Comments on Ferrite Bead in ADC Power supply
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Ferrite Bead in ADC Power supply
I am using AD5941 in my design. The power section of its analog domain (AVDD) is given below. It is obtained from the reference design. You can see the schematics, BOM etc in bottom of this page.
The LDO used is ADM7155. From LDO the output is connected to a ferrite bead (MPZ1608S601ATA00).
I found an Q&A in ti e2e community which suggests don't use ferrite beads in ADC power supply.
I simulated the AVDD power supply section in LTspice. The response is given below. The output is kind of a square wave.
CIRCUIT:
SIMULATION RESULT
May I know do I really need to place a ferrite bead in my board?
The response I shown is that expected. Why it is pulsating?
In the reference board they used single LDO for AVDD and DVDD. In my board I will be using separate LDO's for AVDD and DVDD, but same P/N.
Post
Digital side of the A/D converter injects noise into the digital supply rail. LDOs can regulate out low frequency oscillations. But LDOs don't block high frequency noise very well. Some specially designed LDOs can smooth out ripple from switch mode converters up to 2 MHz. But that's as high as LDOs go.
Ferrite beads block high frequency noise better than LDOs. The ferrite bead along with the decoupling capacitors prevent the noise generated by the digital section of the A/D from leaking through the power supply rails to the analog section of the A/D.
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