Voltage translation from 48V battery to TTL

[KRAPman]

I need to translate 36V-58V DC voltage of my battery pack to my 10bit ADC on MCU in TTL range of 0-3.3V.
I want to use the cheapest solution (most passive elements possible instead of dedicated IC).
How can I do that (opamp, resistor ladder)? How it effects the accuracy (drift, non-linearity) of my output, what can I expect?

 

[KlausST]

Hi,

the cheapest solution is using two simple resistors as divider.

Then 0..58V will be 0...3.3V.
(about 56.6mV resolution of battery voltage, or 3.3mV ADC resolution)

I know you wanted to start at 36V.
Then up to 58V this gives a range of 22V of battery voltage or 21.5mV resolution.

But tracking a battery voltage you don´t need that much resolution, you need accuracy.
And accuracy of your system is determined by:
* resistor tolerance
* ADC accuracy
* and VRef accuracy

And - because you say 0...3.3V ADC input voltage I assume you use VCC as VRef. But VCC in most cases is not accurate. It drifts with time, with temperature and with load.

* 1% of resistor deviance generates about 580mV wrong battery voltage reading. ( = 26 times your 22mV resolution! The exact value depends on the used circuit)
* 3% of VCC (3.20V instead of expected 3.3V) deviance generates about 100mV of ADC reading error ( = 31 times your 3mV ADC resolution)
You may calibrate (a lot) of resitance error, because the error is very stable, but VCC drifts are not preidictable.

****
My recommendation:
Either use cheap "two resistor" solution and be satisfied with the 56mV resolution
Or use the "precise" solution with OPAMP and VRef_IC. (Without VRef_IC it is useless)

Klaus

 

[betwixt]

There is a 'dirty trick' way to do this which only requires one extra component but might be more accurate.

You use a two resistor divider but instead of dividing the whole voltage down, you add a Zener diode in series with the top (input side) resistor. The range is 22V but the minimum is 36V so if you can subtract a constant from the 36 before dividing, you can reduce the division ratio and thereby increase the resolution. For example, if you add a standard 33V Zener to the top of the divider, the voltage range becomes 3V to 25V and the ADC resolution is effectively almost doubled. All you have to do is add the Zener voltage back to the answer to get the true voltage. Given that Zener tolerances tend to be quite wide, it would be worthwhile measuring the actual Zener drop rather than relying on the specification.

Brian.