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Psoc3 - PGA Error!! | Cypress Semiconductor

Psoc3 - PGA Error!!

Summary: 14 Replies, Latest post by danaaknight on 06 Jul 2015 04:03 PM PDT
Verified Answers: 4
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rom17's picture
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91 posts

 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...)

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.

 

 

user_1377889's picture
User
10803 posts

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
 

rom17's picture
User
91 posts

here is the zip.

Thanks

 

Regards

 

user_14586677's picture
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7648 posts

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.

 

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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.

 

 

rom17's picture
User
91 posts

 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.

rom17's picture
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91 posts

 the correct archive is there:

 

user_14586677's picture
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7648 posts

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.

user_14586677's picture
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7648 posts

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

 

rom17's picture
User
91 posts

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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user_14586677's picture
User
7648 posts

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.

user_14586677's picture
User
7648 posts

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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