Passive I/V Vref

[dsavitsk]

When using a passive I/V stage with a TDA1543 and the like there is a resistor from Iout to ground for each channel and a resistor from Vref to ground as well.

I am now trying to make a passive I/V stage for an AD1865N and I am not sure if there is an equivalent pin to Vref, or exactly how to go about this. Can anyone enlighten me on this?

Here's a link to the datasheet: http://rocky.digikey.com/WebLib/Analog Devices/Web Data/AD1865.pdf

Thanks,

-d

 

[dariusku]

As far as & know only TDA1543 hasn't diodes at the output.Other's dacs with current output voltage is limited.For passive i/u converter you can use resistor(about 20 ohm) or transformer and after output buffer with tube or opamp(with resistor you need analog filter). Good example is here http://www.geocities.com/yury_g/dac.htm.

 

[bocka]

Hi dariusku,

unfortunately the link doesn't work...

For passive i/u converter you can use resistor(about 20 ohm) or transformer and after output buffer with tube

Although I don't like transformers in the audio path for a passive I/V stage it has its merits and transforms the voltage as well as the resistance to a (more reasonable) higher level. Instead of using an OP-Amp as buffer it's better to use a single emitter follower

 

[dsavitsk]

The link works, you just have to remove the "." from the end of it (this is a bug in DIYAudio's software that puts it there.) The link, however, is to someone making a balanced DAC with 2 chips, so I am not sure if it is strictly applicable. Also, I am trying to avoid a transformer on the output. Perhaps I will try it for a later incarnation, but for now I am trying to keep things simple, small, and inexpensive as this is just a learning project.

The AD1865's datasheet does not say that there are protection diodes at the current output and they are not there in the block diagrams, though it also doesn't say they aren't there.

If I do just use a resistor, or perhaps use an opamp, then do I need a Zobel filter afterward?

Another thing that I am not at all clear about is the SJ (summing junction) pin. The above linked DAC ignores it. The datasheet example schematic that uses the Ioutput connects it to the Iout pin, and AudioNote's schematic connects it to ground. Any ideas?

Thanks,

-d

 

[dariusku]

& listen to balanced AD1865N-J dac with Lundahl transformer and AD797 buffer and it sounds very good.Transformer have several advantages:separate digital part from analogue,linear with small primary impedance,increase voltage so we need only one gain stage and no need additional filter.But output impedance is big so we need buffer(opamp,follower,tube).With tube output impedance is big too and better use transformer at the output(it increase quality and with opamp).Problem is get good transformer the best are NOS(UTC and others).AD1865 are very precisious so balanced connection is preferred(we get less noise because of no direct ground connection and twice bigger output current).But difference from single dac is small.
Some dacs don't like inductive impedance(like AD1862) but AD1865 works great with transformer.
With resistor we need several gain stage,analogue filter and results are worse.With relatively big resistance(100 ohm) sound is much worse because we get big voltage at the dac output and dac becomes non linear.