DAC Output Impedance - QuantAsylum/QA40x GitHub Wiki

The DAC Output Impedance Automated Test allows you to determine the output impedance of a DAC, using two test runs with different load resistors.

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.

For this test, a breakout board was shown below. This board takes the balanced output from the DAC, allows a selection of loads to be applied via a jumper, and the makes available the balanced signals via BNCs for direct connection to the QA40x audio analyzer. For stereo measurements, a pair of these boards are needed.

Settings

The settings options are shown below. You can opt to step from a starting to a stopping frequency, and that step can be in log (with a selected number of points per octave) or linear steps.

Note there are two impedance levels you must specify. The sweep will run twice. The first time it runs, you will be prompted to apply the first-pass load impedance (in this case, 100kohm). And the second time it runs, you'll be prompted to apply the second-pass load impedance (in this case, 1000 ohms). The 100kohm is used to effectively specify an open. That is, no load is connected except for the 100K of the audio analyzer.

Test Results

The plot below shows the output impedance of a Topping E70 Velvet DAC from 20 to 20 kHz. This was a balanced measurement. Topping specifies the output impedance of this DAC at 100 ohms (XLR). And the measurement below shows a 51 ohm impedance per branch, which is a 102 ohm balanced output impedance.