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Jonathan Harris

Measuring LDO Power Supply Rejection Ratio for a Novice

Jonathan Harris
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10/10/2016 11:27:32 AM
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Re: Add: PSRR Considerations
Hi hansaresta, some very good points. 

1a. I found that the BNC to alligator clips worked equally as well as the high impedance probes in this case. 

1b. Indeed, you want to probe as close as you can to the input pins.

2. Input capacitors must be removed in order to get an accurate measurement of the LDO, that is definitely an important point.  The output capacitors need to be left in place as often times these affect device stability.

3a. Yes, the input signal needs to be small.  Notice the input amplitude setting is pretty small and may need to be lowered depending on the device saturation.

3b. Indeed the specified BW is up to 10MHz, but the measurement seemed to work pretty well and I was able to calibrate it up to the 30MHz limit of the T/R ports on the E5061B.

Thanks for your great comments!  You make some excellent points! Thanks for reading and thanks for taking the time to write in such a detailed and thorough response!

10/8/2016 8:40:44 PM
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Add: PSRR Considerations
Nice read. Some other important points to consider in measuring PSRR: 1.a. Avoid long wires in your set-up since these will add unwanted inductance that may affect your result. 1.b. Probe Vin and Vout (T and R) using high impedance probes and as close as possible to the DUT pin (again to minimize inductance effect) 2. Remove input capacitor/s of LDO before measurement. It could affect your line injector to go unstable at higher frequencies. Note that the input capacitor (and input impedance), depending on its value, also acts as a low pass filter to your AC signal that may introduce a lower cut-off frequency and may lead to an earlier 3dB roll-off of your PSRR (eg. at around 100kHz or less, PSRR may start to roll-off because the filter starts attenuating your input signal.) 3. and basic considerations like: 3.a. Vdc + Vac must be less than the absolute maximum supply of your LDO 3.b. Vdc - Vac must be greater than the UVLO of your LDO BTW: The usable bandwidth of J2120A Line injector is 10Hz~10MHz (set-up requires 5Hz~30MHz). Consider other methods if data outside 10Hz~10MHz is a crucial part of your measurement.

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