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PCM1794A - IV stage example
PCM1794A - IV stage example
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Old 12th February 2018, 03:59 PM   #11
Koifarm is offline Koifarm  Netherlands
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my pcm1794 has 47ohm I/V. That should give a lot of distorsion, but no distorsion at all. Just super sound.
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Old 12th February 2018, 05:18 PM   #12
00940 is offline 00940  Belgium
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Read the table p25. It swings to -7.5v because there is a 6ma negative offset.
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Old 12th February 2018, 07:14 PM   #13
00940 is offline 00940  Belgium
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Originally Posted by Koifarm View Post
my pcm1794 has 47ohm I/V. That should give a lot of distorsion, but no distorsion at all. Just super sound.
Actually, 47r for a passive I/V is ok. You're not loading an opamp and your resistor is small enough to not develop too much voltage at the dac pins. If you were using a larger resistor, the protection diodes inside the pcm1794 might start conducting, creating large distortion.
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Old 12th February 2018, 07:35 PM   #14
diyralf is offline diyralf
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Quote:
Originally Posted by 00940 View Post
Read the table p25. It swings to -7.5v because there is a 6ma negative offset.
Thanks I was not aware of that. Raising the resistor to 1.2k*10.1mAp would be possible and is within 15V minus margin.
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Old 12th February 2018, 10:41 PM   #15
DPH is offline DPH  United States
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I'd personally feel a lot more comfortable with 18V rails on that old of an opamp. Can't remember how well it behaves that close to its supply voltage.

Also, any reason you need this much voltage swing? Especially if you have to subsequently attenuate it, it'd be better to lower the transimpedance gain (remember that low feedback impedances are okay when you're not flowing a ton of current!)
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Old 13th February 2018, 06:57 AM   #16
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I'd personally feel a lot more comfortable with 18V rails on that old of an opamp. Can't remember how well it behaves that close to its supply voltage.

Also, any reason you need this much voltage swing? Especially if you have to subsequently attenuate it, it'd be better to lower the transimpedance gain (remember that low feedback impedances are okay when you're not flowing a ton of current!)
The voltage swing is not that high. You have +-3.9mAp around a constant offset of 6.2mA. The offset is a problem. Eats your valuable headroom.
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Old 13th February 2018, 03:10 PM   #17
DPH is offline DPH  United States
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I have no idea what sorts of impedances you're thinking of for the differential amplifier, but 1.2K is 1.5x the I/V circuit on page 21, which is swinging 4.5 Vrms. Given most amplifier's input sensitivity is closer to 1-2 Vrms, 4.5 Vrms means you're going to waste a ton to subsequent attenuation and pitch a huge amount of SNR right out the door, magnitudes more than you're gaining by "tricks" here. Unless, of course, you know that your downstream electronics need that kind of voltage swing. You do want to optimize your global gain structure, right?

Remember the LT1028 is a pretty hefty beast and *must* be run at low feedback impedances to get the best out of it (also why they have C3/C4 to keep it stable). Realistically speaking, raising the feedback network impedances to minimize loading is backwards. As Ben (00940) wrote -- swinging >12V across a 1.2K load is very likely greater distortion than swinging 8V across a 800 ohm load, or 6V across a 600 ohm load. Pay closer attention to the power demands on the opamp than the feedback network.

I'd be running the differential amplifier as close to unity as I could, and tweaking the I/V transimpedance until I hit the sweet spot of my amps input sensitivity. Maybe a bit higher transimpedance gain and some attenuation in the differential amplifier, as TI shows. But definitely an output voltage swing that matches the input sensitivity of my amplifier.
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Old 13th February 2018, 06:59 PM   #18
diyralf is offline diyralf
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As Ben (00940) wrote -- swinging >12V across a 1.2K load is very likely greater distortion than swinging 8V across a 800 ohm load, or 6V across a 600 ohm load. Pay closer attention to the power demands on the opamp than the feedback network.
Op amps have lower THD with higher voltage swing than with lower. e.g. 5-9V into 1k is no problem at all.
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Old 13th February 2018, 11:20 PM   #19
DPH is offline DPH  United States
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They sit higher above the noise floor, i.e. measurements are THD+N. I just had a quick look at some of the better measured opamps on TI's site (not the NE5532, I'll have to look at Samuel Groner's work for that), which points to me that a 1.2 k swinging 6 or so volts is limited by the noise of the opamp and swinging 3 V across a 600 ohm load is also limited by the noise of the opamp.

I mean, do what you want, but unless you need all that voltage swing on the output at the end, you're throwing any potential benefit you may realize away across a later volume control.
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Old 14th February 2018, 12:01 PM   #20
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We have 2 problems here. 1. The TI chip has a very high output current of -10.1mAp. 2. An OP should not be loaded under 600 ohm. The TI sample circuit is pretty poorly designed in this regard.
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