High Voltage input ans external attenuator


I want to measure ripple/noise on some high voltage DC supply’s
DC voltage are 50 - 500 VDC, single ended

I want to put an external divider in front of my QA403 (div by 100)
R1 = 99 kOhm
R2 = 1 kOhm
I know this will introduce a small error due to the QA403 input impedance of 100 kOhm

But, do I need to add some extra input protection?

Any possibility to enter the external added attenuation in the software?

Any recommendations?

Hans Eriksen

Hi @DDTech. I, too, built a circuit to be able to examine the noise and ripple of power supplies. Keep in mind that the inputs of the QA403 are AC coupled and therefore it is useless to apply any DC voltage which would not be detected but could be a source of stress to the input capacitors. I opted for a 1st-order high-pass filter R C with a cutoff frequency at about 3Hz. I used 100V capacitors in series so as not to have any problem with voltages up to over 100V DC. I had no problem with this system. The only caution is to connect the filter to the power supply to be examined, turn on the power supply, and then connect the BNC to the QA403 so that the transient from the power supply cannot get to the inputs of the QA403

Yes, it is possible. Right-click on the “dBV” button and under “External gain” "Input Gain (dB) enter the attenuation in dB.

@Claudio I looked for any other posts you had made on power supply measurement but didn’t find the actual setup you used. Is there a post I missed? If not can you show us the setup?

Hi @Moto. The schematic I use and have made on a PCB is very simple. The 2 resistors RA and RB are easily changed to match the characteristics of the DUT power supply. In the picture 2 16 Ohm 100W resistor each are mounted, because I wanted to test a (unstabilized) “virtual battery” 55V power supply with about 2 A current. Here is also a photo of the load I use for the power supplies. The whole thing is mounted on a 5mm thick aluminum plate that acts as a heatsink for RA and RB. Connecting the QA403 to the BNC connector completely eliminates the DC component, which would not be detected and would only stress the inputs of the QA. With these values for the RC filter, I have tested power supplies with voltages up to over 100V DC. IMPORTANT. The circuit is very simple and works well, but it does not provide any protection for the inputs of QA403. As I wrote in the previous post, connect the DUT power supply first, turn it on, but without connecting the BNC to the QA403. After a few seconds, make the connection to the QA403. This is all to prevent the transient from damaging the QA403.

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@Claudio thx for the information!