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Ripple Voltage and PSRR

Last Updated: August 12, 2009

Question: For analog measurements, what is the maximum allowed ripple on VDD and what is the PSRR of the PSoC?

Response: There is no explicit specification for maximum power supply ripple. The opamp and the reference circuits have power supply rejection guaranteed better than 50 dB. This means that 100 mV ripple on the power supply will result in a maximum of 300 uV at the output of the opamp or reference. With knowledge of this rejection, you can calculate your maximum allowed power supply ripple from your system requirements. For example, if you have a 14 bit converter using reference = 1.6*Vbg + 1.6*Vbg, the resolution is 0.25 mV. If you want the noise in the system from the power supply to be less than 0.5 bit, then the maximum allowed power supply noise = 3.2*Vbg / (2^14) * 0.5 * 50dB = 38 mV. For lower resolution ADC, the requirement is less restrictive. The numbers above are for the WORST case, typical numbers are 10 to 30 dB better.

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