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From: Roger Johansson
Subject: Re: How to make/solder a circuit fast?
Date: Fri, 27 Sep 2002 19:44:39 +0200
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firstname.lastname@example.org (The little lost angel) wrote:
>>Wouldn't it be easier to use a voltmeter to measure the DC and use the
>>soundcard for the AC content of the same signal?
>I thought of that too... except I really would prefer to do something
>else during the expected 15~30minutes test run, like have a shower
In that case the chopper solution suggested by Bob is a possibility.
>instead of staring at a DVM and hoping that no spikes when I blinked
"Spikes" will be recorded by the soundcard.
Only DC and slow fluctuations thereof would be missed by the
soundcard, as its response does not go down to zero.
>I'd probably be doing a lot of these, so it's going to be a lot of
>hours spent staring at a DVM... I'd rather spend a couple of days
>coding a program to run through a data logged and let it tell me where
>spikes occured, where ripples/noise exceeded tolerance, where voltage
>changes exceeded allowed rates etc etc.
Again, it depends on what you are looking for, spikes and all kinds of
fast changes will be caught by the soundcard (up to its upper
If you are not interested in DC and slow fluctuations, as it seems you
are not, then there is no reason to measure it.
You can look at the signal with a voltmeter at the signal once and see
that it is stable, and thus you only have to measure the non-DC
content of the signal.
Or to say it more simple, if you are pretty sure that the DC voltage
does not change very much, and you are only interested in spikes and
noise there is no reason to measure the DC during the measurement
It doesn't sound like you have any problems with the DC part of the
signal, so why measure it.
What you are interested in will be recorded by the soundcard.
Or to take an analogy, you don't need a 15" bass speaker if you are
waiting to hear if there is a piccola flute present in the music.
There is no reason to look for spikes in the low frequency range,
because a spike is a high frequency event.
>Also, the other problem is I haven't managed to find a cheap DVM that
>can do 1~100mV good enough for measuring current.
It is maybe not the fault of the voltmeter, if you use cables suitable
for such low level measurements that voltmeter might work as it
You have to take special precautions to avoid noise from the
environment when measuring such low voltages at high impedance.
Keep the cables very short, use coaxial cable, screen off the circuit
and voltmeter from electrical and magnetic fields, choose a suitable
input impedance (by putting a resistor over the input terminals),
Cheap multimeters are usually not expected to give good response at
these low voltages even if you do everything right. They generate
noise internally which is coupled to the input signal.
If you want to measure low voltages you might need an amplifier/buffer
close to the measured object.
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