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How do you get rid of noise on the K20 PGA?

Question asked by Andrew Brown on May 7, 2014
Latest reply on May 8, 2014 by Andrew Brown

I need to measure some small AC signals (~100 uV) and was planning to use the PGA however the PGA seems to introduce a lot of noise in the signal path.

 

I have a custom board that exhibits this behavior and was also able to write a small Processor Expert program that reproduces the noise on the TWR-K20D72M demo board.

 

Components used:

ADC, PGA, Vref, PDB

 

The PDB triggers a conversion in the ADC at a 3000 Hz sample rate.

PGA Gain = 1x

VREF Mode= tight, Regulator=Enabled

ADC   Mode=differential, Conversion Time, single sample, short sample time, low-power mode disabled

 

V+ = V- = 0.6 V

 

If the PGA is enabled i get about 7 bits of noise

    1:     52                                                             

    2:    -51                                                 

    3:     54

    4:    -45                                                                

    5:     53

    6:    -45                                                                

    7:     51

    8:    -54                                                               

    9:     57                                                 

   10:    -46                                                                

   11:     55                                                                

   12:    -46

 

With the PGA disabled and reading DP directly I get about 5 bits of noise:

    0:      1                                                                

    1:     33                                                                

    2:     19                                                                

    3:     12                                                                

    4:     27                                                                

    5:     28                                                                

    6:     32                                                                

    7:     24                                                                

    8:     28                                                                

    9:     26                                                                

   10:     11                                                                

   11:     24

   12:     16

   13:     31

 

On my board I get slightly smaller values.

 

One thing I noticed is that when the PGA is enabled, every other sample oscillates between positive and negative. So either noise is getting coupled in from the conversion clock or I have something configured wrong (or?).

 

Any other thoughts? I can attach the PE project if that helps.

 

-andy

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