# Cypress Developer CommunityTM

## Why does ADC sample value change with sample frequency?

Summary: 5 Replies, Latest post by danaaknight on 02 Sep 2015 03:23 AM PDT
User
24 posts

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?

User
9287 posts

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

User
7646 posts

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

do you see ?

Regards, Dana.

User
24 posts

Hi, Dana.

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.

User
24 posts

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.

User
7646 posts

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.