# passive I/V directly into transformer ?

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#### Bernhard

without I/V resistor ?
directly Iout --> transformer.

what you think ?

#### jan.didden

Paid Member
Bernhard said:
without I/V resistor ?
directly Iout --> transformer.

what you think ?

Sure, but you will have a reflected impedance from the secondary to the porimary, and that will become the de facto I/V resistance, so to speak. If you leave both prim and sec unterminated, the prim voltage (from the DAC current) will be very large, probably too large to have the DAC work properly.

Jan Didden

#### Bernhard

I once read in another thread that there must be a resistor because the current needs to develop a voltage over a resistor.
A current alone through a transformer would not work.
Seems to be wrong, I tried that and it works.

#### jan.didden

Paid Member
Bernhard said:
I once read in another thread that there must be a resistor because the current needs to develop a voltage over a resistor.
A current alone through a transformer would not work.
Seems to be wrong, I tried that and it works.

But there probably is a resistor secondary, that will be down-transformed toward the primary. And that works. But if you leave the secondary open, I think it will not work (there may be output, but far from optimum quality). Simple electronic engineering.

Jan Didden

#### dddac

Paid Member
You have to look at 2 things here...

one DC bias: Transformer is actually very good, as it reduces the DC off set you will find with a single resitor. This means that if the primary (dac side so to speak) has a low dc resistance the DC bias over the current output from the dac, the bias voltage will be low as well. This is actually what most opamp I/V circuits does. But take care: this will only work when the required (read datasheet) bias offset from the DAC = 0 Volt. When you need a certain DC bias you cannot use the transformer solution, unless you add a DC source in series.

Secondly you need to look at the AC resitance from the transformer. Indeed this depends as Jan wrote from the termination from the secundary. Lets assume at the secundary is a resistor of 10kOhm and the TX ratio is 1:10. This results in a impedance transformation of 1:100 (10x10) giving 100Ohm for a DAC load. Depending on the DAC there are optimal values to not let the AC voltage become to high for the DAC to work properly...
For a PCM63 this is 100Ohm, for a 1541 I think it is 25 Ohm (so load there should not exceed 2500 Ohm...

at my site www.dddac.de you can read about a few examples of transformer dac outputs....

best regards
doede

#### analog_sa

I think it is 25 Ohm (so load there should not exceed 2500 Ohm...

At 25ohm the dc resistance of both primary and secondary may become an issue

#### Bernhard

dddac said:
Depending on the DAC there are optimal values to not let the AC voltage become to high for the DAC to work properly...

And for PCM56 it is 1k and if you use balanced it gets you 2V output