Assuming a design case where an Ultralow Noise Amplifier like AD8331 is used. This amplifier has an input voltage noise of 0.74 nV/√Hz and input current noise of 2.5 pA/√Hz. The amplified signal will be digitized with a 24 bit ADC (Vref=3V) in order to perform an spectral analysis.

I have some questions regarding the selection of a suitable LDO for this application. I need to get 5V from a 15V source.

How would affect the selection of the following LDOs on the overall noise performance of the amplifier?

Example1: Ultra Low Noise LDO: ADM7150 RMS Noise @ 10Hz to 100 kHz (𝛍V rms): 1.6μVRMS

Example2: Low Noise LDO: LT1761 RMS Noise @ 10Hz to 100 kHz (𝛍V rms): 20μVRMS

Example 3 LDO: LM3480 RMS Noise @ 10Hz to 100 kHz (𝛍V rms): 150μVRMS

It makes sense to use an Ultra Low Noise LDO in this application?

  • \$\begingroup\$ The problem here is that only you can answer the question; you do seem to have an understanding for the Power Spectral Density of Noise, and now it's up to you to define whether your LDO noise will contribute significantly to your measurement result's quality. \$\endgroup\$ – Marcus Müller Oct 31 '16 at 15:12
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    \$\begingroup\$ You need to know what you are doing with that supply. Bear in mind that for many uses you can add further filtering to reduce the noise for a particular use of that supply, e.g. as a reference voltage. \$\endgroup\$ – Brian Drummond Oct 31 '16 at 15:14

You can start by looking at the noise spectrum of the LDO and the PSRR (power supply rejection ratio) of the op-amp over that spectrum. It's not unusual to add some passive filtering on the supplies (R+C or ferrite bead + C) for sensitive parts or a capacitance multiplier so even a relatively noisy source may not be an issue. The IC makers would like to sell you a boutique part, of course, so they are not an unbiased source of information.

  • \$\begingroup\$ Hello Spehro, 'expectral' analysis was a mistake, is spectral analysis :) \$\endgroup\$ – jcambra Oct 31 '16 at 19:48
  • \$\begingroup\$ Ah, I guess that was my expectation or expectoration. Will edit appropriately. \$\endgroup\$ – Spehro Pefhany Oct 31 '16 at 19:52
  • \$\begingroup\$ Ok, the question was because I am starting the design of a new PCB and wanted to know which would be the best choice a priori. I will add the filtering on the supplies as you mentioned. \$\endgroup\$ – jcambra Oct 31 '16 at 19:55

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