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IOUT DAC capacitive coupling?
IOUT DAC capacitive coupling?
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Old 30th May 2019, 11:20 AM   #1
dreamth is offline dreamth  Romania
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Default IOUT DAC capacitive coupling?

Did anybody try to use capacitive coupling between IOUT of a DAC and the I/V stage?
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Old 30th May 2019, 12:01 PM   #2
Mark Tillotson is offline Mark Tillotson
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A current output DAC totally relies on the output current flowing to the appropriate rail, with limited voltage compliance - it cannot function without that current path.
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Old 30th May 2019, 12:12 PM   #3
dreamth is offline dreamth  Romania
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You mean that it needs a dc current path?
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Old 30th May 2019, 12:13 PM   #4
Ken Newton is offline Ken Newton  United States
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Assuming that I understand what you're asking, you're thinking that the capacitor would be there to block any DAC current output pin D.C. flow? The problem with that is the blocking capacitors then would be subject to some net D.C. charging current. Which in turn would offset ramp the current output pins to their maximum compliance voltage. Severe distortion of the signal current would result.

If there is no offset current from the DAC's current output pins, then the blocking capacitor would not be needed. However, DAC offset currents are commonly zero'd out via application of an D.C. correction current, or are first converted to voltage before being addressed post the l/V circuit.

Last edited by Ken Newton; 30th May 2019 at 12:28 PM.
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Old 10th June 2019, 01:25 AM   #5
Trident is offline Trident  Spain
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I do not feel comfortable, separating my ears from the high voltage by a capacitor ... I feel more comfortable putting a transformer through.

Something to meditate, I know nothing has happened, but you would leave everything to the reliability of a capacitor, I think not .... in my case.

¿More expensive transformer than a capacitor?: YES

¿Safer? ¡¡¡¡¡¡¡¡¡yeeeeeeeeeeessssssssssss¡¡¡¡¡¡¡¡¡¡ .... Without a doubt

Call me paranoid.

Last edited by Trident; 10th June 2019 at 01:45 AM.
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Old 10th June 2019, 02:04 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by Trident View Post
I do not feel comfortable, separating my ears from the high voltage by
a capacitor .. you would leave everything to the reliability of a capacitor,
I think not .... in my case.
There are film capacitors that are actually two capacitors in series.
Or, you could use two discrete capacitors in series. Transformers can
short primary to secondary, especially wide bandwidth types.

Last edited by rayma; 10th June 2019 at 02:07 AM.
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Old 10th June 2019, 05:02 PM   #7
DF96 is offline DF96  England
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The current output pin of a DAC chip is unlikely to have dangerously high voltages.

In many cases there will be a DC offset current, which means that a capacitor cannot be used.
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Old 10th June 2019, 07:32 PM   #8
lcsaszar is offline lcsaszar  Hungary
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I am just thinking loudly: let's take a current out DAC with, say, -2 mA (sink) idle current. Now you compensate this idle current by injecting 2 mA DC into the output. The AC current output will be -2 mA to +2 mA. You have an I/V resistor 100 ohms, then you get an AC voltage 400 mVpp. The question is why you would want to AC couple between the DAC output and the I/V resistor? OK, if you want to do so, you need some 33 uF capacitor. The issue is that the reactance of this capacitor will increase with falling frequency, and it adds to the I/V resistor. At a certain point you will exceed the output voltage compliance of the DAC.
With transformer coupling you don't have this issue.
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Old 15th June 2019, 07:29 PM   #9
DF96 is offline DF96  England
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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.
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Old 15th June 2019, 09:01 PM   #10
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by lcsaszar View Post
I am just thinking loudly: let's take a current out DAC with, say, -2 mA (sink) idle current. Now you compensate this idle current by injecting 2 mA DC into the output.
Injecting a correction current, while effective in nulling any current offset, then obviates the need for D.C. blocking in the first place.

Last edited by Ken Newton; 15th June 2019 at 09:05 PM.
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