Questions on Vss for op-amps (differential amplifier)

Question

I was working on a voltage amplifier and confronted the following problem regarding Vss of the op-amps.

The op-amps I am using are MCP603T-I/Ps. The circuit diagram is as shown below.

(INSTRUMENTATION AMPLIFIER CIRCUITS)

When Vss of each op-amp, denoted as Vss and Vtt in the diagram, is -3.7 V, amplification works fine. However, when I set Vss of both op-amps to zero (connect both Vss to ground), I don't get any amplification at all (extremely small voltage (5.6 μV) detected on the output of the amplifier).

I have looked at the datasheet, checked the absolute maximum ratings, and made sure that the analog inputs stay within Vss - 1 = -1 V and Vdd + 1 = 4.7 V. The datasheet is as follows.

I have checked the common mode input range of this diode so that all inputs are within range of Vss - 0.3 = -0.3 V and Vdd - 1.2 = 2.5 V. Moreover, as written on the datasheet above, I made sure that Maximum Output Voltage Swing of my circuit is between Vss + 15 = 15 mV and Vdd - 20 = 3.68 V, but I still can't get amplification from this circuit.

I would be grateful if someone could tell me what I did wrong and what I should correct to get amplification when the Vss of both op-amps are connected to ground (zero voltage, not -3.7 V).

Answer 1

Your input is floating with respect to the power supplies, in fact, you don't know what is common mode voltage; it is not reliably set.

In other words, you lack a DC path between the op-amps' non inverting inputs and GND to allow for input bias current.

Being functional with negative Vee is more a simulation issue than a good result. I don't expect a reliable behaviour in real life.

So, as far as simulation is concerned, try referring one of two inputs to GND or build the usual Vcm/Vdiff three (or two) generators input block.

For the real world you have to think of ground referring of your actual input, at worst add two high value resistors between inputs and GND.