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Let's think DC only (no AC ripple) An op amp datasheet says the PSRR for Vs = ±3V to ±18V is 120dB. I understand that if Vs+ increases by 1V and Vs- decreases by 1V, the input (when RTI) will increase by 1uV. But increase from what reference level? Is it that ±3V means no PSRR effect is added to the input voltage offset, ±4V means 1uV added, ±5V means 2uV added, etc? Or is the reference level from which this 1uV/1V is taken the average supply level (10.5V)?Or am I missing something?

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2 Answers 2

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There is no specific reference level. The decibel is just an unitless value that expresses a factor, or a ratio of two values, in logarithmic scale.

The PSRR means how much the power supply affects the output value As you already said, 120 dB equals to a ratio of one million to one, so 1V of change gets attenuated by 120 dB, or to one millionth, thus equaling 1uV of change.

Of course the PSRR itself may have a different value at different DC supply voltages.

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  • \$\begingroup\$ Maybe my question is not clear enough. Let's take this example: I supply this op amp with Vs=±5V. So is the PSRR contribution to input offset (5-3)*1uV=2uV or is it (5-18)*1uV=-13uV? If this (Vsreal-Vsref)*1uV calculation is valid, what is Vsref? \$\endgroup\$
    – Hyp
    Dec 17, 2021 at 6:37
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Example: I supply this op amp with Vs=±5V.

Take this as reference/starting point.

So is the PSRR contribution to input offset (5-3)*1uV=2uV or is it (5-18)*1uV=-13uV?

PSSR tells you how much the input offset (as a model abstraction) will change exclusively due to a change in the power supply. It is one of several "non-idealities" which are represented by a input offset voltage, so the PSRR adds to the uncertainty of the \$V_{OS}\$, given a possible variation range of the power supply.

Take the initial conditions (\$\pm5V\$). Assume you have an opamp with an input offset specification, due to all factors, which varies between \$\pm100uV\$.

Example: a specific real component of this model, under specific conditions (temperature, common mode voltage, input currents etc) presents a real equivalent input offset of \$+37uV\$. You change only the power supply to \$\pm6V\$. You have \$2V\$ of change in the power supply, which gives you \$2uV\$ of uncertainty (for the 120dB you mentioned).

Now the real total/equivalent input offset for this specific circuit can be anywhere between \$+35uV\$ and \$+39uV\$.

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  • \$\begingroup\$ So would it be correct to say that if the datasheet says the input offset is ±100uV at ±5V and I change only the power supply to ±6V, the resulting offset would be ±101uV? \$\endgroup\$
    – Hyp
    Jan 3 at 9:44

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