## PGA

Summary: 6 Replies, Latest post by naru on 12 Jul 2012 05:28 AM PDT
User
43 posts

hello sir/mam,

I went into sample project of Programmable Gain Amplifier(PGA)  in PSoC creator, when i give input as

Freq:500 hz,,input voltage=.2v,,Gain =1,,Medium power,Offset=0,,Vref=internal vss  the output  obtained is not exact as input as i shown in Pic..

But when i  Increase the Offset to 1v its giving the correct output .

Later i increased the gain to "2" and for every offset value(0,1,2,3) the output voltage i am getting is equal to the input voltage.....plz help me in getting  the correct output....

Attachments:
User
43 posts

i am not getting the output Sir .......can i get some more solutions for my previous question.....plz......

User
10803 posts

When you upload the complete project here, we all can have a look at and probably see what's going amiss.

To do so: Build -> Clean Project

File -> Create workspace Bundle(minimal)

and then upload the resulting ZIP here.

Bob

Cypress Employee
730 posts

Hi naru,

Providing the right offset is important in application like this. Else, the output will be clipped.

The operating voltage of the device should be greater than the input/ output of the PGA. Are you operating at 3.3V or at 5V?

Thanks,

Gautam

User
7648 posts

The input CM range of the PGA is

Vdda >= Vin  >= Vssa

The output PGA CM range is

(Vdda - .3V) >= Vin X G >= (Vssa + .3V), the .3 is loaded spec, see datasheet for load.

Example, Vssa = 0, Vdda = 3.3, You want a 2.5V Pk-Pk swing on output, your input =
swings .1 V below ground on negative peak (assuming sinewave), we need to offset the
input or AC couple it to PGA to meet input CM range. To DC offset, hence avoid a coupling
cap, use the prior post R divider to Vdda. To AC couple bias with an R divider from Vdda
to PGAin to Vssa, which biases input such that peak output does not exceed PGA output CM
range.

So G = 2.5 / .2 = 12.5. Closest G = 8 in configurator. If you picked the next higest G the
output of the PGA would be in clipping. Therefore output swing will be 8 x .2 = 1.6V.
Nominally the DC component of the input signal should be offset such that ouput swings
symmetrically about (Vdda - Vssa) / 2 = 1.65 V. You can do this by placing a resistor
divider, on PGA in and AC coupling signal. Or offseting the ground referenced input signal
with a DC offset.

So Vout = G x ( Voff + Vsig), or Voff = Vout/G - Vsig. For Vsig = 0 (output then at Vdd/2),
Voff = Vout/G = Vdd / ( 2 * G) = 3.3 / 16 = ~.21 V. So your divider should set that up as
offset.

This then satisifies both input and output CM range of the PGA.

Regards, Dana.

User
43 posts

I got output of PGA ....Thank you for all( SIR) your support .....

User
7648 posts

Input CM range for analog is Vssa to Vdda, so you need to effect a DC offset

into the PGA. One approach just using resistors -

Attached is excel file to calculate R values.

Regards, Dana.

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