I'm designing a select-able gain circuit using a multiplexer and an inverting amplifier. Here's an example circuit, using a multiplexer.
I've read that if you provide a voltage larger than the supply voltage it can latch-up and/or damage the circuit due to too high of a voltage.
My question is do these effects occur when the multiplexer is switched to off? For example, if the input voltage has an amplitude of Va = 40V, the multiplexer is supplied with a 5V source, and the third switch (10k ohm) is the only activated line? I'm trying to target voltage ranges from 50mV <= Va <= 40V with gains of 0.02 <= G <= 20 and the output voltage from the op-amp should be ~1Vpp or ~2Vpp (referenced to the virtual ground). The frequency response should be anywhere from DC to ~20MHz.
If not, is there another way I can achieve this wide gain range (note: the value of R5 should be small since the circuit has to be able to handle high frequency signals)? I'd prefer to stick with only solid state components if possible.
Best Answer
Your circuit doesn't make much sense: for two of the inputs you use an amplifier as attenuator, for a third one a x1 amplifier. Only for the R4 position you use it as a x10 amplifier.
Besides, why did you choose such low resistance values? For the 10 Ω most analog switches will have a significant on-resistance, even if they're as low as 1Ω: that's 10 %. If you scale all resistances up by a factor 100 you'll have the same attenuation/amplification.
Analog switches with low on-resistances exist, but their inputs are limited to the power supply, and then 40 V is an issue.
You say you want solid-state, but a relay may be the better performer, especially a reed relay. Low on-resistance of 150 - 200 mΩ, low off-capacitance, in the order of 1 pF, capable of switching high voltages and high frequencies.
Then your configuration. I would replace R2 and R3 by a resistor divider with taps at :10 and :100. Your third relay (for :1) comes at the top of the divider, from where you also go to the x10 amplifier, whose output goes to the last relay. You'll want to protect the input of that amplifier against overvoltage with low-capacitance clamping diodes.