How to correctly understand the op amp input offset voltage?

[Introduction]Digital power system management (PSM) controllers are typically targeted for high-current point-of-load (POL) applications. Lower current applications (with up to 6A load current) can also benefit from the PSM feature. The LTC3815 is a 6A monolithic synchronous step-down converter with digital power monitoring and control.

The input offset voltage Vos (Voltage – Input Offset) refers to the compensation voltage that needs to be added between the two input terminals to make the output terminal of the operational amplifier 0V. The Vos of an ideal op amp should be 0V.

The input offset voltage is equivalent to an additional voltage source loading the input. for example:

If the closed-loop gain G=1 (the left side of the figure below), the offset voltage Vos is conducted to the output terminal Vout = Vos

If the closed-loop gain G=1000 (the right side of the figure below), the offset voltage Vos is conducted to the output terminal Vout = 1000Vos

How to correctly understand the op amp input offset voltage?

Figure 1: The effect of offset voltage Vos on output voltage at different closed-loop gains (Image source: TI)

Therefore, the greater the closed-loop gain, the greater the effect of the offset voltage on the output.

In addition, we have to pay attention to one thing. The offset voltage is not constant, and the largest offset voltage change generally occurs at the extreme value of the output.

To keep the offset voltage under control, we also need to pay attention to the open-loop gain. When we define the open-loop gain, such as TI’s TLV8542, we will find that the working conditions of the open-loop gain are defined at -0.6V in the positive rail of the power supply and +0.6V in the negative rail.

Figure 2: TLV8542 open loop gain (Image source: TLV8542 data sheet)

As shown in the figure below, the abscissa output voltage Vout varies from the negative power rail -Vs to the positive power rail +Vs. The blue line represents the open loop gain, we can see a sharp deterioration in the open loop gain as we approach the positive and negative power rails.

Therefore, for the definition of open-loop gain, many manufacturers (such as TI) will calculate the average value over a large output swing range to achieve good linear operation. For example, select the positive rail of the power supply -0.6V and the negative rail +0.6V to calculate the open-loop gain, as shown in the red line above.

The largest offset voltage changes typically occur at output extremes, such as near the positive and negative supply rails. In the middle section, the incremental open-loop gain is high and then drops off as the output approaches near the positive and negative rails of the power supply. If the op amp is pushed to its swing limit (overworked), the offset voltage will rise more dramatically.

Let’s do some specific calculations. If the DC open-loop gain is 100dB, it is equivalent to:

Therefore, for every 1V swing from the neutral voltage output, the input voltage must change by 10µV. Think of it as an offset voltage that varies with the DC output voltage. The output swings 9V with a change of 90µV.

Therefore, in order to ensure that the offset voltage is within the controllable range, try not to overload the amplifier, resulting in a larger offset voltage.

(Source: DigiKey)

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