Psoc3 - PGA Error!!

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Anonymous
Not applicable

 Hi,

   

I have a problem with noise. Can check the image that the blue is a sign almost no noise (https://www.dropbox.com/s/ijswxkjcemlxt0m/High%20frequency%20noise%21%21.jpg?dl=0)

   

The purple is a signal just outside Two Opamp Topology (equal to figure 3 of this article http://www.cypress.com/?docID=41827) with gain = 2.

   

As you can see the signal have some strange peaks. 

   

They know how I can fix this?

   

Best regards.

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Bob_Marlowe
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Can you post your complete project, so that we all can have a look at all of your settings? To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file. Do not forget the scope settings and the pictures.



Bob
 

View solution in original post

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Bob_Marlowe
Level 10
Level 10
First like given 50 questions asked 10 questions asked

Can you post your complete project, so that we all can have a look at all of your settings? To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file. Do not forget the scope settings and the pictures.



Bob
 

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Anonymous
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here is the zip.

   

Thanks

   

 

   

Regards

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ETRO_SSN583
Level 9
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This ap note is in major error. You cannot use 2 PGAs to create an IA as their

   

G matching accuracy is so poor it virtually destroys all CM which is the reason

   

for using an IA in the first place.

   

 

   

    

   

         

   

http://www.cypress.com/?app=forum&id=2492&rID=82151     IA CMR Analysis

   

 

   

 

   

www.dropbox.com/s/plck7e95v7pw33c/CMR%20Analysis%20IA.pdf

   

 

   

The only way to build an IA in PSOC is to use a thin film precision R network to set up

   

G in the IA front end, using PSOC OpAmps. Even then the Aol matching of the OpAmps

   

will limit your CM performance.

   

 

   

Regards, Dana.

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Anonymous
Not applicable

 ok. 

   

now i create IA with one PGA and one Opamp and the error is same.

   

How can i resolve it?

   

thanks

   

Regards

   

 P.S. The gain =4 and use external resitor for fist opamp.

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Anonymous
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 the correct archive is there:

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ETRO_SSN583
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Looks like R1 and R2 are swapped in value. But you still have

   

a basic problem, you cannot use a PGA in an IA because its G

   

accuracy is so poor part to part. If you want CM rejection either you

   

use precision Rs external (thin film array) coupled with onboard

   

OpAmps or an external IA, the latter emminently preferred.

   

 

   

What CM rejection spec do you want ?

   

 

   

In the ref material I posted is extensive analysis of 2 and 3 OpAmp

   

IA CM performance. And requirments on the Rs.

   

 

   

 

   

Regards, Dana.

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ETRO_SSN583
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Here is an analysis due to Aol differences onchip between OpAmps -

   

 

   

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Anonymous
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ok.

   

Thanks.

   

Now i only test the one PGA with G=2. the input signal is a sinwave from external function generator and the output is very strange...

   

Check this:

   

the purple is a output signal from PGA.

   

is it normal???

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ETRO_SSN583
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Several considerations -

   

 

   

1) Input signals to any pn in PSOC must meet Vdda >= Vinpout >= Vssa

   

2) OpAmps and PGAs need to be biased up to meet common moide ranges

   

 

   

   

 

   

   

 

   

In your case, G = 2, say you have a 1V sine into part, then AC couple it in.

   

To set bias Ideally you want the 1 V sine to swing around 2.5 V on output as

   

bias (5 V Vdda). So the VDAC would be 1.25 V. x G of 2 = 2.5 output bias point.

   

 

   

Regards, Dana.

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ETRO_SSN583
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Another way of biasing -

   

 

   

   

 

   

The sharp spikes make me think you have some coupling going on into

   

PGA input from ajacent pins with digital signals on them ?

   

 

   

Regards, Dana.

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Anonymous
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 Thanks. 😉

   

problem solved.

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ETRO_SSN583
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What was the error / cause you found ?

   

 

   

Regards, Dana.

Anonymous
Not applicable

 the error is in PGA. i need to use more power...

   

it's strange error...but now it's ok.

   

thanks

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ETRO_SSN583
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When PGA is used on lower power its GBW and slew rate drop. In some

   

control loops this would allow greater transients to occur, ie. its not reacting

   

to change fast enough.

   

 

   

But the nosie youw ere experiencing does not "normally" occur because of the

   

above, so I am stumped as well.

   

 

   

Regards, Dana.