How can I do offset nulling for this opamp circuit?

Question

Circuit below amplifies 0-5mV signals to 0 5V level. But at zero input there is like 0.5V offset. Is there an easy way to offset null?

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Answer 1

You could add a pot as shown. The LM324 is not a good op-amp to use for mV DC signals- offset can be as much as +/-20mV and this will only adjust for +/-5mV offset (you have observed 0.5mV).

^{simulate this circuit – Schematic created using CircuitLab}

If you don't like having to come up with +/- supplies (or having it drift around by maybe 5-10mV/°C at the output), use a better single-supply op-amp.

Edit: Since you are using a thermocouple, and assuming it is floating, you can (and should) offset the input by adding a voltage divider to a reference on the (-) side of the thermocouple. Otherwise you'll never be able to measure below the temperature of the instrument terminal block. It's possible to combine the voltage divider with a temperature sensitive element to both offset and cold-junction compensate the input but that's way beyond the scope of this question.

Answer 2

The LM324 datasheet says

Large output swing: 0 V to V\$_+\$ - 1.5 V.

However it's not very good at close to zero. See my answer to How can LM324 work in dual supply mode when it only has 2 pins for the supply? where this is discussed in some detail.

Try connecting the op-amp negative supply pin to a -2.5 volts supply and run the simulation again.

Answer 3

You may not need to, and I suspect it would be the wrong approach, upsetting the behaviour during normal operation by trimming the input for an output-related problem.

The datasheet (Table 6.5 on page 5) shows that the LM324's output stage can source 20-40mA but only sink as many microamps under similar conditions.

Given this information, you can improve the performance of the LM324's output stage in certain applications by forcing the output stage into Class-A, so that it only ever sources current and never sinks it.

To do this, simply connect a resistive load from the output o V- (0V in your example). 1 kilohm if you can afford a few mA extra current consumption, up to 10 kilohms if you can't. Start with 1k, and if that fixes the problem, experiment with higher values if you need to.

One benefit is the elimination of crossover distortion (to which the LM324 is prone in AC applications such as audio).

But here, the benefit you're looking for is improved output swing when the output voltage is close to 0V.