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My feeble attempt at a discrete I/V stage - Page 2

post #16 of 34
of course I was wrong, both inputs works together, but this is clearly desirable, if they worked against each other, the output would stay at zero volts regardless the differential current input

well, it's really late ya know, kinda sleepy already

on the other hand, why bother with inverting/noninverting, if you have balanced input, all you have to do is switch between them to reverse the polarity.. inverting nature of such circuit would only be important for unbalanced inputs, but that would be easily solved in digital domain anyway..
post #17 of 34
Thread Starter 
If you only put current into one input of your circuit, the output would go down. The idea of my circuit is basically to create a current input op-amp, so that one can take a DAC with single ended or balanced current output and create a simple non-inverting voltage output by feeding back the voltage output to the negative input with a resistor of the desired transimpedence.
post #18 of 34
actually this is all wrong then - if injecting current into both PAD1 & PAD2 developes voltage drop on R11, then balanced input will keep output at zero level.. oh my, time to bed I guess

no, this is wrong and I was right actually injecting the same current into both won't disballance the sources/sinks, only differential input will do and thats right.. uf, and now for the bed before I make another mistakes

final thoughts: one of the inputs increases sinking, while the other increases sourcing, these two stated will develop oposite voltage drop accross R11 so they are balanced after all!
post #19 of 34
Thread Starter 
Quote:
Originally Posted by Glassman
final thoughts: one of the inputs increases sinking, while the other increases sourcing
This is not correct.

That aside, if you have an inverting output stage on a standard current output DAC, will the output be 180 degrees out of phase with the intended output?
post #20 of 34
geeze I'm gonna sleep, hopefully I'll wake up smarter
post #21 of 34
ok, I apologize for yesterday's lack of sense, I've reasured myself yet another time that trying to think or work late night is only asking for troubles..

yes you are right, this can't be made nicely complementary, this is all I can do with it to gain differential input.. not as cute, but probably not as bad either:

post #22 of 34
Thread Starter 
Heh, yeah it didn't even occur to me you are a good 6 or 7 timezones ahead of me!

Yes, that design definitely looks like the most compact solution you could get.

My only remaining question then pertains to whether or not a standard inverting output stage on a standard DAC will result in an inverted or uninverted output. I had always assumed the DAC would output positive current for a positive signal, but I guess it makes more sense for the output to be inverted since all of the active output stages I have seen are inverting.

Anyone have a verdict on this?
post #23 of 34
yes DAC's outputs are not inverted, usually there is two stage analog stage used, both inverting and the result is non inverted..

in the design above, it would be also possible to add another output with opposite phase to gain balanced input - balanced output system.. there could be two feedback paths then, from PAD3 to PAD2 and then from the inverted output, say PAD4 to PAD1.. I wonder how would such a double-feedback behave
post #24 of 34
ok, here is fully cascoded version, feedback included:



VR1 is precision 2.5V reference, RIV is the I/V resistor, RFB is the feedback resistor.. noninverting current input on the left, inverting on the right.. the feedback would in fact be taken from the output of a unity gain buffer connected to V out, because V out is a high impedance node..
post #25 of 34
Pretty Cool Glasdude ( ) now you gotta build and report ! not getting off that easy man........

BTW-the Hawksford Papers are serious resource.One that can give a person a headache trying to cram everything but invaluable.Very cool that someone took the time to gather everything into one place
post #26 of 34
Quote:
Originally Posted by rickcr42
Pretty Cool Glasdude ( ) now you gotta build and report ! not getting off that easy man........

BTW-the Hawksford Papers are serious resource.Oone that can give a person a headache trying to cram everything but invaluable.Very cool that someone took the time to gather everything into one place
Yes at least one person is going to try this out should be just the trick for a AD1955
post #27 of 34
Thread Starter 
Argh I didn't get an email when people replied to the thread! I'll likely build this in the next couple weeks when my Monica 2 DAC parts (FINALLY) arrive. I would, of couse, point out that my hearing is not so hot, so perhaps I am not the best judge of sonic quality...
post #28 of 34
Thread Starter 
Well my Monica 2 finally arrived, so I am going to do some final tweaking on this design and get to buying parts! I'll send a shout out when I finish it!
post #29 of 34
Hi Glassman and others.

My question is why you chose to omit the capacitor in parallel with the LED from Hawksford's paper? It seems handy given the high frequency nature of the input.

Aside from that I'd like to congratulate you on a nice practical design. I'm using it in my latest DAC.
post #30 of 34
Thread Starter 
No reason. We focused more on the overall design, and not so much on the implementation. Caps across the cascoding LEDs would probably help with distortion at high frequencies, and putting a cap between the current mirror's bases and their respective power rails would probably help prevent oscillation.

I recommend you take your queues from Malcolm's paper about where to add capacitors, and of course determine WHY the capacitor is there and what the impedences are in the surrounding circuit so you can choose values appropriately. I never did build this thing ("life" stepped in and has kept me busy since :-/ ) so I would be curious to hear how to do.

'soda
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