How to clamp opamp output to +-12v when supply is +-15v

Question

I have the following circuit. It's purpose is to take a 0-3.3v signal from an MCU and provide a square, -12v to 12v wave across R3, R4 and either R5 or R6. The idea is to read +-9v when only R4 is connected, +-6v when R4 and R5 are connected, and +-3v when R4 and R6 are connected. The peak-to-peak voltage of the square wave are reduced depending on the configuration of R4/5/6. I have no control over the values of R4, R5 and R6. My problem is that, considering the +/-15v supply, the output of the opamp is somewhere around +/-13.7v. This offsets the values at the "Output" point by a few volts and is out of spec.

I thought of using a rail-to-rail amplifier with a +/-12v supply, but even the rail-to-rail amps can't truly swing to the rails. Also, I'd have to re-work a whole bunch of other subcircuits to work with +/-12v or add a whole other AC/DC converter and I'm not willing to do either.

I then thought of using two back-to-back zeners right after R3, but that would only clip the max voltage, all of the other points would still be too high/low.

Then, I thought of using simple voltage divider for the power inputs of the amp, but that would add 4 resistors to the board and I'd rather avoid that if possible.

In the schematic, assume SW2 has another unconnected pole so that having only R4 in series with R3 is an option.

^{simulate this circuit – Schematic created using CircuitLab}

Can anyone think of how I can limit the output of the opamp to +/-12v and maintain the 9-6-3v ratios with the regards to the output?

Answer 1

You're using an op-amp as a comparator, which isn't all that great an approach.

Consider replacing the op-amp with an analog switch such as an ADG1419.

You can either reduce the supplies to +/-12V or replace R3 with two resistors that give you the Thevenin equivalent. eg. Ra = 1137\$\Omega\$ series and Rb = 4550\$\Omega\$ to ground.

Ra || Rb = 910\$\Omega\$

15V * Rb/(Ra+Rb) = 12V

---

If you insist on using a quad op-amp you could add a series resistor and back-to-back zeners before the 910 ohm resistor, but you'll need an op-amp that swings a lot closer to the rails under load than the TL081, and the tolerance on the voltage will not be great. Another option would be a Schottky bridge and a single zener or a TL431 with resistors to set the clamping voltage (at least that would be very symmetrical).

Answer 2

Run back-to-back 11V zeners from the output (the node where you want the 12V swing) to the negative input. If you choose the node by the switch (that you've labeled +/-12V) the voltage divider won't do much, I'm guessing you want it on the op amp output.