Q403 variable input impedance

Hi there,

I’m testing some transformer coupled/ balanced out audio gear and I would like to take some measurements with different loads (other than the QA403 100k input impedance). Say, 600, 10k, 20k, etc…

What would be the correct way to do this? For the 600-ohm load example, is it better to connect two 600 ohm resistors from the BNC pins to the cells or just one 600-ohm resistor across the two BNC pins?

I made a wonderful drawing that I’d like to share with you…

So, what’s the correct approach? The Left input or the right input?

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As long as you’re using both + and - inputs in differential mode, the right channel is the correct way. Keeping the common mode impedance high will provide better CMRR.

If you don’t know the configuration of the output stage, you might load one side to ground to investigate.


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Thank you daleshirk

So, for a transformer output device (i.e differential output) the right connection would be more suitable, right?

I’m only asking because on the “DAC Output Impedance” guide, Matt recommends the left connection so I got a bit confused.

When loading the DAC outputs, you will want to use a load that is a shunt to ground, in parallel with the QA40x audio analyzer inputs. For example, if you have your balanced DAC outputs going directly into the QA40x audio analyzer, then the test loads you apply will resistors tied from the L+ input to ground, and from L- input to ground.