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jwb 17th May 2002 07:41 PM

Drawing of I/V converter
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I'm a total newbie at such things, but I thought this would make a fine I/V converter for my new DAC. The DAC output is fed into a virtual ground between T1 and T2, trimmed to 0.0V. The current source T1 draws 10mA and V+ and V- sit at 6.75V steady state. The DAC output swings +/- 1.2mA giving a voltage swing of 2Vpp at the voltage terminal. R11 is the volume control.

The voltage swing could be brought up to 4Vpp by doubling R1/R6 and using a 5mA steady state current.

A few weeks ago I thought I might like to try the I/V stage from the Pass D-1, but it has too many parts (and MOSFETs) for my liking. I believe this design retains the advantages of the D-1 design without the overkill IRF610 MOSFETs.

Please post your scathing criticisms here. I already ordered the parts for it so I'll get to see it it works soon enough.

tiroth 17th May 2002 08:17 PM

I think your design has more parts.

jam 17th May 2002 08:26 PM

How about biasing the transistors with diodes or zeners and current sources.


Jocko Homo 17th May 2002 08:29 PM

I would bias the current sources a different way and use decent transistors. What is the 50 K resistor for? Ignore the guy from Pittsburgh. [joke]


jam 17th May 2002 08:51 PM

I suppose the 50K resistor is for DC balance?


jwb 17th May 2002 08:57 PM

I searched Google for "decent transistor" and didn't come up with anything. Maybe you can suggest something with the lofty specs of Ic > .0112mA and Vce > 10V? I just picked a decent one from a 2Nxxxx spec table.

How about something like Zetex 869? Geez $67 for 100! Okay okay, it's a lot less than the 4xPCM1704U-K :)

As for biasing I could bias with diodes or what-have-you but I calculate the current source should be within %0.1 with 1% resistors. What's the advantage?

jwb 17th May 2002 08:59 PM

The 50K R11 is the attenuator.

Jocko Homo 17th May 2002 09:05 PM

LEDs make good references. Cheap and quiet.

Yeah, Harry would be proud that you looked on Google. He did a list somewhere on this forum. Do a search for "new law" (one of my more notorius threads) with HarryHaller. It is there somewhere. Stop using that simulator and just build it. It will work.

Those '1704s are getting pricey. The ungraded ones aren't that hot.


jwb 17th May 2002 09:13 PM

What are you kidding? Simulate?! Do they even make those for Linux? I'm just glad I finally found something for drawing schematics.

Of course, it was the limitations of this "evaluation software" that inspired me to implement the entire digital receiver + dac section on a 3"x4" PCB!

Anyway the I/V is getting breadboarded first so I'll try resistors and LEDs and see what I like.

Thanks for the comments folks, I was fully expecting something along the lines of "That won't work because of X, fool."

Jocko Homo 17th May 2002 10:04 PM

That's plenty of room. Unless you like to build large power


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