Amp Crosstalk

The amp crosstalk plug-in allows you to quickly characterize the crosstalk of an amplifier using stepped sines. The sines are stepped over frequency and amplitude.

Connections

The connections are straightforward: Connect the left and right QA40x outputs to the amp inputs, and the amp outputs to the QA40x inputs. These can be either balanced or unbalanced connections.

Crosstalk Options

The options for the crosstalk plugin are shown below. Like other plugins, you can specify start and stop levels and start and stop frequencies. You can specify if you'd like log or linear steps. And you can also specify that the input range can be autoset. If this is selected, then the test will start with a tone being used to determine the amp gain on the left channel. Subsequent measurements will then compute the QA40x full scale input level required to ensure the amp is measured with the specified amount of headroom.

In the options above, you can measure the left to right crosstalk, the right to left crosstalk, or both. The left to right crosstalk is the amount of signal from the left channel coupling into the right channel. These measurements are made by muting the appropriate channel. So, if you specify you'd like to measure the left to right crosstalk, then the right channel will be muted on the QA40x and the left channel will be driven at the specified amplitude and frequency. And the crosstalk will be measured by seeing how much of the left channel can be seen in the right channel.

Issues to be aware of

The FFT size you select will have a lot to do with the amount of crosstalk that can be detected. If your FFT is too small you won't be able to discern crosstalk from the noise floor. And if your FFT is too large, the test will take a long time to complete. When the test is running, you can see the measurements in real time. Make sure the crosstalk channel is showing a tone that is above the noise floor by 5-10 dB. If it's not, use a larger FFT.

The QA40x output, while muted, will still have some noise present. Make sure you understand the limits of this noise. For most amps, this won't be an issue. But on extremely low-noise opamps, the noise level of the QA40x may mask the level of the crosstalk tone. You will know this during the run if you cannot see the tone.

For example, in the plot below we can clearly see the crosstalk tone (red) at about -125 dBV.

However, in this plot the crosstalk tone (red) is in the noise

and if we switch to a larger FFT (128K) then tone is still tough to discern. This is bumping up against the limits of the QA40x hardware.

Sample Plot

The plot below is the QA40x in single-ended loopback, run with a 32K FFT.

The default plot will show all left measurements in blue, and all right measurements in red. You can pick the Graph Tool menu option Traces->Make all Color Distinct to get a better view of some outlier data. Additionally, the thickness of traces can adjusted to highlight further. Here, the +15 dBV output levels have been set to a width of 4. What we can see from the plot below is that as the drive level increases, the crosstalk of the QA40x increases too. The mechanism here coupling via the shared rail LDO between the left and right channels. That is, the currents required at the higher drive levels (15 dBV) manifest as a tone on the LDO output as the LDO regulation fights to keep the rail voltage constant. And that tone on the LDO is then picked up by the other opamps sharing that same rail.