Ad797: PSRR definition

Q 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? A 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.

AD797: Offset Null configuration

Q What range of potentiometer values do you recommend for the Offset Null Configuration? The data sheet (Page 14) shows a schematic with a 20kohm used; but there are no further notes? A We recommend the 20k pot. The pot is used to balance the collector current sources of the input stage and compensates for mismatches in transistor sizes. A higher value pot will give you a more coarse adjustment (higher offset per degree of rotation). A lower value pot will give you higher sensitivity but generally reduce the linearity. The offset adjust mechanism is not explicitly tested during production therefore we do not provide numbers for resistor values vs. amount of offset adjust. Do note that the offset adjust on the AD797 should only be used to remove the offset due the AD797. It should not be used to remove offset in your system. System offset should be removed by summing in an appropriate voltage into the non-inverting terminal. There are two reasons for this: Trimming the AD797 for minimum offset, also trims for minimum temperature drift. We only guarantee enough offset adjustment to compensate for the AD797 offset, the absolute amount of offset adjustment available is likely to vary significantly from part to part.

Do you have the ESD levels for the AD797?

Q Do you have the ESD levels for the AD797? A Here are the ESD results for AD797: Passing level for HBM is 1.5kV and FICDM is 1.25kV

AD797: Maximum junction temperature Tj

Q I can’t find the maximum junction temerature of AD797 in its datasheet. What is the value? A The max junction temperature is 150°C, same as other parts on the same process. By the way, the AD797 came out in 1992 and is on a very old process. The AD8597 came out in 2008. It is on a much newer process, lower supply current, has a max voltage noise that is lower than the max on the AD797. The AD797 is being used by hundreds of companies successfully in production, but because of the high bandwidth of the output stage is difficult to use. It requires a very clean pc board layout, and two bypass caps very close to each supply pin. The AD8597 is much less touchy.

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AD797 Production
The AD797 is a very low noise, low distortion operational amplifier ideal for use as a preamplifier. The low noise of 0.9 nVâˆšHz and low total harmonic...
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AD797 on Analog.com