# What is full subtracter circuit

## Forum: Analog electronics and circuit technology Subtracter circuit with OPA244

Hello, I tried to build a subtractor circuit with amplification according to http://www.elektronik-kompendium.de/sites/slt/0210153.htm. The resistors (link) were chosen so that R1 = R3 = 1k and R2 = R4 = 10k. A gain of 10 should therefore be set. The voltage drop was measured via a 200 ohm resistor and applied to the non-inverting and inverting input of the OP. The high-current side was measured. The OP was connected to ground and 3V, so it should be able to output everything between 0 and 3V. Unfortunately, the circuit structure always gave me the full supply voltage (3V) at the output of the OP. Has anyone ever built such a circuit with an OPA244? Or what is possibly wrong from my description? Thank you for your answers.

See data sheet: INPUT VOLTAGE RANGE: Common-Mode Voltage Range 0 ... (V +) - 0.9 V That means: the voltages at the inputs must not be higher than 2.1V (at V + = 3V). The function is no longer guaranteed above this. You need another OP, e.g. LMC6482. Greetings Dietrich

Hello Christian, what are the input signals at Ue1 and Ue2? What do you mean by "The voltage drop was measured via a 200 ohm resistor and applied to the non-inverting and inverting input of the OP. The high-current side was measured." The statement "The OP was connected to ground and 3V, so it should be able to output everything between 0 and 3V." is not correct. It is not a rail-to-rail opamp. See data sheet under OUTPUT. Greetings, chaos

Hello Dietrich, thank you for your comment. Now I understand why a voltage divider is connected upstream of the high-side current sense in the data sheet (to reduce the supply voltage to (V +) - 0.9). Greetings Christian

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