# Scaling -75 to 75VDC to 0-5V for ADC

#### mcgyvr

Joined Oct 15, 2009
5,394
I'm attempting to build a voltmeter of sorts and want to be able to measure from -75VDC to 75VDC and need to scale that to a 0-5V signal into a single ADC channel on a micro.. I guess the typical way to do that is to set 2.5V as 0 volts and then -75V would be 0V and 75V would be 5volts.. Is that correct? and how would I accomplish that. or is there better ways to accomplish the task. I've done very..very little work with op amps and just don't know where to start with this because of the negative voltages too. Thanks in advance for any help you guys provide..

#### Brownout

Joined Jan 10, 2012
2,390
I would first use a 30:1 voltage divider, to get +/-2.5V. Then feed that through a summing amplifer, with a 2.5V reference.

#### crutschow

Joined Mar 14, 2008
23,514
You are on the correct path. You could offset an op amp inverter circuit with an accurate 2.5V reference voltage to the plus input with an inverting gain of 5/150 = .0333 (Rin/Rfb = 30) . If the micro has its A/D reference voltage brought out on a pin, you can use that for the offset voltage (use a resistive divider to get the 2.5V).

Note that the inverter op amp will mean that +75V would be 0V and -75V would be +5V from the op amp, but you can invert that in your software that reads the voltage. If you use a rail-rail type op amp you can operate the op amp from +5V.

The circuit you referenced would also work if you select the resistors to the range you need.

#### mcgyvr

Joined Oct 15, 2009
5,394
Here is what I've got so far (see attached)..comments/concerns??
Is the LT1677 rail-rail op amp a good choice or is there another you guys would pick..

and to Brownout..any reason to go with your solution versus what I've got here..

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#### Brownout

Joined Jan 10, 2012
2,390
I see nothing wrong with your solution.

#### mcgyvr

Joined Oct 15, 2009
5,394
One more question.. The firmware on the micro/ADC is more or less set in stone and offers no ability for calibration. (I'm actually using a pre-programmed NKK LCD smartswitch as the display and the button only functions to toggle between multiple inputs)

What could I do to my circuit to allow for some sort of calibration? Should I make certain resistors actually be potentiometers? if so which ones? or what else?

oh and wow that op amp is expensive ($2.35 digikey).. any reason something like a LMV321SEG-7 can't be used ($0.13)? Basically what should I look for..or will any basic general purpose rail-rail op amp work just fine for this application.

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#### crutschow

Joined Mar 14, 2008
23,514
Here is what I've got so far (see attached)..comments/concerns??
Is the LT1677 rail-rail op amp a good choice or is there another you guys would pick..

and to Brownout..any reason to go with your solution versus what I've got here..
The resistor values for that solution gives a maximum output +4.68V for +75V in. Is that what you want?

oh and wow that op amp is expensive ($2.35 digikey).. any reason something like a LMV321SEG-7 can't be used ($0.13)? Basically what should I look for..or will any basic general purpose rail-rail op amp work just fine for this application.