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PCM1794 vs PCM1793 balanced output?
PCM1794 vs PCM1793 balanced output?
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Old 12th January 2020, 12:25 PM   #1
abza is offline abza  South Africa
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Default PCM1794 vs PCM1793 balanced output?

I want to use a PCM179X DAC to output a line-level analog signal to a pair of balanced XLR connectors. The PCM1794 provides differential current output, while the PCM1793 provides voltage output.


However, I've only seen examples and schematics where the outputs are summed into single ended. I'd like to keep the outputs balanced.



1. If I understand correctly, in the case of the 1794 I'd first have to convert the current differential output into voltage. However, on the datasheet it shows the use of opamps for this purpose, which also convert the differential output to single ended. I want to retain the the positive and negative signals and route them to the XLR connectors - how do I go about I/V conversion in this case, without converting to single ended? (Figure 1).


Figure 1:
Click the image to open in full size.

2. In the case of the 1793, I assume this step is completely unnecessary, and I can just wire it like below? (Figure 2). If so, what is the disadvantage of using the 1793 over the 1794?


Figure 2:
Click the image to open in full size.

Thanks!
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Old 13th January 2020, 04:49 AM   #2
PaulFrost is offline PaulFrost  United States
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Hi Abza,

The output of the I/V stage (also called a trans-impedance amplifier, or TIA) is technically differential voltage. The problem is that given the nature of the current output bias, the output of these amplifiers is negative. That is to say that the common mode output of the two amplifiers is negative. You could leave them like this and just use a DC blocking cap or you could use some other kind of final amplifier stage, such as a fully-differential amplifier, or two single ended but one is inverted.
Note that TI recommends an amplifier on the output of the PCM1793 because the minimum load for the internal amplifier is not that much, and that a out-of-band filter is still recommended.

A TI DAC like the PCM5242 has a differential output, reasonable load capability, and only needs a simple RC filter.
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Old 13th January 2020, 04:12 PM   #3
abza is offline abza  South Africa
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Thanks so much @PaulFrost! I really appreciate it.


Yes, I suspected the output may need pre-amplification, thanks for confirming my suspicions.

Okay, so clearly fewer hoops to jump through with the PCM1793.

Would I be on the right track using something like the below topology for the PCM1794A? My idea is to follow the I/V stage with a non-inverting opamp amplifier (plus high and low pass filters):



Click the image to open in full size.
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Old 13th January 2020, 04:33 PM   #4
PaulFrost is offline PaulFrost  United States
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That would work, but you still need to remove the DC offset from the TIA stage. Remember that the output of that stage would be negative, so adding a DC blocking cap between the two stages would be a good idea. It does not really matter if the final stage is non-inverting or inverting, though in my experience it is easier to achieve a lower distortion output with an inverting amplifier configuration.
http://www.ti.com/lit/an/slyt595/slyt595.pdf

Here is a little article by John Caldwell talking about common-mode distortion. The benefit of an inverting amp in the final stage is that the common-mode will not be changing.
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Old 14th January 2020, 07:40 AM   #5
abza is offline abza  South Africa
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Okay, I see - and thanks for the link! I'll definitely tuck into that PDF with gusto.


Right, so the idea is to place series capacitors between the I/V stages and the filters/amplifiers. Any clues or pointers as to how I would calculate (or thumbsuck!) the values of the DC blocking caps?
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Old 14th January 2020, 10:57 AM   #6
sesebe is offline sesebe  Romania
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In order to take full advantage of the benefits of the differential connection, the amplification stages and the output filters must also be in differential configuration.
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Old 14th January 2020, 12:57 PM   #7
Ken Newton is offline Ken Newton  United States
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I would place the D.C. blocking cap. that PaulFrost correctly points out as needed in-between the RC low-pass filter and the non-inverting amplifier. Then add a shunt resistor from the non-inverting input pin to signal ground. The optimum value for the added resistor depends on the bias current needs of that op-amp's input stage, unless it's a FET input stage op-amp, however, the exact value usually is not critical. The higher the value of the shunt resistor, the greater will be the offset voltage it will produce at the output of that op-amp. The value of the blocking cap. will then depend on the chosen value of the of the shunt resistor, as the two define a high-pass pole. A rough rule-of-thumb I use is below. What typically works acceptably for op-amp coupling is, 1uF : 100K. However, that is not an exact requirement of values. Since this is audio, it is not critical at the deep bass frequencies that are affected.

100nF : 1M
1uF : 100K
10uF : 10K

Since the following component box must feature an balanced input, it should perform the common-mode noise rejection function. You shouldn't need any differential amplifiers within the DAC box to reject common-mode noise from transferring across a balanced signal interface.

Last edited by Ken Newton; 14th January 2020 at 01:25 PM.
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Old 15th January 2020, 01:16 AM   #8
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by abza View Post
Any clues or pointers as to how I would calculate (or thumbsuck!) the values of the DC blocking caps?

3dB roll-off point is when f = 1 / (2 pi R C)
R is the load resistance the capacitor sees.
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Old 15th January 2020, 07:36 AM   #9
abza is offline abza  South Africa
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Thanks everyone, and cheers @Ken Newton - this is extremely helpful.


Here's what I have. I'll likely be changing the preamplifier section from non-inverting to inverting based on @PaulFrost's feedback, but does this block diagram look a little more complete?


Click the image to open in full size.
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Old 15th January 2020, 02:58 PM   #10
Ken Newton is offline Ken Newton  United States
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Yes, that basic topology looks fine. However, your circuit inverts phase, not necessarily a crime. To correct that, however, route the signals so that waveform absolute phase remains non-inverted out of the XLR connectors. The easiest ways to do this is by either swapping the wires going to the XLR pins, or by routing the DAC's negative current outputs to drive the positive XLR pins and the positive current output pins to drive the negative XLR pins, thereby rectifying the inversion.

As an seperate experiment, an possible alteration which I find to provide excellent sonics is to replace the active I/V transimpedance stage with a fully passive one. By that I mean, with a resistor connected in shunt between each of the DAC's current output pins and signal ground. Also, place a capacitor in parallel with each resistor to form the initial first order low-pass filter pole. To compensate for the gain lost from not having an active I/V stage, you can increase the gain of that final output op-amp stage. I use 75R in parallel with about 30nF in my own experimental DIY PCM1794A DAC. A passive resistor I/V such as this will produce higher measured THD than will an 'proper' op-amp active I/V, but the dynamic behavior is usually superior in exchange. As for the sound, your ears would have to judge which sounds better to you.

As I recall, Audio Research utlized the PCM1794A with 200R passive I/V resistors in one of their tube DACs. Although, I don't like the sound of the PCM1794A with passive I/V resistors that high. I find the sound improves (becomes slightly more focused sounding, likely due the the slight commensurate drop in THD) slightly as you lower the I/V resistor, until down around 20-35 Ohms, after which, I could not hear any improvement. Remember, as you lower the I/V resistor value you must increase the gain of one of the active blocks to compensate if you wish to maintain signal level.

Last edited by Ken Newton; 15th January 2020 at 03:24 PM.
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