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Ground lifting dac-i/v
Ground lifting dac-i/v
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Old 19th February 2020, 06:59 PM   #1
dreamth is offline dreamth  Romania
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Default Ground lifting dac-i/v

I have question for anyone willing to help:
I need to end the Iout of a DAC into roughly a 1.8v potential caused by 3 BE junctions as part of the input of an I/V converter .Would a ground lifting done with 3 diodes (or a green LED or just other 3 BE junctions paralleled with some capacitors work?
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Old 19th February 2020, 08:09 PM   #2
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Old 20th February 2020, 12:54 AM   #3
kevinkr is offline kevinkr  United States
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Ground lifting dac-i/v
You need to revisit that circuit, you don't have an I/V converter at all, and the diodes short the output of the dac through their dynamic resistance directly to ground.

Connect I/V converter to inverting input of opamp, add resistor between inverting input and output to set the I/V ratio.

Connect your 1.8V reference directly to the op-amp non-inverting input.

I don't understand why you have diodes in series with the gnd pin of the DAC chip, this shifts a whole lot of things inside the chip away from ground and adds the dynamic resistance in series with all the of dac current.

Perhaps you tell us what you are trying to do?
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Old 20th February 2020, 01:14 AM   #4
abraxalito is offline abraxalito  United Kingdom
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Interesting indeed that I myself, for the first time built a 'ground lifter' for my DAC only a couple of days ago, so I can see the intent even though the circuit isn't drawn up correctly. You need to get the output voltage of the DAC to be both within its allowed compliance range AND compatible with the I/V opamp, which means roughly mid-rail. I used a TL431 to give a 2.5V lift to my DAC, rather than diodes.

The drawback is we now need level shifters to put the DAC's digital input signals into the right range. Fortunately that's not very hard to do - I use current sources and voltage references (TL431s).
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Old 20th February 2020, 02:16 AM   #5
dreamth is offline dreamth  Romania
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Quote:
Originally Posted by kevinkr View Post
You need to revisit that circuit, you don't have an I/V converter at all, ...................
I'm sorry for the sloppy figuration of a different circuit actually! I'm still working on it. I don't have a conventional virtual ground circuit...

Last edited by dreamth; 20th February 2020 at 02:29 AM.
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Old 20th February 2020, 02:34 AM   #6
dreamth is offline dreamth  Romania
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Quote:
Originally Posted by abraxalito View Post
Interesting indeed that I myself, for the first time built a 'ground lifter' for my DAC only a couple of days ago, so I can see the intent even though the circuit isn't drawn up correctly. You need to get the output voltage of the DAC to be both within its allowed compliance range AND compatible with the I/V opamp, which means roughly mid-rail. I used a TL431 to give a 2.5V lift to my DAC, rather than diodes.

The drawback is we now need level shifters to put the DAC's digital input signals into the right range. Fortunately that's not very hard to do - I use current sources and voltage references (TL431s).
Thank'you! I'm using completely different ground planes coming from different power supplies .I'll be using PCM1798 or 1794 as dac so i'll need to use its Vcom or I ref...I don't really know how i'll be using them .I just found out that i also need variable ground lifting too ...

Last edited by dreamth; 20th February 2020 at 02:38 AM.
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Old 20th February 2020, 02:46 AM   #7
abraxalito is offline abraxalito  United Kingdom
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In a typical case, the opamp is going to be fed from balanced (+/-) supplies and the DAC will be referenced to 0V so this issue doesn't arise. In my case though, as I'm using a single supply, the 0V is where the -ve supply would normally be and then to avoid using a coupling cap into the I/V we become limited in the supply the opamp can run from in order to keep the mid-rail voltage within the DAC's compliance range. Raising the DAC's 0V gives much more flexibility in choosing the supply rails of the opamps.
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Old 20th February 2020, 10:44 AM   #8
dreamth is offline dreamth  Romania
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i don't gave an op-amp...at least not a normal one...
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Old 20th February 2020, 04:46 PM   #9
dreamth is offline dreamth  Romania
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Does anyone know the voltages and currents for VcomR/L , I ref and how they relate with Iout ?
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Old 20th February 2020, 11:02 PM   #10
dreamth is offline dreamth  Romania
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Thanks for your help!

I measured them myself : Vcom R, Vcom L are 2.3v and Iref also 2.4v for pcm1798, so it's Vcc/2 as usual...
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