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Audio Converters Forum
Output impedance of current sourcing DACs like PCM1795
the issue appeared before, but was never answered. What are the impedance values of the DACs current-outputs, i.e. the PCM1795 and how do they behave over digital-code resp. current-amplitude?
So far I model the DAC outputs with 2k2||22pF in LTSpice. I saw that there´s an IBIS Model of the PCM1795. I had a look at the IBIS-file with an editor, but couldn´t figure out how one could maybe extract the output current behaviour. The outputs seem to be not modelled at all. My idea was to generate a step param List of current values for LTSpice to improve simulation results, because I don´t use OPamps as IVs because of soundquality reasons, but discrete current-conveyor-IVs instead.
We won't release the info you guys are all looking for wrt the output impedance...
As for IBIS models, these are meant for digital interface, and the models only deal with digital pins. Since the outputs are analog, they are not modeled.
Audio Applications Engineering Manager
Dallas, TX USA
A graph of output current vs input code is shown in Figure 75 of the d/s (pg 53):
Yes, the DS gives the current/word relationship and one should and could expect clean straight lines.
I assume that You, Don, are bound to company policies, which I perfectly understand. But I don´t understand, what secrecies could be revealed by an impedance plot or figure? Obviously not every DAC is created equal, but some are more equal. If You look for example at App-note slyt360 and the corresponding DS, the output impedance of the DAC5674 is stated as 300kOhm/5pF. And this is not the only DAC for which this value is given.
I would like to know the ouput Bias Voltage on the terminals which are Iout_+/Iout_- of PCM1795 because the data sheet of PCM1795 does not have detail blockdiagram.
If no input data for PCM1795, What is the DC Bias voltage on the ouput terminals? Zero or 5V?
THanks and Best Regards
It should be mid-rail, 2.5V if running from a 5-V supply.
I don´t think that the offset voltage is at mid-rail. I´d say it depends. ;-)
Beeing a current sourcing DAC with a centre- or Bias-current the output will settle at a voltage level, depending on the impedance the output will see connected from Iout to a more negative potential (usually 0V/gnd or a negative supply voltage). Restrictions in voltage level are due to the voltage compliance limits of the DAC.
If we connect simple resistor from Iout to gnd, the voltage at the Iout-pin will be Vout = -Iout * R
A value of 714Ohms would lead to an output voltage of +2.5V. The 820Ohms in Fig52. "Measurement Circuit for PCM" of the Datasheet would result in +2.87V
The PCM179x family requires low impedance values though, the best beeing a dead short to gnd, otherwise THD rises (keep Vout peak below ~100mV).
So we could only use small valued resistors up to say 50Ohm, before the voltage rises too high and THD gets worse. If we want more voltage we need active circuitry or transformers.
This is where an OPamp comes into play. In Fig.52 the DAC sees the inverting input of the Opamp, a virtual gnd, a low impedance node (well, at least at low frequencies).
Since a Opamp strives to null out any difference in potentials at its inputs by steering its output accordingly, the inverting input takes on the same potential as the noninverting input.
In Fig.52 this is 0V or gnd-potential. Hence the Iout-Pin also settles around 0V. The Opamps output then settles at Iout * R = -2.87V.
The Opamp kind of level-shifts the voltage over the Resistor. The third Opamp connected as difference amplifier, sees the -2.87V at both its inputs and rejects it from its output signal due to its CMRR.
If one connected current sinks towards a negative supply voltage, say simple JFET-ccs, that sink the -3.5mA centre currents, the DAC outputs would also settle at 0V.
Then the outputs of the IV-converters would also settle at 0V and one could omit with the filters and difference amplifier alltogether and could use the two IV-outputs either in a balanced or single ended mode directly. With high-clocked DACs the need for analog post-filtering has become obsolete.
My customer checked the Voltage on the ouput Io+/Io- of PCM1795 and It is +5V.
The customer told me when put the resistor on the output, the DC voltage 2.1 ~ 2.4V with Audio signal decoded.
I think that Digital audio signal is decoded digitally but output I/V Opamp is some problem in terms of application of customer.
What kind of OP amp should we use for I/V converting in PCM1795?
Would you give me the reason why +5V is on the output?
Thanks and Best Regards
if Your customer just measured the DAC output with a multimeter (nothing else connected to the DAC´s output), the DAC would ´see´ the internal impedance of the meter (probabely 10MOhm) for its current path. The voltage then must stick to +5V.
In case of a measurement with resistor , the voltage at idle will be the 3.5mA centre current times resistor value.
2.1V-2.4V suggest a resistor value of probabely 680Ohm. The PCM179x-series of DACs will distort under these conditions.
You need to think in current terms not voltage terms with this kind of DAC.
The datasheet of the PCM is freely downloadable. You can find suggestions for IV-converters and OPamps within.
Thank you very much for giving me the clear answer.
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