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'[EE] Sample and Hold circuit'
2003\01\07@105856 by

I want to know if I could build a simple sample and hold circuit with just a couple op amps on a small bread board with a +5 volt DC power supply that would do this:

I am watching 2 voltages. When one voltage crosses +.412 Volts DC, I want to be able to hold the other voltage for about 3 seconds so that I can read it with a DVM. The held voltage would be anywhere from about +.255 volts to +.500 volts dc, and I need the .001 volt resolution.

Is this do-able? Practical? Is there an inexpensive DVM that has this ability built-in? Any other ideas as to how I might do this?

Thanks
John

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There are a series of PIC that have built in A/D converters - I'm using the
PIC 16F877, for example. The A/D converter is a sample and hold converter
and is quite fast. It has a resolution of 1 part in 1024 (0.1 %).

You could use op amps to increase the two voltages to the range 0 to 5
volts, then use two channels of A/D to measure them. The A/D conversions
take only microseconds, so there is no need to hold the voltage for 3 seconds.

All you would do is read the two A/D channels in sequence; when the first
one gives a voltage above the threshold, you process the second one -
display it, or whatever. There are inexpensive LCD displays available (\$10
or less).

Larry

At 07:55 AM 1/7/2003 -0800, you wrote:
{Quote hidden}

Orleans (Ottawa), Ontario, CANADA

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> I want to know if I could build a simple sample and hold circuit with
> just a couple op amps on a small bread board with a +5 volt DC power
> supply that would do this:
>
> I am watching 2 voltages. When one voltage crosses +.412 Volts DC, I
> want to be able to hold the other voltage for about 3 seconds so that I
> can read it with a DVM. The held voltage would be anywhere from about
> +.255 volts to +.500 volts dc, and I need the .001 volt resolution.

Hi John,

Yes, it wouldn't be that hard. You'd need a JFET or an analog switch as well as
the two opamps(actually one opamp would be used as a comparator, if the slew
rate of your signals is very high you would need to use a comparator rather than
an opamp).

Doing it with a PIC would require the two opamps to get the signals in range,
but no FET or analog switch. But your speed would be limited by the PIC.

Cheers,

Bob

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