🚀 go-pugleaf

Mark-Tarbell@suite.com writes:

> Have you ever tried to get a DMM to measure its own battery voltage?

> It seems that one must decouple the input from the battery terminals,
> but there must be a trick to it (I'm using a .01mfd disc cap).

It won't pass DC in series configuration. Used in parallel, it might
help filter out any aliasing that might occur as the power demands
on the battery shift during the integration period.

> What happens is that the display cycles +/- 2 volts around the true
> value, but never settles down, seeming to indicate a ground loop.

> Any ideas?

I recently bought some low-voltage detector/warning chips from Digi-Key
(still waiting on them, back-ordered), that run off the voltage that
they are sensing. Until I got the data sheet, I wondered how they'd
deal with the very big problem of what to compare the voltage to. After
all, everything is shifting as the voltage drops.

It's very simple: there's an on-board regulator to run the chip, which
of course sets a lower limit of the voltage you want to monitor. There's
also a very stable 1.3V reference inside the chip, which is compared
to an external resistor divider that you design. Now the chip can run
accurately off a voltage lower than the sense setpoint.

If you use a divider and probe that point, you will be measuring a
voltage closer to the battery (-): previously, you tried to measure
battery (+). Your errors probably won't disappear, but they will be
different. It will be a trade-off between dividing the error and 
approaching the LSB of the display.

For better isolation, you might need to increase the input impedance of
the DMM, perhaps with an op-amp.

tonya@hpldsla.sid.hp.com