amplifiers can be configured to a voltage input ...  so

determine what hardware is installed in your nexus and RTFM

And about the  independence  of source impedance  (transducer capacitance) .. it depends ( 🙂 )how close you look, the ideal charge amplifier looks like a short-cut to the transducer so the capacitance doesn't matter. REAL CAs well .. shameless link to own paper

Haven't tested the nexus ... we have some in the lab....  no time ..

[Henrik_Volkers]

If you have access to a AC voltage source of known amplitude (or a soundcard and AC voltmeter) and a capacitor (air, mica , polystyrene*) in the 100pF to 1000pF range , you can  do a test to determine your sensitivity.

(I wouldn't call it calibration 😉 )

*) air capacitors are rare ... , mica and polystyrene  are easy to get and sufficient stable and low loss for a test. Since Q=C*U

you need to know C and U for a test.   Common mica caps are in the 5% to 0.5% range .. polystyrene ~5%   (air down to some ppm :D)