Using ADC to measure battery voltage

[victorf]

Perhaps someone would help me out here. I have a 12 volt battery and i would like to measure the it's voltage from time to time. I have the following circuit set up on my system board:

VBAT ---R1------R2---GND

The junction between R1 and R2 goes to Pin 20 of my ZX-24. R1 = 8.06K 1% and R2 = 4.02K 1%. The values were chosen so that the voltage at Pin 20 ranged from 0 to 5V. If the voltage at Pin 2o is VADC and the voltage of the battery is VBAT, I wrote VADC = (VBAT * R2) / (R1 + R2). Using this equation I computed that for an VBAT of 15.02 volts I would see a VADC of 5 Volts and for a VBAT of 0 volts I would see VADC = 0 volts.

Here is my function to compute the battery voltage:

[pdubinsky]

The ZX-24 ADC is 10 bit not 12 bit. You need to use 1024 NOT 4096. That will give you a final voltage of 12.12 (3.03 * 4096/ 1024).

Paul

[dkinzer]

In reality, a 10-bit ADC has only 1023 quantization levels (plus zero) so each step is equal to Vcc/1023, or 4.89mV per step at 5V.

[pdubinsky]

I agree: 4.882813 mV per step is different than 4.887586 mV per step <gg>. I was staying with the "count" (4096) that Vic had used for 12 bit resolution - mostly so that the 4096 he used in his equation divided by 1024 would give an integer mulitplier of 4.

Thx for, as usual, keeping us on our toes(all 10 to the 1 of them).

C8-)

[victorf]

Here is the discussion of GetADC from the SysLib.

[mikep]

I would suggest that the documentation has an error on both GetADC pages and it should read 10 not 12 bits. The underlying ATmega32 only supports 10 bits. I'm sure Don will fix this mistake asap. I never caught it in proof-reading the drafts either

[dkinzer]