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JStuff (Computer) (OP)
4 Sep 03 14:29
I have a circuit that is converting a 4-20ma signal to 0-5v
the 0-5v was originally connected directly to the 0-5v input of an a/d converter. This Converter is now obsolete. The only drop in replacement is a converter that has a -2.5 to +2.5 input. I have an opamp on board that I can direct the signal tough before it gets to the A/D. Is there an easy way to use the opamp to convert the 0-5 to -2.5v to  +2.5v. I am a computer programmer by trade. But my major was electronics. The problem is Its been 10 years since I have worked with an op-amp.
nbucska (Electrical)
4 Sep 03 15:17
if you have +5V:
(R3=R2=R1;  R4=R5=2*R1 ;  lets say R1=10K0hm)
One line === one net
---------------------------------
inp --- R1.1       ; noninv circuits
R1.2--- R2.1 --- opa.NONINV
R2.2 --- GND

+5V --- R4.1         ; inverting circuits
R4.2 --- R5.1 --- R3.1 ---opa.INV
R5.2 --- GND
R3.2 --- opa.OUTPUT --- ADC.inp

IN[V]   OUT[V]
0        -2.5
2.5        0
5        +2.5
You may want to use voltage reference or measure the 5V
and rescale the result in software.

<nbucska@pcperipherals.com>

JStuff (Computer) (OP)
4 Sep 03 16:09
Sorry. I can't quite follow your email. Are you saying R1,R2,R3 are 10k?

A little more info. This op-amp is currently supplyed with +5v and -5v.

nbucska (Electrical)
4 Sep 03 17:19
I don't know the OP.AMPs type or your requirements, so
I can't design for accuracy, offset VTG, etc.

R1,2 &3 are the same, R4 and 5 are twice as large.
The value and OP.AMP determines the offset error.

I would guess, you can go as low as R1,2,3 =1K
-------------------------------------------------
You could use just two resistors from current input
to + and -5V to get -2.5 to +2.5  --- if the power supplies
are accurate enough for your purpose.




<nbucska@pcperipherals.com>

JStuff (Computer) (OP)
4 Sep 03 17:47
Ok is op.amp the gain?  the part I am using is a tc913 (microchip part). As far as accuracy the best I can get with what I have. The 4-20ma signal is from an ultra-sonic
sensor we would like to be within .1 inch overall error (This is the spec on the sensor) however it could be as high as +-.3


Thanks alot for your help.
nbucska (Electrical)
4 Sep 03 18:46
Connect 1250 ohm to GND and 416.6 ohm to -5V and the current
to their junction -- it will change from -2.5 to +2.5.

<nbucska@pcperipherals.com>

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