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we are designing a circuit with inverting inputs in the order of millivolts. The aim is to build an amplifier so that the signal can be raised to a digital level (3.3V)

Now, the question is this: What happens if the inverting signal is below the input offset? How does the op-amp gain behave? Will it be predictable?

This circuit is part of a pressure sensor. So if the transducer is designed with a resolution of say 10mV per Hg delta in pressure, do we need to factor in the input offset voltage?

Another related question is whether the CMRR too becomes a criterion in this design.

Please help.

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  • \$\begingroup\$ I guess you mean "differential inputs" not "inverting inputs", don't you? \$\endgroup\$ – Curd Jan 14 '17 at 13:28
  • \$\begingroup\$ I take it as "designers" you've done your research and you've looked at "instrumentation amplifier design" along with basic op amp theory . \$\endgroup\$ – JIm Dearden Jan 14 '17 at 13:28
  • \$\begingroup\$ Yes Differential inputs is the appropriate word. Thank you. We (indeed are two of us) have gone thru the CMRR criteria. Hence we have included it in the tail part of the question rather than as part of the main body. More critical is the question on the offsets. What happens if the input offset is more than the signal strength itself? Would it make a difference to the answer of this were measuring a varying pressure that gives out an AC signal? \$\endgroup\$ – Venkata Rahul Satuluri Jan 14 '17 at 13:42
  • \$\begingroup\$ For cable interface to sensor use twisted pair with shield grounded at one end with Op Amp that senses below ground. (V-) if output goes to 0V. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 14 '17 at 15:30
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The input offset is little to do with the closed loop gain so long as the input common mode is within the specification of the opamp. It is true, some opamps do do odd things if the CM range is exceeded.

As your input is in mV and if you are intending this to be a digital signal then the eventual output is 3.3V, you can bias the input to ensure the CM limits are not violated, regardless of the input offset.

There are opamp topologies that allow you to tune out the offset, though any offset can drift with time and temperature so this might not be what you want.

Chopper amplifiers or self calibrating opamps are used where you want to completely eliminate offsets.

There are many opamps available these days that have offsets in microvolts, so you should have no trouble sourcing one.

It's impossible to give an exact answer as you don't specify what type of sensor you are using; what output it produces, what range of pressures you want to measure, what accuracy and if you want only positive pressures. Because of this I will answer the main part of your question, "what will happen if the input offset exceeds the magnitude of the signal"

The simple answer is that the output will saturate, clamp itself to the maximum (or minimum) output and stay there. Some opamps behave oddly if the output saturates, e.g. suddenly gain a 180 degree phase shift (a positive clamp suddenly becomes a negative one or visa versa).

My suggestion would be to choose your opamp carefully to avoid the offset.

The suggestion of a differential opamp configuration offered by others is misleading, you only need this if your sensor produces a signal that can be either positive or negative without biasing.

If you want best accuracy you may want to consider an instrumentation amplifier, this allows you to have much higher gains, so less gain stages and will have a low input offset.

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