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# Sensor Circuit Design, need help with Op-Amp circuit design

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#### Deleted member 155794

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Hi Everyone
I am trying to measure a variable resistance(Rv) which changes with input (X) according to the following equation

Rv = 0 Ohm at Xin = 1000
Rv = 1000 kOhm at Xin = 0

I am using the feedback resistor as 10kOhm. The input resistor is (Rv + 10kOhm). Hence, as per the close loop equation, I should be getting

At Xin = 1000
Vout /Vin = - Rf/Rin = - (10)/(10+0) = -1

with Vin = Vcc = 5V
Vout = -5V (or 5V with phase -180)

At Xin = 0
Vout/Vin = -Rf/Rin = -(10)/(10+1000) = 0
with Vin = Vcc = 5V
Vout = 0 V

I am using LM324 which is a standard Op-Amp and getting 0.6V for Xin = 1000 and 0.0V for Xin = 0.

Please can you let me know if I am doing something wrong.

Aliter
Using a simple voltage divider, between 10kOhm and Rv, I can see the voltage change from 0 - 5 V. The reason for using the OpAmp is to act as an inverter so that Vout ~ Xin rather than it be Vout ~1/Xin.

Best
Chandra

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• Screenshot from 2019-09-27 20-31-49.png
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No schematic, no mention of the power supply rails, no schematic, no circuit topology, no schematic ...

ak

AnalogKid
Thanks for correcting me.

Sorry wrong schematic attached

Best
Chandra

#### Attachments

• Screenshot from 2019-09-27 20-30-45.png
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Check the input common mode voltage range and the output voltage range on the LM358 datasheet. Your feedback loop might not be closing. It is common for simulation programs to get this wrong.

ak

Your schematic shows missing resistor values, missing the opamp part number and missing a power supply for the opamp.
I corrected your schematic and guessed where Rx is. The output is not +0.6V.
EDIT: The opamp does not have a negative supply and is trying to drive its output to a negative voltage but it can't. Then the output will be about +0.01V as shown on its datasheet.

#### Attachments

• inverting opamp.png
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Last edited:
Thanks audioguru and AnalogKid

I understand that not having -Vcc is the issue.

Changed the design.

This thread can be marked as closed.

Best
Chandra

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