Sudden strong noise from the analog OpAmp

[Anonymous]

Hi,

   

I am building a device that works like a phonograph. I am using a piezo disc as a microphone and a pcos3 kit for the amplifying, filtering and processing . But I a getting a terrible sudden noise at the output of the OpAmp for some seconds.

   

Maybe somebody knows the reason?

   

I attached the amplifier circuit.

   

THANKS!

[Bob_Marlowe]

The project you supplied misses the main loop. main() will start over and over again including PSoC component initialization.

   

Place at least a while(1); infinitive loop after your initialization.

       

Bob

[Anonymous]

Ok. Thanks!!!

[ETRO_SSN583]

To Bobs point you commented out your for(;;) so application falls out of main().

       

Note you use 2 Vrefs and 2 OpAmps to buffer the two Vrefs. You really only need to do

   

the left one and route its output to OpAmp2's NI input. Saving an OpAmp resource.

               

Regards, Dana.

[Anonymous]

Great! Thanks! I thought is not possible...

[ETRO_SSN583]

One additional consideration I am sure you are aware of is the exceptionally high Z levels

   

at the input amp, fdbk R values, etc.., very subject to noise pickup. So PCB layout very

   

important.

       

https://www.dropbox.com/s/2h96beh1fbvz4e2/noise_notes.zip?dl=0

       

http://www.cypress.com/?rID=39157&source=an61290     AN54181 - Getting Started with PSoC 3

   

http://www.cypress.com/?rID=39677     AN57821 - PSoC® 3, PSoC 4, and PSoC 5LP Mixed Signal Circuit Board Layout Considerations

   

http://www.cypress.com/?rID=40247     AN58827 - PSoC® 3 and PSoC 5LP Internal Analog Routing Considerations

   

http://www.cypress.com/?rID=39974     AN58304 - PSoC® 3 and PSoC 5LP – Pin Selection for Analog Designs

       

Regards, Dana.

[Anonymous]

Thanks!

   

As regular you are very helpful 🙂

[odissey1]

Piona,

   

attached an example from Analog Devices AN-106 on how to interface Piezo transducer with OPAMP.

[ETRO_SSN583]

You can't use that AD circuit as shown, its meant to operate on split supplies.

   

You can do a single supply version of it, eg. still bootstrap with + fdbk to raise

   

Z levels, but then bias circuit Z becomes the problem for practical values. Maybe

   

a real JFET current source would permit that.

       

As an aside the 10 uF block cap in that circuit can't be an ordinary electrolytic,

   

it has to have super low leakage to avoid latchup in the OpAmp.

       

Regards, Dana.

[Anonymous]

HI,

   

To interface a piezo transducer I used a single supply circuit of charge amp.

   

Regarding the big Z in the circuit

   

"still bootstrap with + fdbk to raise Z levels, but then bias circuit Z becomes the problem for practical values" - what does it mean?

   

All piezo interfaces containt big Z impedance. Maybe it is reasonable to use external Opamp and not Psoc?

       

Anna

[ETRO_SSN583]

The judgment to use external OpAmp or IA should be made on

   

noise, bias currents, offsets, etc.. The primary issue here is the

   

biasing solution and its affects on loading of the Piezo.

       

You might consider doing some spice simulations to get a general idea

   

of how to proceed.

       

Note board prep for real hi z applications not trivial. Google "Bob Pease"

   

on ap notes for bias current and pcb design factors.

       

Regards, Dana.