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IOUT DAC capacitive coupling?
IOUT DAC capacitive coupling?
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Old 19th June 2019, 05:40 PM   #11
lcsaszar is offline lcsaszar  Hungary
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Quote:
Originally Posted by DF96 View Post
You would still need to ensure that the offset current exactly matches the mean signal current. If not you get a rising voltage on the output capacitor.
It is possible to design a servo circuit: measure the DC voltage on the I/V resistor (use low pass filtering), and apply negative feedback on the offset compensation current.
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Old 19th June 2019, 08:44 PM   #12
MarcelvdG is offline MarcelvdG  Netherlands
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Just take the reciprocal of a normal AC coupling circuit: not a capacitor followed by a DC bias resistor and then an amplifier with a high-impedance voltage input, but a DC bias resistor followed by a capacitor and then an amplifier with a virtual ground input. Don't ask me what the advantage will be, because for a normal DAC chip, I can't think of any.
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Old 19th June 2019, 09:00 PM   #13
gabdx is online now gabdx  Canada
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you just take a divider of a +/- regulated supply with good rejection ratio, that will nul the DC out, like a servo but without any feedback and no interference. It is very simple!

then you feed the Iout directly to a transistor or in the middle of 2 transistors
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Old 19th June 2019, 11:59 PM   #14
Mark Tillotson is offline Mark Tillotson
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I still don't see any reason to even think of doing this, current outputs are differential, so you don't need to null anything out, the signal is balanced already. Current output DACs work by steering discrete packets of charge to one output or the other, allowing excellent linearity as the number of packets is effectively the signal, and charge cannot be created or destroyed (leakage aside).



You can pass the currents through two resistors, a centre-tapped transformer (commonly done for RF DDS chips using current mode outputs), or a pair of I/V converters. The result is a differential voltage signal (except maybe single-ended for RF transformer). Converting to single ended is then an option, as is DC-blocking (many DACs are inherently DC capable). Seems sensible to only block DC at the point where the desired bandwidth is known, ie the amplifier input.
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Old 20th June 2019, 12:09 AM   #15
Ken Newton is offline Ken Newton  United States
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Originally Posted by gabdx View Post
you just take a divider of a +/- regulated supply with good rejection ratio, that will nul the DC out, like a servo but without any feedback and no interference. It is very simple!

then you feed the Iout directly to a transistor or in the middle of 2 transistors
Just for clarity about what you propose, any DAC which needs to source or sink bias current will then require asymmetric divider resistance values in order to obtain the D.C. null, is that correct?
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Old 20th June 2019, 12:15 AM   #16
Ken Newton is offline Ken Newton  United States
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Originally Posted by Mark Tillotson View Post
I still don't see any reason to even think of doing this, current outputs are differential, so you don't need to null anything out, the signal is balanced already...
Mark, the issue is that some DACs must source or sink bias current through their output pins. For example, T.I.'s PCM1794A requires D.C. paths to source an 6.2mA bias current per output pin.

Last edited by Ken Newton; 20th June 2019 at 12:17 AM.
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Old 20th June 2019, 01:43 AM   #17
gabdx is online now gabdx  Canada
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Originally Posted by Ken Newton View Post
Just for clarity about what you propose, any DAC which needs to source or sink bias current will then require asymmetric divider resistance values in order to obtain the D.C. null, is that correct?
? I did this for tda1541... I have a regulated +/- supply for the circuit. maybe 15 years ago.

So, if i place DC at the input, it screws up the transistors and gives around the same DC at output, my iv is like unity gain for DC.

So, the simple answer is to (explicitly) 1. use 50k trimpot

2. connect trimpot legs 2. to a 7.5k ohm to tda output

connect leg 1 to power supply +
connect leg 3 to power supply -

3. adjust trimpot for 0 dc at the iv output, it is almost 0V at the tda output then, because I used matched transistors

result : a good sounding dac, a nice working IV without need for an horrible sounding DC servo or DC problems.

for tda1541 it is preferable to 'sink' the current like this, my regulated supply is a constant current source and effectively acts as a current ground for the DAC.

However the tda1541 can work up to 100ohm of resistance before THD rises much.

For other dacs, the same thing applies.

you can build a regulated supply with just 10x caps, 4x transistors, 2x LED, 4x 4007 diodes, 8x resistors.

no need to have exotic regulators, like dexa etc, waste of money, they sit in the basement
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Last edited by gabdx; 20th June 2019 at 01:51 AM.
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Old 20th June 2019, 12:13 PM   #18
Ken Newton is offline Ken Newton  United States
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Originally Posted by gabdx View Post

2. connect trimpot legs 2. to a 7.5k ohm to tda output
If I read the above correctly, there is an 7.5k fixed resistor connected between the DAC output and the 50k trimpot wiper? Is that to isolate the DAC ouput from any trimpot parasitics, or perhaps to reduce the sensitivity of trimpot adjustment?

Last edited by Ken Newton; 20th June 2019 at 12:15 PM.
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Old 20th June 2019, 08:13 PM   #19
gabdx is online now gabdx  Canada
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I am not sure, I know nothing about dacs! I understand that it is there for protection of the dac from excessive current draw, reduce parasitics, reduce the sens of the trim, it's there for many reasons... you could place 10k or nothing, it doesn't matter much I think.

I like the fact that there is always a minimum of 7.5k from the dac to the + - 16 V rails.
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Old 20th June 2019, 08:31 PM   #20
Ken Newton is offline Ken Newton  United States
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Originally Posted by gabdx View Post
I am not sure, I know nothing about dacs! I understand that it is there for protection of the dac from excessive current draw, reduce parasitics, reduce the sens of the trim, it's there for many reasons... you could place 10k or nothing, it doesn't matter much I think.

I like the fact that there is always a minimum of 7.5k from the dac to the + - 16 V rails.
Agreed, all of those reasons are valid.
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