dropping a variable voltage by 1v

I'd like to offset a variable voltage by 1 volt. What I have is an LM34 temperature sensor that outputs a linear voltage based on temperature (+10mV/F). I'd like to change that to (+10mV/F - 1V). What's the easiest way to drop the LM34's output by 1V?

I tried a potentiometer but my understanding is that a pot is a divider so it doesn't give me the result I'm looking for. I also tried a diode that was supposed to have a 1V drop but that didn't seem to work either (not sure why). Is there a simple solution?

Thanks, Ray

Reply to
Ray Manning
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Hi, Ray. Get an op amp and four equal value resistors (match values with your ohmmeter -- the closer they are, the better), and make a difference amplifier like this (view in fixed font or M$ Notepad): ___ .-|___|-. | 22K | | | ___ | | Vin o--|___|---o VCC | 22K | |\\| | '-|-\\ |Vout = ___ | >--o----o 1VDCo--|___|-o---|+/ | 22K | |/| | .-. GND .-. 22K| | 22K| |(not matched) | | | | '-' '-' | | | | === === GND GND (created by AACircuit v1.28.5 beta 02/06/05

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This circuit will do the job for you. If you don't need anything below about +50mV DC output from the op amp, you can use a single supply and half of an LM358, or 1/4 of an LM324 (available at Radio Shack and many other places). If you need a negative voltage output, you'll need a dual supply (say, +/-5V) and you can use an LM741 or any other standard op amp.

Good luck Chris

Reply to
Chris

Oops. Wrong labels -- try this:

___ .-|___|-. | 22K | | | ___ | | 1VDCo--|___|---o VCC | 22K | |\\| | '-|-\\ |Vout = ___ | >--o----o Vin o--|___|-o---|+/ | 22K | |/| | .-. GND .-. 22K| | 22K| |(not matched) | | | | '-' '-' | | | | === === GND GND (created by AACircuit v1.28.5 beta 02/06/05

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Inputs backwards -- sorry.

Chris

Reply to
Chris

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Instead of connecting it to ground, connect it to -1V.
Reply to
John Fields

John,

Are you suggesting that if I applied -1v to the GND terminal of the LM34, the output would be offset by -1 volt? If I understand correctly, this would work because the output would still be referenced against the original ground. If thats the case, can I produce the -1V using the input +Vs (12v in this case) and the orginal ground? Wouldn't that solution (offseting the input) be the same as offseting the output or am I missing something?

Thanks, Ray

Reply to
Ray Manning

An opamp with 4 resistors configured as a subtracter (difference amplifier) outputs the difference of two voltages plus a third voltage. A 2 opamp version has two of the inputs with high impedance, so you don't have to worry about loading high impedance inputs.

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This opamp might be a good choice:
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Reply to
John Popelish

I understand what you are saying but still don't see how to accomplish it. The curcuit that is driving the LM34 hands me the ground reference (0V) and a +12V source. How do I produce the negative voltage to connect to the LM34 to change its reference point?

It would seem to me that I would have to use the same difference amplifier others have suggested for dropping the output by 1V to create the negative

1V for the "holy" ground pin. Are you saying there is an easier way?

Thanks, Ray

Reply to
Ray Manning

Sure, you just need an op-amp difference circuit. One input is your signal, the other is 1V, and the output is the difference. You will need a known voltage source, but it doesn't have to be

1.00 V or even close, since you can scale it with the input resistors or a separate divider. You can even use a negative reference and sum it with the LM34 to get a difference.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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Home of DaqGen, the FREEWARE signal generator

Reply to
Bob Masta

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Yes.
Reply to
John Fields

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Yes. How\'s the LM34 going to be hooked up to the rest of the
circuit?
Reply to
John Fields

The LM34 is driven by a circuit that converts the LM34's voltage to a frequency that is read by a remote computer. The computer supplies the circuit with a ground and +12V and receives a frequency on a third wire. The circuit in turn supplies the LM34 with the same ground and 12v supply and monitors the output voltage of the LM34.

- Ray

Reply to
Ray Manning

drop it's the supply and "ground" by one volt.

or something with an op-amp

in ----[100k]--. | +|\\ 0V-----[100k]--+--| \\ | >----+--- out 1V-----[100k]--+--| / | | -|/ | | | `--[100k]--'

a silicon diode ansd two germanium diodes in series will give approx 1V drop but it's terperature dependant...

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Bye.
   Jasen
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Reply to
Jasen Betts

Hmm, you could use a dual osc, one that generates your freq to the computer and the other to generate a small low power - source for the Vee (common of the LM34). using a couple of caps and 2 diodes and a divider for the common to have the common of the LM34 connect to this -1 output. since there is very little current involved here i think you could get away with it. i don't know if your using a mpu or what but, either an extra osc or a clocked output can generate the constant you need for the - voltage source.

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Real Programmers Do things like this.
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Reply to
Jamie

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OK.  So what you have is a voltage-to-frequency converter and an
LM34 wired like this:
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Reply to
John Fields

not to criticize because i know your a smart guy and all but shouldn't the Diode be going to ground on the K side and A of the diode to the C1 and maybe another Diode in series to C2 to isolate the 0.7 average ripple ? or have i over looked something?

>
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Real Programmers Do things like this.
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Reply to
Jamie

it may be possible to modify the VCO (voltage to frequency circuit) by raising its ground by one volt

OTOH can you account for the 1V offset by modifying the software to read the frequencies differently?

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Bye.
   Jasen
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Reply to
Jasen Betts

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