PGA does not use reference

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Anonymous
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In a PSoC 3 design I need a differential amplifier but it does not seem to work.

   

I created a simpler design for the CY8CKIT-030 (as attached):

   

Two voltage VDACs drive the two inputs of a PGA the output of the PGA can be observed through

   

a voltmeter and the ADC.

   

I can control the VDACs and PGA settings through USB.

   

I checked the settings of the PGAs (PGA_1 in the schematic is implemented by SC0 as seen by the chip so the

   

relevant registers are 0x5800 pp):

   

0x5800:

   

'0x0c', '0x2e', '0x01', '0x00'

   

So the GNDREF - field in SC0_CR2 is 0, which  -- according to the register TRM means the external input is used.

   

And still, the output of my circuit depends only on the voltage of VDAC8_1, i.e. the positive input of the PGA.

   

The output is as if the reference input was at 0.

   

I also checked the analog routing representation and this seems to be correct, the output of SC2 goes to the input of SC0.

   

I did not find many examples that use the PGA, but maybe someone in the forum has more experience with them.

   

If not, I need to open a case, I guess.

   

Andreas

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