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? on opamp I-to-V converter schematic

The schematic at the link below is from analog's AD1955 DAC reference schematic with IV stage using opamps.

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What is the purpose of R80 and R81 and also R60 and C42 in the feedback path? Appreciate if anyone explains their working and how to calculate those values.

Thanks in advance Ma.jnk

Reply to
Mahen K
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R80, 81 are current source for the D/A.

R60 and C42 make a low pass filter.

tm

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Reply to
tm

Thanks tm. Could you please elaborate little bit on "R80, 81 are current source for the D/A"? Are they not part of I-V conversion but part of DAC current output?

TIA Ma.jnk

Reply to
Mahen K

From the data sheet:

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The AD1955 audio outputs sink a current proportional to the

input signal, superimposed on a steady bias current. The current-

to-voltage (I/V) converters used need to be able to supply

this bias current, as well as the signal current, or a resistor or

current source can be used to a positive voltage to null this

current in order to center the range of the I/V converters.

If pull-up resistors are used to bring the output of the I/V converters

to 0 V for maximum headroom and THD balance, as shown

in the applications circuits, the following equation can be used:

HTH

tm

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Reply to
tm

R80 and R81 are as "tm" stated -- note too that they add some compliance to the current outputs, so the "IOUT+" and "IOUT-" aren't perfect current sources (if they were, it sounds like they wouldn't work with the DAC).

R60 and C42 are either there to make the circuit low-pass from input to output, or they are there to stabilize the amplifier circuits against the loading to the loop imposed by C16*, or both.

  • In fact, I'm a bit surprised that the circuit is stable at all -- normally any capacitance on the V- input to an op-amp is death on stability. I can only assume that because C16 is connected between two like points on the two op-amps that it's effect gets washed out, but I can't imagine it's a terribly good thing. You certainly wouldn't want to put anything over a few pF at C15 and C17.
--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

Thanks a lot for all the responses. Its pretty clear now.

Ma.jnk

Reply to
mr

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