Comments on: Differential Amplifier Calculator

By: Adrian S. Nastase

Adrian S. Nastase — Fri, 28 Jun 2024 20:39:16 +0000

In reply to Al Testani. Let's look at it this way. If Vin = 0V, R1 is connected to ground, in parallel with R2. There is no current going through them. So, in effect, the non-inverting input of the op amp is connected to ground and the circuit becomes an inverting amplifier. With V2 being a positive value of +2.5V, how can the op amp output be positive when it inverts? When V1 = 0V, the output is V2 = -R4/R3 * 2.5V= - (4kohm / 10kohm) * 2.5V = -1V exactly as the calculator shows.

By: Dave

Dave — Thu, 06 Jun 2024 10:25:06 +0000

In reply to Graham Toal. Hi Graham. If you are still grappling with this problem reply here. I have the solution... and it's not a differential amp.

By: Al Testani

Al Testani — Wed, 03 Apr 2024 03:38:50 +0000

In reply to Adrian S. Nastase.

I am seeing the same contradiction to the following link as Mihai Beffa
https://www.daycounter.com/Calculators/Op-Amp/Op-Amp-Voltage-Calculator.phtml
I get the same results to that link in a simulator.

With your default values, an input of +2.5V the above link and simulator gives Vo = +1V but with an input of 0V, Vo = +2V not the -1V as your calculator requires.

What is the problem?

By: Adrian S. Nastase

Adrian S. Nastase — Mon, 29 Jan 2024 19:50:33 +0000

In reply to Graham Toal.

5V reference will not work. If you use 5V reference, you will get negative values, no matter the power supply level. You need to drop the reference voltage below 3V. I recommend 2.5V.
With a 2.5V reference and your input/output requirements the resistors are R1= 1.4k, R2= 10k, R3= 10k, R4= 12.8k.

The power supply values are not needed in resistor calculations. When choosing power supplies, you need to make sure that the op amp can reach the maximum output trip. For this, look in the op amp datasheet for the Output Voltage Swing parameter. It is given for a certain power supply level. This will give you an idea if your power supplies are the right choice for your application.

By: Graham Toal

Graham Toal — Mon, 22 Jan 2024 22:17:42 +0000

I’m not clear on how to use this without entering the values for V+ and V- which don’t have a parameter box above. I’m trying to build a circuit where my analog input signal varies between 0V and 3.2V, and the output signal will between proportional between -3.2V and +3.2V. I also have 5V and -5V reference input supplies (and a 0V ground). My (limited) understanding of these is that I need to set V+ to 5V and V- to -5V in order to have an output range that is within those bounds. I’m having trouble designing this circuit so was hoping your calculator would help! (I’m OK with digital electronics but weak on the analogue side – in particular a negative output isn’t usual in most of the tutorial examples to be found online) Without specifying V+ and V- your calculator suggests a negative resistance for R1!

By: Adrian S. Nastase

Adrian S. Nastase — Fri, 19 Jan 2024 04:43:39 +0000

In reply to Christian Schmitz. If you need to calculate the amplifier for the other input, you can follow the calculations shown in my article Solving the Differential Amplifier - Part 1.

By: Christian Schmitz

Christian Schmitz — Tue, 03 Oct 2023 12:07:48 +0000

Hello.
I am trying to use the calculator but i found a problem.
In my appplication i need measure the consumption over 5v ( currents fron 3mA to 100mA). I made this with i use a low resistor between 5v main and 5v to the load and an ADC to read the Vresistor (and calculate the current).
Now i want improve the accuracy so i want amplify this difference with an differential AmpOp.

So in my case Vref is V1 and is fixed at 5v, and V2 is mobile from 5 to 4.95v

By: Adrian S. Nastase

Adrian S. Nastase — Sun, 02 Apr 2023 22:42:18 +0000

In reply to Mihai Beffa. What is the contradiction? I do not see any.

By: Adrian S. Nastase

Adrian S. Nastase — Tue, 28 Feb 2023 00:51:20 +0000

In reply to Ayyappan A.

If you drop your ref voltage to 1.5V, then you have a solution:
R1= 3.37k, R2= 10k, R3= 10k, R4= 71.54k.
Of course, you need to choose standard resistor values as close as possible to R1 and R4 calculated values. Your power supplies have to be greater than 10V, so you need to select an op amp that can take +/12V or even higher, depending on its maximum output voltage swing specification.

By: Ayyappan A

Ayyappan A — Fri, 10 Feb 2023 04:15:40 +0000

Hi in my design I am supplying2.5 vref to noninverting terminal. And input to the inverting terminal ranges from 120mv to 3.4.v. To the min input(120mv) I need output of -10 and for max input (3.4)I need +10V.

It will be grateful if you could share me design calculations.