What are charge amplifiers and how they differ from normal amplifiers?

[Kinshoro]

can anyone tell me what are charge amplifiers and what is the difference between normal amplifiers and charge amplifiers and can you suggest me a good component or opamp for charge amplifiers for low frequency sound using piezo crystals????

 

[karesz]

charge amplifiers

Hi,
://en.wikipedia.org/wiki/Charge_amplifier
www.etesters.com/see/Charge_Amplifiers
www.mikrocontroller.net/topic/151789/
www.iup.uni-heidelberg.de/institut/.../Physik-II-S-2005--1-Skript-Elektrostatik.pdf
K.

 

[Kinshoro]

Re: charge amplifiers

can you tell me a good opamp which would be good for this application like OPA128 or something for analog devices???

 

[karesz]

charge amplifiers

Hoi,
Ein LEV ist kein OPamp ...
The best CSAs are from Amptek, then R & S... .
K.

 

[keith1200rs]

charge amplifiers

I have used the MAX4250 with good results as a charge amp. You need to make sure it would be suitable for you (bandwidth, gain, noise etc.) I used it for a hydrophone amplifier with 21dB of gain from 3850pF up to 100kHz. It beats the performance of a B&K charge amp and runs from 3.3V!

Keith

 

[karesz]

charge amplifiers

Kystler (CH) has good piezo sensor (dedicated)CSAs too...
K.

 

[shouvikroy]

Re: charge amplifiers

hey keith ..i too am working on a pressure gauge. I dont have any data about the piezoelectric transducer..help would be appreciated..

 

[keith1200rs]

Re: charge amplifiers

I would need to know something about the pressure transducer. The gain depends on the piezo capacitance. This is one I designed for a B&K hydrophone which worked well.

Keith,

 

[shouvikroy]

Re: charge amplifiers

Thats the problem Keith. The transducer is scavenged from old stock. I dont have any data sheet and no info. about the transducer. All I can show you is the photograph of the transducer if that's of any use. Thanks. Needed your help.

 

[FvM]

Re: charge amplifiers

I dont have any data sheet and no info.

Measuring the capacitance and a rough calibration factor shouldn't be a problem.

 

[shouvikroy]

Re: charge amplifiers

Thanks for replying FvM. I really appreciate your help. Yes I would try to measure the capacitance. But I have no clue about the calibration factor. As par NI website ,CF is the output of the transducer expressed in mV/V. But how should I measure that? I know the sensitivity of the transducer is 100pC/psi. Here is the transducer I am talking about.

---------- Post added at 13:57 ---------- Previous post was at 13:48 ----------

I checked the capacitance with a DMM. It showed 1nF. Do you suggest the reading is correct?

 

[erikl]

Re: charge amplifiers

I know the sensitivity of the transducer is 100pC/psi. ... I checked the capacitance with a DMM. It showed 1nF. Do you suggest the reading is correct?

The order of this value seems quite right (Keith's B&K hydrophone has 3850pF).
Hence your transducer's output voltage will be
δV/δp = (100pC/psi)/1000pF = 0.1 V/psi = 1.93 mV/Torr = 14.5 µV/Pa

 

[FvM]

Re: charge amplifiers

The said 0.1V/psi applies, if the transducer is operated in voltage mode, acquiring the output voltage with a high impedance buffer. With a charge amplifier, the amplifier's conversion factor would be put into the calculation instead of the sensor capacitance.

 

[shouvikroy]

Re: charge amplifiers

Thanks for your replies guys. But I am still clueless. Maybe it would help me if I can run some simulation with Proteus. The problem I am facing with simulation is that I dont know how to model the transducer accurately. I know it can be modeled as a voltage source with series capacitance or as a current source with parallel capacitance but are these models accurate enough?How do I model the charge generated by a voltage source?(which values i consider?) Moreover what kind of waveform should I expect? Some say charge amp is basically an integrator. Is it so? Sorry for my long post..am just totally lost here. Thank you

 

[FvM]

Re: charge amplifiers

In my opinion, your suggested models are representing the transducer sufficiently. In the current source variant, the current represents the pressure change dP/dt while the voltage in the other model represents the pressure. In both cases, you have to decide about the actual waveform of the pressure variation.

A charge amplifier is an integrator with current input respectively low input impedance. And for practical reasons, it has a low frequency roll-off, introduced by the resistor in parallel to the integrator capacitor, in other words, it's designed as a non-ideal integrator. As a practical consequence for a pressure sensor, it can't measure static pressure.

 

[shouvikroy]

Re: charge amplifiers

ok i got it. The input voltage waveform will be the pressure waveform I am going to apply. And the capacitance would be the capacitance measured at the transducer output. Thanks. I would let you know about my progress. Thanks guys. You are *** sent..!!

 

[shouvikroy]

Re: charge amplifiers

Ok guys, finally I was able to figure out the charge amp. Its working fine now. I used TL082 opamp.