🚀 go-pugleaf

In article <BuAB37.6I6@suite.com> suite!tarbell@uunet.uu.net 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).
>

Yes, one can manually simulate a kind of sample & hold circuit
using a capacitor. Use a 1000 microfarad capacitor to sample the
battery voltage of the DMM. Since DMM's usually have an internal
resistance of about 1 to 10 MOhms, you should most likely be able to get a
reading within +/- 1 least significant digit for a 3.5 digit DMM
(1/2000 of full scale) off the charged capacitor for at least a duration of
approximately 1MOhm*1000uF/2000= 500 ms (1/2000 of the time constant).
This time is barely enough to observe a stable digital display,
unless there is a bar display, or a hold-display function.

One can even go further and design an electronic sample-and-hold
circuit that has a periodic sampling interval. A simple one can be designed
by using a pair of MOSFET SPDT bilateral switches ( If I am not mistaken,
CD4069 would do it) to alternately connect the capacitor terminals
to the battery poles and the measurement probes. The switching frequency
can be increased in order to use a smaller holding capacitor.

>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?

IMHO, the fluctuation in the display might be the side effect of a feedback
mechanism built into the DMM circuitry to offset a common mode voltage
appearing in the differential inputs of the DVM circuitry.

Another possibility is that one of the probe terminals does not have a
high series resistance in its connection to the DMM, and connecting it to
a battery terminal causes considerable battery current to flow through this
path, causing a drop in battery voltage, and therefore, a malfunction
in circuitry sensitive to the supply voltage
(a ground loop, using your terminology). A high current through
a probe may also drive the input op-amp into saturation, with displayed
results depending on the type of ADC and display control methods used.

Yours,
-- 
Mustafa KOCATURK  mustafa@seas.smu.edu   Electrical Engineering Department
Home:(214)706-5954  Work:(214)692-2541   SMU Box 753190, Dallas, TX 75275-3190