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Old 27th April 2013, 09:11 AM   #1
billbo is offline billbo  Australia
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Default TDA 1545a confusion

Hi all. I was looking at the TDA1545a datasheet and I am a little confused in regards to Figure 1 where the DAC is connected to a couple of op amps with no current sinking resistors.
Are these resistors only required when you do not use Op amps? Does the VREF connected to the non inverting inputs of the op amps maintain the sinking current for the DAC'c outputs?
I have seen circuits with 2.2k ohm resistors on the outputs but with no op amps connected. Which would be the best way to connect this DAC?
The data sheet can be looked at:
http://www.datasheetcatalog.org/data...A1545A_T_3.pdf
Does any one have any suggestions on this please.
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Billy D...
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Old 27th April 2013, 09:40 AM   #2
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I don't recommend using opamps with the TDA1545A - its going to sound better with passive I/V. That's what you would have seen with only 2k2s connected. I use TDA1545A a lot but haven't tried with such large resistor values - normally I'm using something an order of magnitude smaller then subsequently have a gain stage, after some passive filtering.

The current sinking is provided via the outputs of the opamps, through the feedback resistors. The Vref connection to IN+ gives the correct offset so the opamp output swings symmetrically between the supplies.
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Old 28th April 2013, 10:25 AM   #3
billbo is offline billbo  Australia
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Default Tda1545a

According to the data sheet a 3.9k ohm resistor is used. Is this enough to sink the right current? Can any one shed more light on this. Can any one recommend an appropriate output stage for the tda1545?
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Old 28th April 2013, 10:55 AM   #4
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3k9 is used with the opamps, not to 'sink the right current' but rather to define the voltage swing at the output of the opamp - 3.9V p-p because the DAC's swing is 1mA.

I'm currently using AD8129 as the output stage with my TDA1545As - I'm using them balanced with a digital inversion to create the negative phase. Between the DAC and the amp there's a steep passive (LC) filter.
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Old 28th April 2013, 11:09 AM   #5
billbo is offline billbo  Australia
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Default Tda1545

Thanks for the insight mate. Can you possibly share some of your circuits so I can can get an idea how this dac functions. I am trying to couple this into a tube preamp as an exersise.
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Old 28th April 2013, 11:44 AM   #6
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I don't have any schematics detailing the function of the DAC itself, that's rather a black-box. I have a thread here though with some example filters that I've designed for my DAC - Digital that sounds like analog - Page 16

In using passive I/V with the TDA1545A its important to respect its output compliance voltage limits. What this means in practice is you can't connect a resistor direct to GND, rather the resistor needs to go to a 2.5V shunt as a reference. If that's a bit tricky to understand you'll be better off using a TDA1387 which is essentially the same (however uses I2S as the interface) but whose outputs can operate happily down to 0V.
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