AD7849: critical item for power supplies

Document created by analog-archivist Employee on Feb 23, 2016
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Is there any critical item for power supplies of AD7849 which could result in
behaviour as described above ?  Or do you have any other idea what happens with
the converters. We are using this devices in other applications for years and
there we do
not have problems like this. So it seems that there is an application dependent
reason.

 

Firstly, I would like to make sure that the part is not going into latchup. The
fact that the part is drawing too much current could suggest that the part is
latching up and this DAC is particularly vulnerable to latchup.

It is absolutely critical that you respect the absolute maximum ratings for the
AD7849 at all times, even during power up.

Any digital input should never be allowed to be more than 0.4V outside the
digital supply voltage, even during power up. If the circuitry driving the
digital pins of the AD7849 is powered separately and is likely to power up
before the AD7849, you can protect the digital inputs using Schottky diodes to
Vcc and Dgnd. For extra protection, you can also add a series resistance prior
to the digital inputs.

Vcc(digital supply) must never exceed Vdd(analog supply) by more than 0.4V. If
the digital supply is likely to power up before the analog supply, you should
protect the AD7849 using the circuit shown on page 4 of the datasheet.

ROFS and the DAC output pins must never have a voltage applied to them which is
outwith the analog supplies.

All analog and digital pins on an IC are protected by parasitic ESD protection
diodes. These diodes will turn on when the voltage applied to a given pin
exceeds the supply voltage. You must limit the current through these protection
diodes to less then 10mA otherwise there is a danger of them shorting out and
blowing the part.

It's probably worth while examining the power supplies in your system to ensure
that they turn on in the correct order i.e. so that the Absolute maximum
ratings are respected during power-up.

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