Post by thread:[EE] Sample and Hold circuit

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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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}>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 > >-- >http://www.piclist.com hint: PICList Posts must start with ONE topic: >[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads Larry Bradley Orleans (Ottawa), Ontario, CANADA -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> 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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads