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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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PSoC 5LP
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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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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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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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Sorry, 64000 uV, not mV
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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.