THD "Measurements"
Posted: Sun Mar 24, 2019 8:07 am
I promised to post some THD "measurements."
"Measurements" are in quotes - it's more like THD interpretation.
The measurement chain is:
Soundcard Out > THAT1646 > -60dB U-Pad > ZTX851 Balanced Preamp > PCM4222EVM (96k/24b) >AudioTester
The DUT gain is 62 dB; PCM4222EVM loading pads the output by just under 0.7dB
Because there is 6 dB gain in the THAT1646, the soundcard output is set 6 dB lower.
Rsource is 10Ω; the front-end op amp is an NE5532.
For the FFTs which show the "Generator Monitor" the Right Channel Flow is Soundcard Out > PCM4222EVM.
There is a 6 dB loss through this path due to the high 560Ω output impedance of the soundcard and the same (about) 560Ω termination of the PCM4222EVM.
When the generator output level is reduced by -6dB along with the DUT channel so that they level match, there is a 12 dB offset in overall level seen at the PCM4222EVM inputs.
The gain offset, though not intentional or ideal, does let us see, at different levels, the overall THD signature of the D/A and A/D.
Though not the best, I use what I have and I'm sure the explanation makes everything about as clear as mud.
Having said all of that here are the actual measurements. The first two are the overall THD of the preamp and measurement system.
ZTX851 Moving Coil Preamp THD 62dB Gain
Note that no effort was made to null the induced hum radiating into the ProtoBoard.
ZTX851 Moving Coil Preamp THD 62dB Gain 5 kHz
The next images show the THD signature of the D/A and A/D loop-through in the Right (red) channel.
ZTX851 Moving Coil Preamp THD 62dB Gain 5 kHz vs GenMon
The THD signature at -10 dB.
ZTX851 Moving Coil Preamp THD 62dB Gain 5 kHz vs GenMon at -10dB
The PCM4222 THD starts to head north out of the noise floor around -20 dBFS which corresponds to about -5 dBu.
Its also noteworthy to point out that if something measures lower than the source it's contributing THD by subtracting distortion from the source.
It's one of those cases where "less equals more."
What I don't know is if the slight rise in THD that I'm seeing is the preamp or converter.
The reality is is that its the sum of everything.
That's why this is a THD "interpretation."
What I confirmed is that the preamp's THD contribution, based on what I've seen, is low enough to justify laying out a board.
"Measurements" are in quotes - it's more like THD interpretation.
The measurement chain is:
Soundcard Out > THAT1646 > -60dB U-Pad > ZTX851 Balanced Preamp > PCM4222EVM (96k/24b) >AudioTester
The DUT gain is 62 dB; PCM4222EVM loading pads the output by just under 0.7dB
Because there is 6 dB gain in the THAT1646, the soundcard output is set 6 dB lower.
Rsource is 10Ω; the front-end op amp is an NE5532.
For the FFTs which show the "Generator Monitor" the Right Channel Flow is Soundcard Out > PCM4222EVM.
There is a 6 dB loss through this path due to the high 560Ω output impedance of the soundcard and the same (about) 560Ω termination of the PCM4222EVM.
When the generator output level is reduced by -6dB along with the DUT channel so that they level match, there is a 12 dB offset in overall level seen at the PCM4222EVM inputs.
The gain offset, though not intentional or ideal, does let us see, at different levels, the overall THD signature of the D/A and A/D.
Though not the best, I use what I have and I'm sure the explanation makes everything about as clear as mud.
Having said all of that here are the actual measurements. The first two are the overall THD of the preamp and measurement system.
ZTX851 Moving Coil Preamp THD 62dB Gain
Note that no effort was made to null the induced hum radiating into the ProtoBoard.
ZTX851 Moving Coil Preamp THD 62dB Gain 5 kHz
The next images show the THD signature of the D/A and A/D loop-through in the Right (red) channel.
ZTX851 Moving Coil Preamp THD 62dB Gain 5 kHz vs GenMon
The THD signature at -10 dB.
ZTX851 Moving Coil Preamp THD 62dB Gain 5 kHz vs GenMon at -10dB
The PCM4222 THD starts to head north out of the noise floor around -20 dBFS which corresponds to about -5 dBu.
Its also noteworthy to point out that if something measures lower than the source it's contributing THD by subtracting distortion from the source.
It's one of those cases where "less equals more."
What I don't know is if the slight rise in THD that I'm seeing is the preamp or converter.
The reality is is that its the sum of everything.
That's why this is a THD "interpretation."
What I confirmed is that the preamp's THD contribution, based on what I've seen, is low enough to justify laying out a board.