nRF52832 SAADC and small batteries

[PW]

Hello. 

I try to make audio noise meter on nRF52832. I used analog MEMS MIC with OPA which I turn on via MOSFET while before measurement. I'm using SAADC triggering by RTC and I take samples by about 25ms. Almost everything works fine. When I'm powering up device from LDO or large battery (CR14250), results are correct, but when I try to power up by two CR2430, device works not very well. Results are clearly to high. I tried to correct situation by couple of ceramic capacitors, but it wasn't helpful. I wonder if it possible to use SAADC in longer measurements (several dozen ms) when we use batteries like CR2430? Maybe better solution will be PDM mic? I wanna to do it on possibly the smallest batteries, eg. CR2450.

Best regards, 

PW

[AmbystomaLabs]

The capacitors only help to smooth out peaks in the current consumption. Most CR batteries are rated for 1mA max current draw. So the caps work to store up power during off time such that the during TX/RX activity or other high current times the nRF can draw 10mA to do its job. But you can still never draw more than 1mA from the battery.  So the resulting solution means that for any process you can have at best about a 10% duty cycle. ie, nine 1ms periods at 1mA and one 1ms period at 10mA.

This is an over simplification of the solution but it avoids all the math and works well in this venue.

So, your problem is likely something you are running, or all things put together are drawing more than 1mA and you doing that process for too long or too frequently or a little of both.

You need to figure out your peak and idle current consumption and the duty cycle to come up with a timing for the activity that will work.

[PW]

I can't see other way like sampling MIC circuit at least 15..20ms (maybe I'm wrong here). Additionally it good to make more than one measurement :( I'll try to do something with sampling frequency..... If it won't help, it's looks like I have to change to more complicated analog part or use PDM MIC.

[hmolesworth]

What size were your ceramic capacitors? You need a minimum 100uF, more is better but check the leakage specification on larger capacitors.

[awneil]

PW said:

I tried to correct situation by couple of ceramic capacitors, but it wasn't helpful.

As AmbystomaLabs asked, what values did you use?

Just randomly slapping on capacitors is not a very scientific approach!

You really need to use an oscilloscope to see what's happening to your supply voltage: if it is sagging, you need to fix that; if it isn't, then you need to look elsewhere for the problem.

Use your scope or a current profiler or similar to measure  your supply current...

[PW]

It wasn't randomly, I was increasing capacitance. I started with 22uF, finished with 3x47uF (X5R). Peaks during measurement decreased a bit, but it was still about 5mA/20ms. Much more efective is to decrease sampling, but I didn't finished that part. I'll back to this on Monday.