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Subject: Re: generating calibration voltage
Date: Wed, 08 Jan 2003 02:13:51 -0800
X-Mailer: Mozilla 4.78 [en] (Win98; U)
Getting 24 of anything hooked up with less than
10pF to ground
sounds like a task in itself.
Maybe you need a tiny rechargeable battery,
and a flipflop that's optically triggered. Charge
when it's not hooked up to the system.
A 30 Hz. square wave going into a system with
frequency of 70 hz will be interesting. What you
measure may be
more dependent on the characteristics of the
filter than the input.
Stated another way, square wave calibration of 24
different inputs each with different
filter pecularities may NOT give you 24 systems
that respond in the same
way to the REAL input.
Designing a circuit to do a particular thing is
rather easier than
deciding what particular thing needs to be done.
Bunch of stuff For Sale and Wanted at the link
Tek 2465, TFT projector, ham radio, 30pS pulser
Tektronix Concept Books...
Tom Torfs wrote:
> Hello John,
> > How square must the square wave be? (What rise and fall times do you
> > require?)
> Rise/fall times can be upto a few ms. I'm only interested in the
> amplitude. The reason I can't just use a DC signal is that there is a
> highpass filter with fc=0.1Hz after that (and also a lowpass filter
> with fc=70Hz).
> > and how much capacitance to ground can you tolerate with
> > this floating output?
> Something like 10 pF would be ideal.
> > What precision must the amplitude hold to?
> Preferrably something on the order of magnitude of a few uV.
> > How flat must the wave be (can there be any ringing on the flat portions
> > of the wave?
> I would think the lowpass filter will take care of that, but with such
> high risetimes allowed I don't think there's any reason for it to
> To the people suggesting a transformer: that's very nice, but I do not
> have room for a 30Hz transformer. The footprint of the entire design
> is limited to a few square cm... that's why I was hoping for some
> suggestions of doing this with some sort of opamp circuit.
> My alternative if a floating output turns out to be too difficult is
> to briefly hook up the reference to ground during calibration using a
> switch with low impedance when on and very high impedance when off. I
> could also use a differential mux per channel, but rather avoid that
> as that would add many components (24 channels).
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