ABCs of ADCs - Analog-to-Digital Converter Basics (PDF)

PSRR of the ADC120406050PSRR, dB403020100.1 0.3 1 3 10 30Noise Frequency, MHz30The PSRR of the ADC12040 is excellent, yet PSRR does degrade with higher frequencies. Todegrade just 8dB from 100kHz to 10MHz, however, is exceptionally good for any ADC.This test was performed by providing a 1.2 MHz input frequency to the ADC and a.c. couplingthe a constant amplitude (500 mV P-P ), variable frequency sine wave to the ADC power supplypins. This amplitude was measured right at the supply pins. The ADC reference was 2.0V, soif the PSRR were zero the output amplitude would have been 20*log(0.5/2.0) = -12.04 dBFS.The supply “noise” frequency amplitude was measured in dB. The PSRR was calculated to bethe difference between this output level and 12 dB. So, for example, with 500 mV P-P at 10 MHzon the supply pins,the 10MHz frequency bin was found to be at -62 dBFS, so PSRR was determined to be-PSRR = -62 - (-12) = -62 + 12 = -50 dBABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray30Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved