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Part Number: ADC12D1800
Hi, I'm using ADC12D1800. connected vbiasq and vbiasl to ground with 100nF capacitors as recommended. Check voltage on the caps and it shows -0.9v , negative voltage. While my input voltages of the ADC are positive.
Checked datasheet and no circuit of the VBias pins. On what voltage does the Vbias depend and what may cause the negative voltage?
It would be difficult to tell if there is an issue with your board design which I assume you are taking the measurement from.
There is no way to generate a negative voltage bias internal to the device which is being seen on the board.
Is it possible that one of the caps to ground is shorted to some other supply voltage on the board instead?
High Speed Converter Group
Texas Instruments Inc.
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In reply to Rob Reeder:
I understand that its difficult to answer this question without seeing the board and the design.
This is a one card of many that works.
On a working card I see that the vbiasq and vbiasl are 0V.
That is why I asked if i can get more information about the pins vbiasq and vbiasl to understand where do they get their voltage from and what can effect them.
In reply to michael modilevsky:
Can you please confirm that you are saying that for a good board you are measuring 0V on the Vbiasq/l?
And on a bad board you are measuring -0.9V?
I spoke to design today and -0.9V should be correct. I was in error there is a neg charge pump circuit inside the IC, so plan to measure this in the lab tomorrow to confirm.
Also, in the meantime, if you send me a schematic for me to quickly review this might help as well.
I measured an evaluation board in the lab today.
Both VbiasQ/L pins are -0.9V.
Power must be applied to the converter as well as a clock signal.
Otherwise, the negative charge pump circuits will not necessarily be at that voltage.
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