Analog amplifier with gain switchable between +1 and -1

[hmalissa]

I'm looking for a circuit that would allow me to invert the sign of an analog signal with a digital control signal.
I was thinking of building a unity gain amplifier and an inverting amplifier with gain -1 with 2 op amps, and use a MOSFET or such to switch between them. Does anyone have experience with such a circuit?
Also, I'd need fairly high precision, since I'm looking for a small difference in 2 larger signals.
The switching interval would be at least 4ms.
I think this is a very simplified version of a lock-in detector, is that right?

 

[FvM]

A simple sign multiplier will do
https://www.edaboard.com/threads/272103/
https://www.edaboard.com/threads/246986/

 

[hmalissa]

Thank you very much pointing out the analog multiplier to me. I did some reading and I believe that some kind of analog multiplier or voltage controlled amplifier is close to what I need.
But my control signal that switches the sign is a digital TTL signal. How can I build a sign multiplier that works with a digital input?

 

[crutschow]

Below is a circuit using one op amp that will either invert or non-invert the signal depending upon the position of S1 (S1 closed is inverting).

S1 can be an analog switch controlled by a digital signal.

Note that R1 and R2 determine the inverting gain so need to be matched if you need the gain exactly at -1. You can make R2 a pot to adjust it exactly to match the non-inverting gain if you need to.

What is the highest frequency of the analog signal?

 

[hmalissa]

Thanks a lot, this is very helpful!
The highest frequency is 1kHz, so the circuit doesn't have to be very fast.
So S1 would be some kind of relay?

 

[crutschow]

At 1kHz just about any standard op amp should work. Note that the power supplies require a voltage greater than the plus and minus input voltage peaks. How much greater depends upon whether it is a standard or rail-rail type op amp.

S1 can be a mechanical relay or analog solid-state switch. Note that the ON resistance of the switch has an effect on the inverting gain. You can minimize the effect by using a low resistance switch and/or increasing the value of R3. Also the switch must tolerate the plus and minus peak input voltage.

 

[FvM]

But my control signal that switches the sign is a digital TTL signal. How can I build a sign multiplier that works with a digital input?

Apparently you did not read the linked posts. I was exactly referring to circuits multiplying a digital with an analog signal.