Passive I/V Help

[dsavitsk]

I am looking to use a passive I/V with a TDA1541a and then an opamp for gain. The top version on the attached schematic is what I was thinking of doing, the bottom version is stolen from hagtech (sorry Jim) and uses this concept but with a bit of filter. Jim's original, however, is for a PCM1704 which may have different requirements form the TDA1541A.

So, the questions:
- do I need the filter (is it a good idea?) and if so how do I figure out the component values,

- do I need to AC couple the DAC to the opamp (i.e., is this a difference between the philips and BB chips) since there is offset, and

- is there anything else I should do here?

Thoughts?

-d

[andrei]

I've posted formulas for I/V passive filter values here.

[dsavitsk]

Thanks, that is helpful.

I am wondering about a few other things, however. My primary question is about whether I can direct couple to the opamp, or if I need to block the offset from getting to the opamp. It seems that it should be low enough that I can safely ignore it and do the blocking on the output, but I feel like I should get some confirmation on this. Second, with tda1543 based DACs I never worry about the filter. Is it more necessary here, or can it be safely ignored? Last, Jim uses a resistor in series with the opamp's input. Is this necessary?

[andrei]

From Phillips TDA1541A datasheet, Iout from pins 6,25 is refered to ground, and zero scale current is < 50nA. So it looks like you don't have to worry about offset.

[tmblack]

You will have 2mA flowing thru the passive I/V resistor when there is no signal.
So you only need a coupling capacitor after the opamp.
A 3rd order LPF is often used when oversampled.

Haven't tried the TDA1543 yet.
Anyone have some to spare?

Tom

EE

[dariusku]

Few days ago & tweaked Onkyo dx-6720.It has ad1860 dac(output current 1mA).& used 100 ohm Allen Bradley resistor for i/u and parralel 18nF Wima FKP.After that AD825 (non inverting 100 ohm and 2k).Max output 2V, 1,8 V at 20kHz.No any hum even without capacitor.Connected to battery powered gainclone and loudspeakers with Seas .Every filter only reduces quality.At the output about 5 mV so no need decoupling capacitor too.You only need to use good resistor for i/u conversion and try different values.With small values(10-40 ohm) less distortion and mainly second(more analogue sound) with bigger more dynamics and opamp works better.
Tda1543 & use without any filter.

[dsavitsk]

Another idea, assuming that I do want to strip off high frequency noise, would a cap across the opamp's feedback resistor be an okay way to do this, as opposed to the cap, resistor, inductor method above?