OPA325: OPA325

Guy Berwart

Part Number: OPA325
Other Parts Discussed in Thread: OPA320,

Hello,

I plan to built hydrophones using the OPA325 or OPA320 as charge amplifier. The piezo to be used has a capacitance of 8200 pF operating within a frequency range from 160 Hz to 15 KHz

I designed and tested the circuit on Tina (see attached) but I'm not shure to be right with a 40 dB gain using these op amps still stable.

As I haven't any experience with such ampiric design, could you please let me know your advise and recommendations?

I already thank you for your help

Kind Regards,

Guy

40 dB hydrophone amplifier OPA 325 REV 01.TSC

over 1 year ago

0 Marek Lis over 1 year ago

TI__Guru 61340 points

Guy,

There are couple of issues with your circuit.

1. Using R24 of 1kohm attenuates the gain at 15kHz by 3dB (by about ~30%) - see below.

Shorting R24 maintains the gain of 100 above 15kHz - see below.

2. The input signal is AC coupled and thus it needs to be referenced to signal ground and not mid-supply - see below.

With the above modifications, OPA325 is very stable with 90 degrees phase margin - see below.

A small-signal transient analysis confirms very stable operation with no overshoot - see below.

Likewise, implementing 40dB hydrophone using OPA320 is very stable - see below.

Below I have attached circuit schematics used for AC and transient simulations.

40dB hydrophone amplifier AC stability with OPA325.TSC

40dB hydrophone amplifier transient stability with OPA325.TSC

0 Guy Berwart over 1 year ago in reply to Marek Lis

Prodigy 110 points

Dear Marek,

Thank you so much for your analyse but I'm a little bit confused now because I designed my circuit based on the SLOA033A where the sensor (piezo) is connected between the negative input of the op amp and its positive input with 1/2 Vcc.

I knew that the 1 K resistor will attenuate the signal but this publication indicates that this one provides ESD protection.

Please let me know your comments regarding this topics.

I already thank you,

Kind Regards,

Guy

0 kai klaas69 over 1 year ago in reply to Guy Berwart

Guru 140200 points

Hi Guy,

Guy Berwart said:
Thank you so much for your analyse but I'm a little bit confused now because I designed my circuit based on the SLOA033A where the sensor (piezo) is connected between the negative input of the op amp and its positive input with 1/2 Vcc.

Yes, this can make sense when the equivalent circuit model of hydrophone shows a parallel resistance which is smaller than your feedback resistance. Often the opposite is the true.

Do you have an equivalent circuit model of your hydrophone? Or a leakage current specification? Can you give a link to the datasheet of hydrophone?

Kai

0 kai klaas69 over 1 year ago in reply to kai klaas69

Guru 140200 points

You may want to have a look at these threads:

e2e.ti.com/.../tlv2771-tlv2771-signal-conditioning-piezoelectric-sensors-note-sloa033a-what-s-the-difference-between-voltage-mode-amplifier-and-charge-mode-amplifier

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/986054/asking-about-charge-amplifier

Kai

0 kai klaas69 over 1 year ago in reply to kai klaas69

Guru 140200 points

But check for stability!

Kai

0 Guy Berwart over 1 year ago in reply to kai klaas69

Prodigy 110 points

Hi Kai,

If you carefully read back my reply to Marek, you will read that I was inspired from the SLOA033A publication to set up my design..

C24 is used to simulate the capacitance of the piezo with TINA, nothing else and the position of R24 installed between or before C24 and the negative input of the OPA325 doesn't change anything to simulate the circuit. It just attenuate the gain by 3 dB as Marek mentioned it but I knew that.

On the bench, if I replace the piezo by C24, a 8.2 nF capacitor and connect a LFG on it and the + input of the op amp, it works perfectly.

An other interesting Circuit note from Analog Device regarding such design is CN-0350 witch gives different informations about this subject.

Kind Regards,

Guy

0 kai klaas69 over 1 year ago in reply to Guy Berwart

Guru 140200 points

Yes, is stable:

guy_opa325.TSC

Kai

+1 Marek Lis over 1 year ago in reply to Guy Berwart

TI__Guru 61340 points

In this particular case it makes no difference where you connect the return of your the piezo sensor because it's capacitively coupled but as a general rule the return of the sensor should not be connected to unbuffered reference voltage (1.65V) because it could feed back into the positive input terminal corrupting the output. As far as R24 goes, if it's there for ESD protection, it should be where Kai recommended so the -3dB gain roll-off occurs at 102kHz instead of 16kHz.

+1 Guy Berwart over 1 year ago in reply to Marek Lis

Prodigy 110 points

Thank you so much for your help. I will test it on PCB's with the piezo connected on it now.

Kind Regards,

Guy

0 Marek Lis over 1 year ago in reply to Guy Berwart

TI__Guru 61340 points

ça a l'air bon et bonne chance!

0 Guy Berwart over 1 year ago in reply to Marek Lis

Prodigy 110 points

Bonjour Marek,

Je vois que vous parlez Français, quel plaisir!!!

Merci mille fois pour votre aide.

Bien cordialement,

Guy

0 Marek Lis over 1 year ago in reply to Guy Berwart

TI__Guru 61340 points

Mdr, J'aimerais pouvoir parler français mais je ne connais que le traducteur Google :)

Au plaisir de vous parler,
Marek

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OPA325: OPA325