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# PGA didn't work well

Anonymous
Not applicable

hi,

i configure PGA with this configuration :  PGA_1_Start(); PGA_1_SetGain(PGA_1_GAIN_04);

And the schematic is attach.

I put in P60 a sinwave with 0.5 volt peak to peak(vpp) and the P62 i only have approximately 1.2 vpp.

If i set Gain=08 i get on P62  approximately 2.1 vpp,

is it normal?

1 Solution

# Re: PGA didn't work well

Level 9
Level 9

The Vdd / 2 Vref internal is a R divider off of Vdd. Vdd can be noisy if you

do not have good decoupling, and that noise would appear on the Vdd / 2

reference. Also references tend to be hi Z, so you need to use an OpAmp

configed as a buffer to buffer Vref to the PGA gain setting R input.

Regards, Dana.

6 Replies

# Re: PGA didn't work well

Level 9
Level 9

Does your sine meet the following Vdda >=Vsine >= Vss, ie. there

is no negative portion of sine below ground ?

Also keep in mind the the transfer function is

Vout = Vref + (Vin – Vref) × Gain

What is the sine frequency ?

Regards, Dana.

# Re: PGA didn't work well

Anonymous
Not applicable

the sine work in 12hz with vpp=0.5 and average value = 2.9

regards,

Romil

# Re: PGA didn't work well

Level 9
Level 9

Use the equation, Vout = Vref + (Vin – Vref) × Gain, measure the peak values of Vin and Vout.

and see if the equation predicts what you are seeing.

I set up something close to your case and found my board Vdda was causing a problem, at a

G = 4 the output was saturating because my Vdda was 4.3 V (I need to figure out why this,

another problem).

So I cut the offset of the sinewave in to 2.00 V, and got the predicted output

for G = 4.

Regards, Dana.

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# Re: PGA didn't work well

Level 9
Level 9

Screenshot of PGA in operation.

# Re: PGA didn't work well

Anonymous
Not applicable

OK.

I try it. The problem about gain is form vref. I use internal vref (2.5 V) from PSoC and  it wasn't constant if i connect directly on PGA pin.

is it normal?

thanks

Regards,

Romil A.

# Re: PGA didn't work well

Level 9
Level 9

The Vdd / 2 Vref internal is a R divider off of Vdd. Vdd can be noisy if you

do not have good decoupling, and that noise would appear on the Vdd / 2

reference. Also references tend to be hi Z, so you need to use an OpAmp

configed as a buffer to buffer Vref to the PGA gain setting R input.

Regards, Dana.