[time-nuts] Low noise powersupplies

Alexander Pummer alexpcs at ieee.org
Thu Aug 7 18:06:47 EDT 2014

to measure a power supply noise, better to say the noise spectrum,  you 
would need a very large non polarized capacitor and spectrum analyzer, 
The input of the spectrum analyzer does not like DC, and has low 
impedance. Since spectrum analyzer's input impedance is usually 50 ohm, 
for to be able to see the noise at low frequency you need  C = 1/( 2 x 
3.14 x 50 ohm x f Hz ) capacitor, and you would need a DC level limiter 
to prevent blowing the input of the spectrum analyzer during the charge 
up of that capacitor. If you could get a hold of an old HP 1Meg to 50ohm 
buffer amplifier you would need much lover capacitance or if  the buffer 
has AC input capability with low enough corner frequency like  the 
Tektronix P6201 FET probe, you would not need any capacitor. And that 
would make your life much nicer since capacitors could generate noise to..
Charles Wenzel  in his circuit collection files ha very nice good 
working noise reduction circuits.

On 8/7/2014 1:41 PM, Chris Albertson wrote:
> Just practical question..
> How would one measure noise at this level?  If I were evaluate this what
> would I need?  My scope lacks a nV/dev setting so is there some way to tell
> the difference between this and an LM317?  Seriously, what kind of
> instrumentation would I need before I could measure an improvement over my
> "standard" LM317
> On Wed, Aug 6, 2014 at 11:38 PM, Ole Petter Ronningen <olepr01 at gmail.com>
> wrote:
>> Hello, all
>> I thought it may be of interest to some of the members of this list that TI
>> is selling evaluation modules for some ultra low noise regulators for $20
>> in their estore, shipping world wide included. The specs looks pretty
>> decent to me, and I've ordered up a couple of boards to use as "clean up
>> boxes" for my bench-supplies, to use on noise-sensitive projects.
>> 1.4-30v output TPS7A4701EVM-094
>> 3.5µVRMS (10Hz, 100KHz)
>> <25 nV/√Hz (10Hz, 1MHz)
>> Maximum Output Current of 1A
>> +-15v version TPS7A30-49EVM-567:
>> 15v rail:
>> Noise:
>>   12.7µVRMS (20Hz to 20kHz)
>>   15.4µVRMS (10Hz to 100kHz)
>> Power-Supply Ripple Rejection:
>>   72dB (120Hz)
>>   ≥ 52dB (10Hz to 400kHz)
>> Maximum Output Current: 150mA
>> -15v rail:
>> Noise:
>>   14µVRMS (20Hz to 20kHz)
>>   15.1µVRMS (10Hz to 100kHz)
>> Power-Supply Ripple Rejection:
>>   72dB (120Hz)
>>   ≥ 55dB (10Hz to 700kHz)
>> Maximum Output Current: 200mA
>> Ole
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