ADC to measure battery voltage

RE: ADC to measure battery voltage

MARTIN FR — Fri, 12 Feb 2016 10:12:50 GMT

Hello Zcmm could you please tell me how you measure the voltage present on the analog pin 6 i also followed the procedure exactly similar to you but when i measured the voltage available on PIN0.06 by using mulitmeter it is showing 0volt only so i am bit confussed how to monitor the voltage availble on the ADC PIN because am trying to measure to battery voltage and send over the bluetooth to notify the peripheral about the battery percentage

RE: ADC to measure battery voltage

Nenik — Thu, 18 Jun 2015 03:14:01 GMT

You'd go with higher resistances and lower capacitor - higher resistance to reduce the power consumption, but you may need to reduce the capacity to get reasonable response time - the divider, together with the capacitor forms an RC filter element. If you use 10x the resistance, you'd have to reduce the capacity 10-fold to keep the same time constant. You can safely go as low as 1nF, below that, you'd start loosing the other important property of the filter - supplying the ADC input during the sample phase.

RE: ADC to measure battery voltage

zcmm — Wed, 17 Jun 2015 08:18:31 GMT

thanks.i use 10K and 56K,capacitor is 100nF.it add 45ua to my power consumption.How to reduce power dissipation in this respect?

RE: ADC to measure battery voltage

Nenik — Wed, 17 Jun 2015 04:20:20 GMT

Maybe you're loosing MSB somehow? Take your value of 0xA7, add the (possibly) lost MSB, thus 0x1A7 = 423 and you'll get 0.496V, pretty in line with your expected result.

Anyway, even 0.495 can't really be considered "less precise", as at 8bits, your LSB step is almost 5mV. The fact that you "measured" 0.498 is just a coincidence, it could as well easily be 0.493 or 0.503, both far within the ADC specification (read on quantization noise and all the documented nonlinearities of the ADC).

Also, I would suggest going with much higher resistances (with a capacitor for better sampling) in the divider, this one is adding 50uA to your power consumption, something you can't really afford for a battery powered device.