Measuring PSRR with Vcc seeing a low impedance

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drew_f

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I usually measure PSRR with the DC voltage applied to the DUT through a 50ohm resistor, and the AC voltage coupled in with coupling caps. However, this part I'm working with exhibits positive PSRR, when measuring it this way. I'm told that the Vcc pin needs to see a low impedance throughout frequency. Any recommendations on how to make this measurement? I've heard some suggest using bypass caps...but that would reduce the AC signal to negligible levels once it reaches the DUT. Any thoughts?
 

You can use a wideband buffer amplifier to supply Vcc, load current and the modulating signal from the network analyzer.
Another possibility is transformer coupling the ac modulation signal to the Vcc. Both of these would need calibration and check for resonances caused by the circuit with the transformer especially....
 

I heard of the one where you mix the AC signal onto a DC voltage and pass it to the DUT using an amplifier. However, what if this is a wideband part? How do they even test the PSRR of 1 GHz + op amps?
 

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