QA-401 Differential voltage specs

Hi Matt,

I would just like to confirm the QA-401 differential voltage specs.
I would like to know the maximum differential voltage without the 26dB attenuation engaged.

I would also like to know what is the maximum voltage that can be applied to a single differential input terminal when taking a differential measurement without the 26dB attenuation engaged.

For Example.
I’m assuming that I could not put 300v into the positive differential input terminal and 299v into the negative differential input terminal. For a total differential voltage of 1v.

I can not see this maximum voltage in the spec.

I plan to take a differential measurement across a series of resistors and need to know this information.

Kind Regards


Hi @Stuartmp74, it’s a good question! The relevant section is “Full Scale Input” on page 26. It says

Full Scale Input
Note that the maximum input to the analyzer is +32 dBV, with a max DC of 40V
and a max ACPEAK + DC of +/- 56V. Those limits must always be respected,
even if you are on the +42 dBV input range. The concept of full-scale input is to
help the engineer understand how far away they are from clipping, as this has an
impact on the measurement quality.

The takeaway here should be that you can apply up to +32 dBV to a single input OR a differential +26 dBV to the PLUS input and +26 dBV to the MINUS input. Both conditions will be reported as +32 dBV.

The additional qualifier is that the tip of the sine plus any DC must not exceed +/- 56 volts, And another qualifier is that the max DC must not exceed 40V.

So, your 300V RMS would handily violate the AC peak + DC rule of +/- 56V.

This is also written on the top of the case for quick reference:

And also present on the UI for quick reference:


In short, you needed search far to see the limits!

Hi Matt,

Thanks for the response.
I’m a bit confused. I asked this question in relation to the QA-401 but you have sent me information for the QA-402

Attached is the image from the top of the QA-401

Please confirm the numbers for the QA-401

Ah! Sorry sorry sorry! I misunderstood. Yes, for the QA401 you don’t want any single input to exceed 20Vrms. You could drive L+ and L- both at 20Vrms with any phase relationship, no problem. That will show clipping, but it will be tolerated. I’d not leave 20Vrms applied overnight while you sleep, as that would prematurely age the input cap. Sorry again for the misunderstanding!