voltage scaling

Thread Starter

hardik vamja

Joined Oct 15, 2015
13
Hi I need to convert 3.7 to 4.7 volt into 1 to 5 volt
I know it can be done with opamp but I don't have any idea what can I do in this
Plz any one can help me..!!!
 

crutschow

Joined Mar 14, 2008
23,349
What supply voltages do you have?
What is the output load?
How accurate does the shift need to be?
 
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Thread Starter

hardik vamja

Joined Oct 15, 2015
13
What supply voltages do you have?
What is the output load?
How accurate does the shift need to be?
Actually I'm converting RTD resistance into 4to20 ma current and output of bridged & instrumentation amplifier is 3.7 to 4.7 volts so I need 1 to 5 that can easily converted into 4-20ma
My power supply will be 30v(+15 -15) to opamp
 

MikeML

Joined Oct 2, 2009
5,444
So what is the RTD min and max resistance over your range of interest? There may be a way of doing it more directly.

270.gif
 
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MikeML

Joined Oct 2, 2009
5,444
@MikeML hi id try to simulate your ckt but dont getting exact results
which one is input v1(you show 4.6v) or v2
V1 is a fixed offset of 4.600V. Note that this must be very precise. A 1mV error causes a 3mV error at the output. Watch it if it is a trim pot off +15V.
V2 is the input that varies from 3.7 to 4.7V
 

#12

Joined Nov 30, 2010
18,076
this must be very precise.
May I suggest using fixed resistors to make a trim pot work in a small range of voltages. That's one way to get a more accurate voltage setting.

For instance, if you're trying to get 4.6V out of a 5V supply, put a 4.7 k resistor under a 1 k pot. That way the pot works from 4.1 volt to 5 volts.;)
 

MikeML

Joined Oct 2, 2009
5,444
May I suggest using fixed resistors to make a trim pot work in a small range of voltages. That's one way to get a more accurate voltage setting.

For instance, if you're trying to get 4.6V out of a 5V supply, put a 4.7 k resistor under a 1 k pot. That way the pot works from 4.1 volt to 5 volts.;)
Yeah, but if the 5V supply drifts from 5.000 to 5.050 (1% change), the output of the scaling opamp shifts 3%, i.e., it is ratiometric. In the OP's case, he is likely trying to get 4.600V from his +15V supply. How stable is that?

In any case, I would use a voltage divider, followed by an opamp voltage-follower to drive the left end of R1 (see post #4). Or use a real voltage reference like (wait for it) TL431 to make the 4.600V.
 

MikeML

Joined Oct 2, 2009
5,444
As promised, here is the basis of producing your voltage directly using only one opamp. You would have to tweak the gain and offset a bit, but you could do it

100b.gif
 

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MikeML

Joined Oct 2, 2009
5,444
Try this for a relatively stable adjustable voltage reference.

129.gif

If you keep the instrumentation amplifier, you will need a stable voltage reference for it, too...
 
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