Why does ADC sample value change with sample frequency?

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Anonymous
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I have a custom design which used CY8C4247LQI-BL483 for main control chip. I have two ADC input channel, one for battery voltage detect and another for current channel. When I set different sample frequency, for example 1MHz to 18MHz as user guide told me, I got different return value from the battery voltage detect channel, the faster sample frequency I set, the smaller value I got. But the current detect channel had no affection. What's the problem? How can I fix it?

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Bob_Marlowe
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The 18MHz are not the sample frequency, but the ADC clock. To see if all settings are made correctly, consider to post your complete project. To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.

   

A cause might be that a higher ADC clock will draw remarkably more supply current which can influence the voltage delivered by the battery.

   

 

   

Bob

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ETRO_SSN583
Level 9
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What do the readings look like for both channels ? How much variation

   

do you see ?

   

 

   

Regards, Dana.

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Anonymous
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Hi, Dana.

   

ADC settings are as follow:

   

Channel sample rate 3676sps, Vref select: Internal 1.024 volts, Single ended negative input Vref, so the sample range is 0.0 to 2.048V.

   

One channel is for battery voltage detection, the battery's voltage is 3.798v, I use two resistor 330KOhm and 200KOhm to make sure input voltage is below 2.048v. I measured the input sense voltage is 3.798v*200/(330+200) = 1.412v, but the ADC sample value is 2672. 2672/4096 * 2.048 = 1.336v.

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Anonymous
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Another channel is for circuit current measurement. ADC channel sense voltage is 175mv, and ADC sample value is 340, this 1:2 is match my circuit, and I think 5mv tolerance is acceptable.

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ETRO_SSN583
Level 9
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Keep in mind unless you have buffered the ADC inputs the ADC has a

   

finite input Z of ~ 2.2K ohms, so that will affect your V divider design

   

values and Rsense value for the current measurement.

   

 

   

Regards, Dana.

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