Help needed on DAQ board!

[Jay the Communist]

I am trying to configure a NI Lab-PC-1200 board, to collect analog
input signal from a battery powered photodiode. Both multimeter and
scope gave a DC reading of 1V. However when I attached it to the DAQ,
it saturated at once and showed a straight line at 5V(high limit). I
then tried to attach a regular battery to the board, it did give me the
right reading of 1.5V, but after the battery was removed, it took more
than 20 seconds for the reading to go down. Even more weird, although I
got 0 when the two probes are shorted, I got absolutly wild readings
when there was nothing bewteen the two probes. The voltage curve jumped
back and forth between high/low limits. Well another observation is:
when I attached the diode direc
tly to the DAQ, the voltage reading did
come down after the light source was removed (The number was still
wrong), but if they were connected with a coaxial cable, the reading
just stayed there and never came down again). One of my friends
suggested it could be that the computer was not grounded right, but I do
not know how to find out and how to solve the problem.
It is too bad that I know hardly anything in electronics, and I really
needs your help. Please post your response or email me. Thanks.


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[Walter van Iterson]

Jay the Communist wrote:
>I am trying to configure a NI Lab-PC-1200 board, to collect analog
>input signal from a battery powered photodiode. Both multimeter and
>scope gave a DC reading of 1V. However when I attached it to the DAQ,
>it saturated at once and showed a straight line at 5V(high limit).

Hi Jay,

Is your card configured for single-ended, or differential measuring?

Single-ended measuring. provides you with 8 channels to be measured,
but forces you to use one single ground. It measures the voltage between
AGND and ACH0 up to ACH7.

Differential measuring allows you to measure 4 completely unrelated
signals, each with their own ground. You measure the signal between
ACH0 and ACH1, up to the signal between ACH6 and ACH7.

I
f you measure 4 signals or less, I'd recommend using differential
measuring. If I recall correctly, it can be configured in the Ni-Daq
control panel.

> signals remaining high for a long time
I don't have a hardware background either, but if differential measuring

doesn't solve the problem, if you put a big resistance between the two
channels, it will probably go down faster.

Hope it helps,
Walter van Iterson