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| (pre-)amplifier to measure power supply noise |
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| exe:
Hello. I would like to measure noise of my linear power supply. I think I need a pre-amplifier as my oscilloscope's noise dominates the measurement. I've seen a few circuits before, but, sadly, didn't make a bookmark. So far I only found not too many circuits, one of them is this one: https://www.analog.com/en/analog-dialogue/articles/low-noise-inamp-nanovolt-sensitivity.html . Basically, it's a single IC solution using ad8248. The article suggest to put four of them in parallel to reduce the noise, but, give the price of the device, I think I'll start with one :). So, what do you think of ad8248? Is it good-enough to measure some tens of microvolts? Or may be there is a cheaper alternative? I see there are quite av few low-noise opamps on the market. I don't really need it to be very accurate in terms of gain or something. It's more for comparative measurement. Bandwidth is not that important. I think 1MHz is should be fine. Gain 100-1000x, not sure how much I really need. I think the trade-off here bandwidth and gain accuracy. More gain -- less bandwidth. |
| iMo:
The lowest frequency of interest? |
| exe:
10-20Hz, up to 50Hz if this makes a big difference (I want to see ripple, if any). But I'm open for suggestions. I think very low frequencies I can measure with my keithley 2000 (may be not, never tried). Basically, for me it's a learning experience, I don't really have a need for this. I just want to play with my DIY power supplies and see what and how affects stability and response. |
| David Hess:
For a cutoff frequency that low, a low input bias current operational amplifier should be used to make the size of the input coupling capacitor reasonable. This only applies to the first stage; a majority of the gain can be in the following low impedance stage or stages. If the noise is really that low, then a differential measurement may be necessary to remove noise from ground loops. |
| radiolistener:
if you want to test power supply, you're needs at least 3-10 MHz bandwidth, in order to be able to see possible internal oscillations in the linear regulator circuit or noise in the switching regulator. |
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