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PCM1794A - IV stage example
PCM1794A - IV stage example
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Old 10th February 2018, 03:40 PM   #1
diyralf is offline diyralf
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Default PCM1794A - IV stage example

http://www.ti.com/lit/ds/symlink/pcm1794a.pdf

In the manaul of the DAC 1794a an example of an IV-stage is shown on page 20. The OPs are loaded with 750 and 270 ohms. Isn't that to low? The distortion increases quite below 1 k.
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Old 10th February 2018, 03:58 PM   #2
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They're trying to keep noise as low as possible.

If you want, you can add a small class A buffer inside the feedback loop of the opamps. It keeps the opamps operating in much better conditions.
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Old 10th February 2018, 04:25 PM   #3
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I'm thinking of raising the resistances in the 1k range. This increases the noise a bit, but in the range of <-100db it does not matter. Low thd is more important.

Last edited by diyralf; 10th February 2018 at 04:27 PM.
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Old 12th February 2018, 05:53 AM   #4
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Keep in mind that you need enough headroom in your supplies for the -10.1mA max IOUT current for the first stage.
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Old 12th February 2018, 07:04 AM   #5
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There is also another thing to consider: it's not certain you get much higher thd at 1vrms into 600r than at 2vrms into 1.2k (as a rough example), depending on the opamps.
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Old 12th February 2018, 01:52 PM   #6
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@Paul: Thanks for the hint. Current is not a problem.
@00940: I am refering to the NE5532 (#1 opamp). 1 or 9 V doesn't matter, problem is the load. All above 600ohm is allowed. The diff between 600 and 1200 is 0.0007% and 0.0005%.
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Old 12th February 2018, 02:52 PM   #7
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Quote:
Originally Posted by diyralf View Post
@Paul: Thanks for the hint. Current is not a problem.
@00940: I am refering to the NE5532 (#1 opamp). 1 or 9 V doesn't matter, problem is the load. All above 600ohm is allowed. The diff between 600 and 1200 is 0.0007% and 0.0005%.
0.0002% is hardly something worthy of worrying.

Current is a problem btw when combined with the I/V resistor. If you push 10mA through a 1K I/V resistor, you already get a 10V signal. If you go even higher, you will clip your opamp. The 750r of the I/V stage is well optimized.

Actually, the first opamp is loaded with 750//560. You could play with the resistors around U3 as we're already at line level but I wouldn't touch too much the I/V resistor.
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Old 12th February 2018, 02:57 PM   #8
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Imax from pcm1794 is 3.9mA. No concern here.

Quote:
Actually, the first opamp is loaded with 750//560
That is far to low.

Last edited by diyralf; 12th February 2018 at 03:01 PM.
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Old 12th February 2018, 03:11 PM   #9
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If I were you, I'd check the datasheet more carefully, p.25.

If the output of U1 swings down to 7.575V with a 750r resistor, what does that tell you ?
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Old 12th February 2018, 03:25 PM   #10
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The output of U1 never swings to 7.5V.
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