- Dec 23, 2019
Thank you, but I think this can be applied to all the measurments, I doubt that the grounding scheme would have been changed from one measurment to another. What is in your opinion the correct way to connect the ground to have correct measurments? what is the fault that wold make interference appear that are not present in real life? It's interresting.
You asked an important question. We’ll try to explain it in a simple way
When analysis and the device under test is connected, grounding must be taken into consideration, because it’s related to measurement accuracy. The key factor is “reference ground”, especially in the analysis of AC signals. Ideally, the connection of analysis and signals under test is like this:
However, when it comes to the actual analysis, it’s more complicated, such as the gain adjustment and filters, which makes the data output of the ADC part needs isolated transceiver.And the reference ground of the analysis is floating.
Here we only talk about audio analyzer’s input schematic. Below is the connection of the device under test and AP:
It can be seen that to the signal source, AP’s “reference ground 1” is floating, so all the uncertain interference between reference ground 1 of AP and reference ground 2 of the device under test will influence the measurement. Equivalent principle is like this:
Now, a short and thick wire (for smaller AC impedance) is needed to connect AP and the “reference ground” of the device under test, in order to reduce interference. Like this:
Almost all kinds of analyzers have the independent grounding port, which is for more effective measurement. Take AP for example, it even has one independent grounding port for each channel.
So does DS3:
An analyzer would set its grounding port. As for audio devices, some have it too, and some don’t, so we need to analyze the situation accordingly. What is the “reference ground” to a DAC? It’s usually like this:
In the actual application, such problems will be avoided by manufacturers. Like an amplifier would usually connect input ground to “earth”:
Most DACs’ output ground takes “earth” for reference. And the interference will be small. To measure a DAC above 20kHz is because out-of-band signals and 20Hz-20kHz signals will produce IMD. The principle is as follows:
If the IMD is within 20kHz, it’ll be heard, which is also the reason why a DAC design needs oversampling. For more details please refer to:
DYNAMIC PERFORMANCE TESTING OF DIGITAL AUDIO D/A CONVERTERS, By Larry Gaddy and Hajima Kawai, Copyright 2000, Texas Instruments Incorporated (also attached)