Op-amp comparator and differential signals

Question

On a board I'm working with, there's a 120 ohm termination resistor. I need to measure the voltage across it and compare it against a reference voltage.

I'm wondering how to make the connections.

The op-amp has 2 inputs. The inverting input will be connected to the fixed reference voltage.

The non-inverting input should be hooked to the point where I can measure the voltage across the resistor. Voltage is usually measured between a point and the ground reference, but the voltage I need to acquire is between the two leads of the resistor.

How would I be able to connect it to the non-inverting input? Should I use a differential to single ended buffer or something of this nature?

Edit: ignore the switch.

Answer 1

I hope this circuit configuration helps. The first two opamp acts as a voltage buffer so that extra current doesn't flow through 120 Resistor, which may alter the output. After the first stage, both inputs are passed to the differential amplifier configuration of OpAmp. The resistors R2, R3, R4, R5 are chosen according to the gain required. After altering a little bit with OpAmp IC and other values, you may adapt to your need. I think buffer stages are required to properly measure the voltage.

Answer 2

The following circuit does what I think you want.

Op amp OA1 serves as a level shifter. The output of OA1 should be equal to the voltage across \$R_{term}\$, but referenced to ground. Op amp OA2 acts as a comparator, and gives a output near \$V_{cc}\$ if the output from the previous stage is above 3.3V, and it gives an output near 0V if the voltage from the previous stage is less than 3.3V.

The resistor values are not critical, but they should be matched if accuracy is at all important. There exist monolithic resistor arrays that contain multiple matched resistors. My guess is that you may not have to worry about such things.

You didn't specify \$V_{cc}\$. In the schematic, I used 15V, but 5V works just as well.

The op amps probably do not need to be anything special. Your tripping point is 3.3V and that is in the range of virtually every op amp available. If using "dual-supply" op-amps in this application, such as 741s, you may have trouble with a 5V \$V_{cc}\$, but just about any rail-to-rail op amp should work.

The jumper around the switch can be ignored. I added it so that I could test the circuit. (You can too. There is a little option to the left below the schematic to "simulate this circuit".)

^{simulate this circuit – Schematic created using CircuitLab}