Power supply rejection ratio

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In electronics, power supply rejection ratio or PSRR is a term widely used in the electronic amplifier (especially operational amplifier) or voltage regulator datasheets; used to describe the amount of noise from a power supply that a particular device can reject.

The PSRR is defined as the ratio of the change in supply voltage in the op-amp to the equivalent (differential) output voltage it produces,[1] often expressed in decibels.[2][3][4] An ideal op-amp would have infinite PSRR. The output voltage will depend on the feedback circuit, as is the case of regular input offset voltages. But testing is not confined to DC (zero frequency); often an operational amplifier will also have its PSRR given at various frequencies (in which case the ratio is one of RMS amplitudes of sinewaves present at a power supply compared with the output, with gain taken into account). Unwanted oscillation, including motorboating, can occur when an amplifying stage is too sensitive to signals fed via the power supply from a later power amplifier stage.

Some manufacturers specify PSRR in terms of the offset voltage it causes at the amplifiers inputs; others specify it in terms of the output; there is no industry standard for this issue.[5] The following formula assumes it is specified in terms of output: \mathrm{PSRR} [\mathrm{dB}] = 20 \log_{10}\left({\Delta V_\mathrm{supply} \over {\Delta V_\mathrm{out}}} \cdot A_v\right)\mbox{dB}

For example: an amplifier with a PSRR of 100 dB in a circuit to give 40 dB closed-loop gain would allow about 1 millivolt of power supply ripple to be superimposed on the output for every 1 volt of ripple in the supply. This is because

100\ \mathrm{dB}-40\ \mathrm{dB} = 60\ \mathrm{dB} .

And since that's 60 dB of rejection, the sign is negative so:

1\ \mathrm{V} \cdot 10^\frac{-60}{20} = .001\ \mathrm{V} = 1\ \mathrm{mV}

Note:

  • The PSRR doesn't necessarily have the same poles as A(s), the open-loop gain of the op-amp, but generally tends to also worsen with increasing frequency (e.g. [1]).[citation needed]
  • For amplifiers with both positive and negative power supplies (with respect to earth, as op-amps often have), the PSRR for each supply voltage may be separately specified (sometimes written: PSRR+ and PSRR-), but normally the PSRR is tested with opposite polarity signals applied to both supply rails at the same time (otherwise the common-mode rejection ratio (CMRR) will affect the measurement of the PSRR).
  • For voltage regulators the PSRR is occasionally quoted (confusingly; to refer to output voltage change ratios), but often the concept is transferred to other terms relating changes in output voltage to input: Ripple rejection (RR) for low frequencies, line transient response for high frequencies, and line regulation for DC.
  • Sometimes kSVR (or simply SVR) is used to denote the power supply rejection ratio (e.g. [2])

References

  1. ↑ Maxim Integrated's glossary definition of PSRR
  2. ↑ Allen, Phillip; Holberg, Douglas, CMOS Analog Circuit Design, Oxford University Press, Inc, cc 1987.
  3. ↑ Franco, Design With Operational Amplifiers and Analog Integrated Circuits, McGraw-Hill, Inc, cc 1988.
  4. ↑ Jung, Walt; Op Amp Applications Handbook, Newnes, 2006, page 86 http://www.analog.com/library/analogdialogue/archives/39-05/Web_Ch1_final_R.pdf#page=93
  5. ↑ Op Amp Power Supply Rejection Ratio (PSRR) and Supply Voltages. Analog Devices appnote/tutorial MT-043

External links

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