Timeline for Using op amp to measure battery voltage
Current License: CC BY-SA 3.0
8 events
| when toggle format | what | by | license | comment | |
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| Jul 21, 2021 at 14:27 | comment | added | LorenzoDonati4Ukraine-OnStrike | @Neil_UK some ADCs won't tolerate too much high-impedance sources. Citing from ATMega328 datasheet (ver. J) "The ADC is optimized for analog signals with an output impedance of approximately 10k or less. [...] If a source with higher impedance is used, the sampling time will depend on how long time the source needs to charge the S/H capacitor, with can vary widely. The user is recommended to only use low impedance sources with slowly varying signals, since this minimizes the required charge transfer to the S/H capacitor." So that divider will cause problems with such a MCU. | |
| May 19, 2016 at 12:37 | comment | added | StjepanV | Then I'll definitely go with R1=910K and R2=100K. Thanks so much Neil for your help and advice! | |
| May 19, 2016 at 11:35 | comment | added | Neil_UK | Can you suggest a circuit, I can't think of a simple one, especially if the battery voltage is well above that of the MCU. Once you are up to 100k or 1meg R1, the sense current is negligible compared to battery self discharge, so it's fairly irrelevant. | |
| May 19, 2016 at 10:57 | comment | added | StjepanV | Additional question that now arises is, could I use transistor which is controlled by MCU to periodically switch (e.g. once a min) the measuring circuit so that power is not wasted even when I don't need the measurement? | |
| May 19, 2016 at 10:45 | comment | added | Neil_UK | you'd have to check the electrical specs for your specific part, but generally, yes, most MCU ADCs are high input impedance. However, with a switched capacitor input, most require a big capacitor across the input to source the instantaneous charging current if they are driven from a high impedance. In your battery monitor situation, the bandwidth limit that that imposes is not a problem. | |
| May 19, 2016 at 10:32 | comment | added | StjepanV | Thanks so much Neil for the answer! a) So you are saying that any modern MCUs (I plan to use STM32F4xxx) would not need the op amp, and could just take the output of voltage divider b)You would go with the Mega Ohm values, right? I didn't think of the current (power) being wasted on these resistors... Thanks for great advice! c) Sry for that :( d) will take this into account | |
| May 19, 2016 at 10:32 | vote | accept | StjepanV | ||
| May 19, 2016 at 9:07 | history | answered | Neil_UK | CC BY-SA 3.0 |