Delta Sigma ADC Accuracy

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Anonymous
Not applicable

I am working on a project that measures an analog output from a solar radiation sensor. I am a novice and have some confusion about the various settings for the Delta Sigma ADC. It seems that, depending on the buffer mode, I get wildly different readings, even when the gain is set to 1. Bypassing the buffer seems to provide me with the most reasonable readings, but still not accurate. Using a voltmeter, I measure values of around 1500 microvolts, yet the ADC spits out a reading of only about 600 microvolts. Changing the conversion mode didn't make any difference to the readings, and dropping the sampling rate of course only slowed down the update interval.

   

Has anyone run into issues like this before when trying to use the delta sigma ADC at low amplitudes? The ADC works just fine when I connect the pins to an external voltage source that is much greater (1.5V instead of less than a millivolt), but I think the ADC may be having issues due to the low magnitude of what I'm measuring

I've attached a screen shot of my configuration and my sampling code is below, though it's not much different from the sample project on creator

   

ADC_DelSig_Start();

   

    ADC_DelSig_SelectConfiguration(ADC_DelSig_CFG1,1);

   

    ADC_DelSig_StartConvert();

   

    CyDelay(100u);

   

    for(i = 0; i < 100; i++)

   

    {

   

        if(ADC_DelSig_IsEndConversion(ADC_DelSig_WAIT_FOR_RESULT))

   

        {

   

            output = ADC_DelSig_CountsTo_uVolts(ADC_DelSig_GetResult32());

   

            break;

   

        }

   

        CyDelay(5u);

   

    }

   

    ADC_DelSig_StopConvert();

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HeLi_263931
Level 8
Level 8
100 solutions authored 50 solutions authored 25 solutions authored

Your ADC is configured into differential mode, is this what you want? In that mode, there are some limitation regarding commong mode voltage. Also, did you read the component data sheet with regards to the limitations of the input voltages?

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ETRO_SSN583
Level 9
Level 9
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This might help -

   

 

   

    

   

          

   

http://www.cypress.com/?rID=39677     AN57821 - PSoC® 3, PSoC 4, and PSoC 5LP Mixed Signal Circuit Board Layout Considerations

   

http://www.cypress.com/?rID=43337     AN61290 - PSoC® 3 and PSoC 5LP Hardware Design Considerations

   

http://www.cypress.com/?rID=40247     AN58827 - PSoC® 3 and PSoC 5LP Internal Analog Routing Considerations

   

 

   

 

   

As hli points out look at graphic of CM range in config tool. If you bypass the buffer you will get/exceed true R-R performance.

   

 

   

Regards, Dana.

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Anonymous
Not applicable

 Yes I'm measuring the differential voltage across two pins and I am utilizing pins that, according to the documentation, have higher accuracy for analog purposes.

   

The datasheet doesn't discuss any limitations on input voltage on the low end, only on the high end.

   

With regards to the common mode voltage, unless I'm misunderstanding something, I am getting nowhere near exceeding the input range (+/- 64000mV when I'm only reading in, at most, around 15 mV).

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Anonymous
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 Sorry, 64000 uV, not mV

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ETRO_SSN583
Level 9
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If you read the ap notes bypassing Vref importent as well.

   

 

   

Attached some old but very applicable ap notes on signal path

   

errors and dealing with them.

   

 

   

Is this your own layout or you using a Cypress DVK ?

   

 

   

Regards, Dana.

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ETRO_SSN583
Level 9
Level 9
250 likes received 100 sign-ins 5 likes given

Another area of concern is bulk cap performance, Polymer Tanatalum

   

best, an order of magnitude better z(f)  performance than traditional

   

tants.

   

 

   

Multilayer ceramic as well for higher f noise suppression.

   

 

   

Regards, Dana.

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