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PCM1794A - IV stage example
PCM1794A - IV stage example
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Old 14th February 2018, 01:13 PM   #21
DPH is offline DPH  United States
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Yes, please do show that the sfdr has completely fallen apart...
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Old 15th March 2018, 11:11 AM   #22
zenelectro is offline zenelectro  Australia
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Quote:
Originally Posted by 00940 View Post
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.
Ironically the 5534 appears to be the major noise contributor.

I sat down one night and calculated all the noise contributions for the dual
mono circuit in data sheet. It's quite complex because there are many noise
sources all adding RMS however I came to a figure that showed the 132dB DR
in mono mode was actually (slightly) I-V dominated.

Having said that, no one is going to be using 9V RMS out so in real world
use, getting 132dB DR will not happen.
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Old 15th March 2018, 11:15 AM   #23
zenelectro is offline zenelectro  Australia
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Quote:
Originally Posted by diyralf View Post
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.
Under 600 is fine, it just depends on V swing. +-4mA / phase is within most
opamps capability and easily driven by the better ones.
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Old 15th March 2018, 11:34 AM   #24
zenelectro is offline zenelectro  Australia
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Quote:
Originally Posted by diyralf View Post
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.
If you raise the I-V opamps FB resistor to 1k you will actually get slightly less noise.
The reason: For double the resistance value, noise multiplies by 1.414 but there will be 2 x OP signal so SN will be slightly better all other things being equal.

The distortion will be dependent on a few things. a/ opamps current load so same in all cases but with greater OP swing, given most of the distortion will be OP stage then overall should be slightly lower with more swing.
b/ opamp gain - this will be determined by DAC OP Z versus FB R. The 1794 is almost perfect current source so OP Z is very high resulting in I-V being close to unity gain. Also the I-V has no common mode induced distortion (see Samuel Groners opamp measurement papers.

What this all amounts to is that the 1794 has a good mix of parameters for very low distortion I-V with good dynamic range.
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Old 15th March 2018, 03:13 PM   #25
cdsgames is offline cdsgames  Canada
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Quote:
Originally Posted by diyralf View Post
Op amps have lower THD with higher voltage swing than with lower. e.g. 5-9V into 1k is no problem at all.
So THD will improve if voltage swing is 3Vrms vs 4.5Vrms ?

I have an active stage and I am trying to fine tune it. My feedback R is 1.5K (IV stage) and I have 2.8ma. Would I have any benefit to lower the voltage swing by 30% ?

Also on the summing , I have a Rin of 1k. Is that a bit high ?

Last edited by cdsgames; 15th March 2018 at 03:16 PM.
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Old 15th March 2018, 03:26 PM   #26
Ken Newton is offline Ken Newton  United States
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Quote:
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.
I also find that passive I/V works very well the PCM1794A. It's super transparent sounding. I've measured 0.002% THD at the output pins of my PCM1794A DAC utilizing 75 ohm I/V resistors. In addition, Audio Research utilized 200 ohm passive I/V resistors in one of their recent PCM1794A based commercial DACs. Also, the popular DIY DDDAC1794A project utilizes even larger value passive I/V resistors. So, there doesn't appear to be protection diodes on the outputs of the PCM1794A. The THD does increase as the resistor value is increased.

Search this site for the excellent posts by contributor [smms73], who has performed extensive spectrum analyzer measurements of the PCM1794A driving various I/V circuits, both in single-ended mode and in differential.

The problem with active virtual ground transimpedance amplifiers is that DAC chips feed them a wideband, fast slewing, fast settling signal current which is difficult to track without dynamic errors. In addition, the full amplitude of the DAC chip's output current must simultaneously be handled by the amplifier. This isn't simply an issue of whether the amplifier can output the required current at D.C., or at audio frequencies. It's an issue of the amplifier outputting the required current at high the frequencies of a fast slewing DAC output.

So, with the PCM1794A, an op-amp transimpedance stage must swing up to +/-3.9mA while attempting to accurately track the fast slewing output signal. It makes no difference to the current swing demanded whether the output voltage amplitude is set to a low level. The current which must be processed/handled is always the same as is output by the DAC chip. Such a signal places maximum dynamic demand upon an feedback based transimpedance amplifier.

How does passive I/V help? For some DAC designs, passive I/V is utilized to develop a full-scale amplitude signal, eliminating the need for any output amplifier - along with it's potential dynamic errors. For some other designs, passive I/V is utilized to develop a relatively low amplitude signal voltage, which is then amplified by a zero feedback voltage amplifier, either solid-state or vacuum tube. Passive I/V can even help lower the dynamic errors of feedback based amplifiers, by enabling use of standard inverting, or non-inverting voltage gain stages, thereby significantly lowering the magnitude of the current swings which must otherwise be handled by the output driver of an virtual-ground transimpedance amplifier.
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Old 15th March 2018, 03:38 PM   #27
00940 is online now 00940  Belgium
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Quote:
Originally Posted by cdsgames View Post
So THD will improve if voltage swing is 3Vrms vs 4.5Vrms ?
diyralf was saying the contrary (lower thd with higher voltage) but I suspect he meant THD+N and not simply THD.

THD+N improves with higher voltage swing, not THD by itself. If you look at graphs of THD+N vs output voltage (at a constant load), it goes down first because the noise component improves at higher voltages. Then it usually starts to flatten because the distortion component rises as the opamp is asked to provide more power. And it finally rises when the output nears the PS rails and clipping finally sends THD trough the roof.

Quote:
Originally Posted by Ken Newton
I've measured 0.002% THD at the output pins of my PCM1794A DAC utilizing 75 ohm I/V
Which is good but not great for a pcm1794 (-94db). smms73 suggested keeping a passive I/V resistor under 50ohms (he suggested 22R) to achieve closer to 0.0002% thd and repeatedly said that 200R was a bad idea.
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Last edited by 00940; 15th March 2018 at 03:40 PM.
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Old 15th March 2018, 03:54 PM   #28
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by 00940 View Post
Which is good but not great for a pcm1794 (-94db). smms73 suggested keeping a passive I/V resistor under 50ohms (he suggested 22R) to achieve closer to 0.0002% thd and repeatedly said that 200R was a bad idea.
I wasn't suggesting that 200 ohms is good, but merely pointing out that a well respected DAC vendor utilizes that value. As I stated, lower is better for the resistor value. Never the less, -94dB is an inaudible level of THD. As I had offered, THD shouldn't simply be the main concern with I/V circuits, but rather the overall dynamic behavior.

Last edited by Ken Newton; 15th March 2018 at 03:59 PM.
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Old 15th March 2018, 04:10 PM   #29
diyralf is offline diyralf
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The point is that an OP can not drive 200 ohms or any other low value. TI probably did not care about this fact in the demo application. Anything below 600 ohms will cause the THD to explode.
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Old 15th March 2018, 04:26 PM   #30
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by diyralf View Post
The point is that an OP can not drive 200 ohms or any other low value. TI probably did not care about this fact in the demo application. Anything below 600 ohms will cause the THD to explode.
Hi, ralf. I'm uncertain of whom your comment was addressed. If it was to me, the I/V resistor values I've been mentioning are for a totally passive implementation. The I/V resistor is connected between the DAC chip's current output pin and ground, but is not driven in conjunction with an active stage.
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