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Questions about LDO test

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hung_wai_ming@hotmail.com

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ldo testing

You can measure the transient loading change by conecting a resistor and a transsitor and control the switch on/off of the transistor to make the load changes.

PSRR is defined as dVout/dVDD, so when u change VDD, you observe the change in Vout and the PSRR can then be calculated, it's the simplest method
 

    penghan

    penghan

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test ldo psrr

hung_wai_ming(at)hotmail.com said:
You can measure the transient loading change by conecting a resistor and a transsitor and control the switch on/off of the transistor to make the load changes.
Click to expand...

Good idea! I will try this method to test it! Thank you!

hung_wai_ming(at)hotmail.com said:
PSRR is defined as dVout/dVDD, so when u change VDD, you observe the change in Vout and the PSRR can then be calculated, it's the simplest method
Click to expand...

Since the PSRR of LDO is as high as 80dB. This to say, input voltage should change 10V to make output change 1mV. Thus, this way is not realizable. How do you think about it ?
 

ldo test

Another definition for PSRR for LDO:

PSRR = 20log(ripple input/ripple output)

So you can inject input ripples with say 100mV and measure the output ripple and you can calculate the PSRR, in a simple method. Also, you can vary the frequency to obtain the PSRR vs frequency plot.

Attached is the TI reference for you
 

ldo ripple definition

For input ripple of 100mV, the output ripple is only of 10uV. No availabe intrument can be used to detect 10uV ripple. Correct?
 

how to test psrr for ldo

An spectrum analyzer can do the job. Other method consists of decoupling the the DC component of the output ripple and amplify it through a low noise amplifier.
 

testing ldo

Yes, penghan, you can always amplify the output signal by low noise PGA with some gain factor, say 100 so that you can observe visible output
 

output noise ldo

good idea, have a lesson, 3x
 

ic test ldo

Yes, use low noise amp is good idea.
I haven't got a low noise amp but I know that PSRR curve look like Gain curve so I did like bellow step:
[1] I change VIN and measure Vout --> I got PSRR @ DC(f=1K) then I also got PSRR ~ 80dB @ 1KHz if my LDO has BW=1K (approximation)
[2] I change f=10KHz then I got PSRR ~ 60dB (with 60dB you can see on Osc). PSRR roll-off qickly @ BW frequency.

Hope that can help!
 

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