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PGA2311 high THD, why?
PGA2311 high THD, why?
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Old 10th November 2017, 09:24 PM   #1
Boscoe is offline Boscoe  United Kingdom
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Default PGA2311 high THD, why?

Hi all,

Would like some suggestions from you guys, if you have time!

I've made a small DAC PCB with an FPGA, PCM1754 DAC and PGA2311 for analogue volume control. Everything works well up to the PGA2311, at the input of the colume control (output of the DAC) I measure 0.005%THD (not bad for PC measurement). But after the PGA I measure 0.05%. The PGA2311U is rated to 0.0004%!

These are the conditions:
- 1.4Vrms Vin
- Gain is set to 0dB
- 1.4Vrms out
- 10k resistive load on each output.
- Supplied with +5VA, -5VA and 5VD from seperate regulators. All supplies work very very with noise below the scopes noise floor.
- AGND and DGND are seperated up until the filtering capacitors at the PSU (star GND) about 4cm away from the IC.
- Try tying AGND and DGND together underneath the IC but no difference.
- PCB is 4 layers with inner 1 and 2 being AGND layers, bottom and top being signal.
- SCLK and SDATA are only running when new data is sent to the PGA.

Here's the schematic.

Some notes/mods.

- All AC coupling capacitors are now 100nF film cap win parallel with 2.2uF WIMA film.
- R8, R11, R18 and R19 are 220k.
- All the caps in the low pass filter are now 470pF film and the resistors changed achieve the correct cutoff.
Attached Files
File Type: pdf 016-5_spdif_simple_DAC_reva_sch pcb.pdf (21.9 KB, 87 views)
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Old 10th November 2017, 10:34 PM   #2
BJosephs is offline BJosephs  United States
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Yikes. That is weird, hope you get it figured out. It sounds like you're doing everything right.

I presume you are checking THD at 1kHz like the datasheet?

Any DC offset at the input so of the PGA that might be reducing your headroom?



Brian
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Old 10th November 2017, 11:42 PM   #3
454Casull is offline 454Casull  Canada
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Output impedance of the DAC section?
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Old 11th November 2017, 01:10 PM   #4
Boscoe is offline Boscoe  United Kingdom
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Quote:
Originally Posted by 454Casull View Post
Output impedance of the DAC section?
Good spot, checked the data sheet and the distortion gets worse with higher source impedance. I removed the 39R resistors and now I'm down to 0.02%.

I'm still thinking of other possible issues.
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Old 11th November 2017, 01:38 PM   #5
mt490 is offline mt490
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How much of it is THD and how much is +N ?
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Old 11th November 2017, 06:11 PM   #6
stratus46 is offline stratus46  United States
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Quote:
Originally Posted by Boscoe View Post
Good spot, checked the data sheet and the distortion gets worse with higher source impedance. I removed the 39R resistors and now I'm down to 0.02%.

I'm still thinking of other possible issues.
I don't buy it. The data sheet says below 600 ohms is recommended. Source impedances up to 2K ohms will cause minimal degradation. 39 ohms is not high impedance in anyone's book. There is something else going on. I suspect you have some oscillation going on that you can't hear but clouds your readings. This would be a good time for a scope. If that isn't a possibility, an attenuator back into the analog input of your PC and some spectrum analyze software might do. The attenuator is to ensure you don't damage or overload the PC input. You might also want to capture with the highest sample rate to get some of the out of band noise.

G
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Old 11th November 2017, 06:38 PM   #7
Boscoe is offline Boscoe  United Kingdom
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Quote:
Originally Posted by stratus46 View Post
I don't buy it. The data sheet says below 600 ohms is recommended. Source impedances up to 2K ohms will cause minimal degradation. 39 ohms is not high impedance in anyone's book. There is something else going on. I suspect you have some oscillation going on that you can't hear but clouds your readings. This would be a good time for a scope. If that isn't a possibility, an attenuator back into the analog input of your PC and some spectrum analyze software might do. The attenuator is to ensure you don't damage or overload the PC input. You might also want to capture with the highest sample rate to get some of the out of band noise.

G
Hmmm, I know what you mean.

I've got a 100MHz scope, I went through the signal path before I did anything else - it's fine. I've been doing my THD readings at 192kHz but the PC only has a 20kHz bandwidth.
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Old 11th November 2017, 09:59 PM   #8
Boscoe is offline Boscoe  United Kingdom
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Quote:
Originally Posted by mt490 View Post
How much of it is THD and how much is +N ?
Very little. The noise is around -110dB (very good).
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Old 12th November 2017, 06:21 AM   #9
Monte McGuire is offline Monte McGuire
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PGA2311 high THD, why?
A 1.4V RMS output will have a peak voltage of 2V, and your power supplies are 5V. That sounds like a wide margin, but the LME49720 is not a rail-to-rail device, so you're getting close to the output swing. In the datasheet, they say that with 2.5V supplies, you get 1% THD+N at around 1V RMS output. By this logic, you should get 1% THD+N at 2V RMS with your 5V supplies, but maybe 1.4V RMS is getting too close?

Try the measurements again with significantly lower signal levels (1V peak or so) and see if that changes things?

The LME49720 is a great amplifier, but for this circuit, it might make more sense to use a different amplifier that can swing larger output voltages. The OPA1612 has a rail-to-rail output, but not a rail to rail input - the common mode input range is still 2V smaller than the supply voltages, or 3V for your circuit, which will be problematic for your noninverting stage. You could use two inverters, chained into each other instead, and then get the full voltage swing from the R-R output. since the opamp inputs will be near 0V

The other idea is to use something greater than 5V for the output amplifier, and use extra regulators to make this into 5V for the PGA2311

But, first, see if this matters at all. Other than that, I can't see why things aren't working...!
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Old 12th November 2017, 06:05 PM   #10
rsavas is offline rsavas  Canada
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Or use the PGA2310(20) with its wider supply range and use the same supply for the opamps as well.

You show a balanced line driver, U7,8, I assume you measure your THD at the output of it, what test equipment are you using to do this balanced line measurement?

Last edited by rsavas; 12th November 2017 at 06:13 PM.
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