# Troubleshooting circuit design for sensor input and desired output range

#### cinar

Joined Feb 20, 2021
1
I designed a circuit to process input from a sensor with a range of -300 mV to 300 mV. The desired output of the circuit is 0 V to 600 mV. To achieve this, I used the op-amp equation Vout = Vin + 0.3 as a basis for my design. However, when I simulated the circuit, the output was only around 900 mV.

I am seeking advice on where I may have made a mistake in my design. Can someone help me troubleshoot this issue?

#### LowQCab

Joined Nov 6, 2012
4,293
The first thing is,
Your drawing shows a Plus & Minus Power-Supply,
then, your simulation shows a Single-Supply.

What Op-Amp number are You planing to use ?,
does it have a "Rail to Rail" Output ?,
if i doesn't, then You must use a Plus & Minus Power-Supply to achieve "0" Volts Output.

What is the part-number of your Sensor ?
Is it an "Active-Sensor" or a "Passive-Sensor" ?

When the Output of the Op-Amp is directly connected to the "Inverting-Input",
the Inverting-Input can no longer be used as an Input,
in his configuration, the Op-Amp acts as a "Buffer-Amplifier" with zero-Voltage-Gain.

Is this a Classroom-Homework assignment ?
.
.
.

#### crutschow

Joined Mar 14, 2008
34,841
If the op amp output is connected to the (-) input then how do you expect the output to be different from the (+) input, since in normal operation, the op amp adjusts the output voltage to try to keep the two input voltages essentially the same?

#### Irving

Joined Jan 30, 2016
4,049
The last part of your formulae should tell you something

The two equalities can't both be true. The gain of a non-inverting opamp is 1+Rf/Rg. The only way the gain can be 1 is Rf=0, Rg=∞, ie output connected to -ve input and the resistor to ground an open circuit. In that scenario there is no easy way to offset the sensor.

In order to apply an offset to the opamp and work down to 0v you need a -ve supply rail. (also, as the sensor goes below zero, it must already have a negative supply? - what sensor is it?). Since the opamp gain must be > 1 the following works:

The gain is 2 (can you see why?), and then R5 & R6 divide the output by 2.

Most devices, even those billed as RRI/RRO (rail-to-rail in and out), cannot handle inputs more than 100mV below the -ve power input. This makes it hard to find a solution with commonly available devices that doesn't require a -ve supply rail. Given more info about the sensor other options may be possible.