--- In "Yan Seiner" <> wrote:
>
> I'm going to be using the TS7200 with the on-board MAX197 to measure car
> battery voltages. Load dumping is not an issue - the batteries I am using
> aren't connected to the alternator.
>
> The chip has a 0-10 VDC range and I need to measure voltages from about 8
> to 16 VDC. I had thought of using a simple resistor divider but now I'm
> not so sure. The voltage I'm measuring is pretty steady, so speed of
> measurement is not an issue; I will be recording once per minute.
>
> The MAX197 http://datasheets.maxim-ic.com/en/ds/MAX197.pdf claims a "input
> dynamic resistance" of 21kohms.
>
> Does anyone know what that means? Can I just add 21Kohms in series and
> drop the voltage in half? Should I use a 10.5K + 21K divider, and count
> on the resistance of the MAX197 to cut that in half?
>
> I don't need absolute accuracy; I can calibrate the resulting readings to
> a known-good voltmeter. I do need accuracy over the range I'm expecting.
>
> --Yan
>
> --
> If you have eight hours to chop down a tree
> spend six sharpening your axe.
> --Abraham Lincoln
>
The absolute accuracy will be quite close to the accuracy of your reference
voltage. The internal reference will probably be fine for what you are doing.
Input resistance means you need to be able to supply current to that load
without perturbing your measured voltage. If you want to use a potential
divider with <1% peturbation you will need to ensure the current in your
resistor chain is 100x that drawn by the ADC. This may be possible on a car
battery but is probably going to consume more power than the 7200 !
The simplest solution would be an op-amp as voltage follower: ie 100% neg feed
back. Now you can use 100k+100k resistor divider to get within 0.10V range with
minimal current drain.
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