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PCM1792a Output Current

Other Parts Discussed in Thread: PCM1792A, OPA1662, LME49990, NE5534

Hi there.

I have some questions about the output current of the PCM1792a.
I sense the output current with a resistor. I read, when the dac is stopped, 1,56mV on a 27 Ohm, or 578mV on a 100 Ohm resistor: that means the output current is 5,78mA. Why not 6,2mA as stated on the documentation? I've read many times the data-sheet, but i didn't find any explanation for this strange thing. For example, I see an output pin named "IREF -  Output current reference bias" pin, connected to ground through a 10KOhm resistor in the application circuit schematic. Is there any relationship between the steady-state output current and that resistor? What is, if exists, the relationship between this pin and steady state level of output current?


Other question: if I use a resistor for sensing the output currente, what is the limit value of that resistor after that the quality of the output signal degrades significantly?

Thank you in advance for answer.

  • Hi, Angelo,

    Are you measuring the current in R7 in Figure 36, for example?

    -d2

  • Hi Don.

    No, I'm not measuring the current on R7, I'm not using the circuit of page 36. I put a resistor directly at the output of pcm1792a on pins 17, 18, 25 and 26. This is the way I sense the current (what better I/V circuit of a simple resistor?) and apply the signal to a normal class A amplifier.

    The main line that I follow when designing audio circuits, is never use feed-back, so, no operational amplifier. This is the reason why the second question I put on my post regarding the maximum value of this resistor.

  • Hi, Angelo,

    I apologize, but since you are using the device in a non-standard implementation, we won't be able to provide you with any data as we have not generated it in this configuration.

    -d2

  • Hi Don.

    Sorry, I am very surprised, if you don't want to say frozen, from your answer.

    This means that the circuit only works in the configuration your data-sheet suggest?

    The difference between good dac and common ones is the circuitry attached at their outputs. I want to realize a good dac and I'm requesting only for data not treated within the ordinary data-sheet.

  • Don,

    it is not an issue how I/U convertor is implemented - with simple resistor or with OP, the zero signal current shold be the same.  DAC chip do not know what is connected to its outputs - his task it to provide the output current according to the input data.

    And Angelo asks why it is 5.78mA instead of 6/2A, mentioned in th datasheet.

    I can suppose only one think - what Angelo called "the dac is stopped" was after the power up and after the reset, without I2S signal. According to my measurements, with the "zero data" I2S (for DSD it is not zero, but 0x9696) stream - the output current is exactly 6.2mA, as in the datasheet.

    Ahgelo, max resistor without THD degradation is about 100R, however, some peoples use even 200-300R, I prefer to use not more then 10-20R. The sound is 10^6 better then OP I/V.

    Using datasheet's OP I/V, it is nesessary (at least), to use better OP then NE5534 (LME49990, LME49722, OPA1662, AD8397 - are good).

  • Hi Alto.

    Thank you for your good and polite answer.
    I'm very sorry to have to contradict you, but I've two circuits working and I measure 587-587-572-572 mV on the four outputs of the first and 606-606-610-610 mV on the four outputs of the second one, while they are streaming a "digital silence" signal. The base voltage remains the same, either streaming or in stop mode. "Stop mode" means: Mute, LRCK and BCK running at their proper frequencies, and DATA = 0. A possible reason of that could be the sensing resistors tolerances; we use 1%.
    The sensing resistors value is 100 Ohms, but we are elaborating a circuit employing transistors in the common base configuration, in order to not to have the need of them.

    Anyway, no matter if the zero current is not exactly that expected, the circuits which follow have wide range of working point, so they will adapt their working conditions as needed.

    I agree with you, the results in terms of sound quality are very, very different if you do not use operational amplifiers, even if of the best quality... We obtained very brilliant results this way.

    Regards

    Angelo Adamo