QA40x DC measurement


I’ve been waiting for a QA403 and I would like to ask whether it’s possible to measure things like noise, ripple, etc… of bipolar power supplies(say +/-30Vdc for example) and if so, what is the procedure and what should I be careful of.

As far as I can understand, it is possible with voltages up to +/-40VDC but are there any online examples? I certainly don’t want to destroy a brand new analyzer… :slight_smile:


Hi @peppermint. Some information, to get you started, can be found here: Characterizing LDO Noise: Part 2 – QuantAsylum

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Thank you Claudio.

So, from what I understand on a single supply (up to +40VDC) is just a matter of connecting the V++ to the BNC pin and the 0V to the BNC shell, right? If so, do I use a 50ohm terminator on the unused BNC input like I would do with audio signals?

Also, on a bipolar supply (let’s say +/- 40Vdc) can I use the attached configuration if I want to measure both rails simultaneously?

I don’t want to find out the hard way… :slight_smile:

Hi. Since I think you are not interested in evaluating the DC component of the power supply (which, moreover, would be blocked by the input capacitors of the QA403), but you are only interested in studying noise and ripple, I recommend (at least I do so) that you take the signal to be analyzed from the power supply by means of a high-pass filter consisting of a 47 micro F 100V electrolytic capacitor with an appropriate resistor. In this way you will block the DC component and only the useful signal to be analyzed will pass through, and you will not subject the inputs of the QA403 to unnecessary stress.
Yes, any unused Input (ADC) of the QA403 must be closed with a terminator. Caution! This applies only to the Input. For the Outputs (DAC) of the QA403, those not used should be left open.

Hi Claudio,

I will read more carefully you post later today when I have a more peaceful time to understand better what you suggest.

In the meantime, what Fc should I be aiming for for the high-pass filter?

Also, I’m afraid I’m not sure what you mean by “DC component of the power supply”. On the link you posted, it’s the noise that is being measured, right?

Hi. For example, if you have a 40-volt DC power supply, the “DC component of the power supply” is the 40 volts. Added to this 40 volts DC is noise and ripple. It is these two components (ripple and noise) that it will be able to display with the QA403, while the 40 volts DC will be blocked by the input capacitor of the QA403 (see PDF of the QA403 specifications). As for the fc of the filter, I adopt about 3Hz.

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