Floating input vs. ground to a summing amplifier? 1

[benjaminc]

Hello,

I've got two low-level audio-frequency analog signals -- we'll call them A and B. I'm designing a circuit to give me an output of either A, B, or A+B, with the simple logic being handled by two multiplexers and summing by an op amp inverting summing amplifier (e.g.

http://www.ecircuitcenter.com/Circuits/opsum/opsum.htm).

For other reasons, I want all multiplexing to happen prior to the summing amplifier.

The A+B situation is obvious -- just have both signals on entering the summing amplifier. I see two options for producing just A or just B: I could either turn the enable of one multiplexer off, thus effectively floating that input line to the summing amplifier, or I could use the multiplexer to tie one of the lines to ground.

So, essentially, from a signal-to-noise perspective, any suggestions on which is better: signal+ground summed, or signal+floating summed? My instinct says to ground it, but that instinct is rather undeveloped. Input would be much appreciated.

Thanks!
Benjamin

 

[Noway2]

It has been my experience that all operation amplifiers need a DC bias path to ground. There is a small finite amount of current that flows into or out of these terminals. Without a path for this current, the inputs will charge to the rail, taking the output with it.

You need to weigh the factors of desired input impedance, input offset current, and signal to noise associated with the input offset current and offset voltages. Also be aware that an imbalance in the impedance at the inputs will produce additional error voltage. Whether or not these are a problem for your circuit depends on your design and its requirements. Use of precision components can reduce the effects of some of these error sources, but wanton application of precision components will simply drive up the cost.

In past implementations, I used .1% 100K resistors on the inputs to opamps that had a combined offset error less than the sensitivity of my ADC. I reasoned that 100K was still a good input impedance and was significantly higher than the circuit that was driving it. This is one area you need to factor the loading effect of the input.

 

[IRstuff]

Circuit design is all about CONTROL. A floating input is NOT CONTROLLED. That can lead to spurious noises, amplifier saturation as mentioned by Noway2, and even just plain ornery behavior. That said, grounding an input is still a non-trivial matter, since you still must control the noise that might get into the circuit.

TTFN

FAQ731-376

 

[benjaminc]

Thanks to both of you for the clear feedback. That adds some reasoning to my intuition. Sounds like nothing is ever quite simple when dealing with the analog side -- my experience with digital logic tends to lead me to oversimplify analog design.

Appreciatively,
Benjamin