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I would like to know if the power supply rejection ratio (PSRR) of the AD797 depends on the gain of the amplifier. Assuming a power supply with 100mVrms of 100Hz ripple and the circuit of the figure 44b with a non inverting gain of 1000. According to the figure 13 of the AD797's data sheet the PSRR of the AD797 at 100Hz is 120dB, which means that a 100Hz ripple will be attenuated by a factor 10^6. So the 100mV 100Hz ripple should be present at the output reduced by a factor 10^6 which results in 0.1uV of 100Hz ripple at the output. It can be neglected. But if this ripple is amplified by the gain it becomes 0.1uV*1000=100uV which is much higher and can't be neglected. That's why my question is : Do I need to take into account the gain of the amplifier when computing the effects of the PSRR?
The short answer is yes. For several parameters on the typical op amp datasheet, the errors are all referred back to the input. This is sometimes denoted as “RTI”. Examples would be PSRR, CMRR, noise voltage and noise current. This has been an industry standard for many years, and does allow you to compute all the errors and apply them all at once. As a side note, The electrical table specification for PSRR and CMRR is a DC measurement. We therefore provide curves of PSRR vs. frequency. With the advent of switching regulators at 20 kHz and above, it is very easy to get power supply noise into you system. Having been an audiophile in the past, I can sympathize with the problems encountered with building an MMC pre-amp. PSRR can be measured by moving the +supply up and the –supply down by the same amount. This is what is typically done in the industry. The AD797 is unique in that it has separate curves for PSRR+ and PSRR-. See figure 13 of the datasheet. This is more useful because most people either run an op amp on single supply, or do not have +/- tracking supplies. For your application, it would not be unusual to have separate +/- linear regulators to power only the preamp stage. Another thing that is common is to have cascaded regulators, such as 78L15/79L15 feeding 78L12/79L12, but the regulator itself can have a noisy output, so an RC filter may also be required. Layout and bypassing are also critical. Even if the highest signal frequency is 20 kHz, the op amp can oscillate at 10-20 MHz if you have some stray capacitance in the wrong place. There are some "oldies but goodies" app notes on our web site that may be helpful. Take a look at AN-202*, AN-214, AN-257, AN-345*, and AN-581. I would read the * app notes first. Even if the +/- supplies are clean, noise can sneak into your system through analog ground. In general, “ground” is not the same as “ground” a few inches away.