AD7812 power-up problem

Document created by analog-archivist Employee on Feb 23, 2016
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We are using AD7812 and during the tests the ADC stopped working and it returns
the same values on all channels (for example 1.58VDC). We need to turn off the
equipment to recover the ADC. We also tried to shut down only the regulator
(the equipment is kept turned on) that supplies voltage to the ADC, but the ADC
doesn't turn off completely...it has always 2.2VDC on its VCC pin. What could
be the reason behind? As we need to turn off the ADC when it stops working,
could we use another ADC(with the same pinout and footprint) that have a “reset
pin”?

 

There have been cases in the past that the AD7812 does not power up correctly
if power is removed from the device and the supply voltage doesn’t go all the
way to ground. This is what you could be seeing here. In order to fully reset
the device, you should write to the control register and set the power down
bits (PD0, PD1) to zero followed by a write to the control register setting the
power down bits to 1. This action of cycling the power bits has the effect of
resetting the AD7812 and should return the part to normal operation under all
conditions. We recommend in the datasheet that customers do this software power
cycle in all power up cases. This recommendation is included in the datasheet
on page 12 top right corner.

I expect you are seeing 2.2V on the VCC pin because another pin is pulling it
up via the internal ESD diodes, for example if there is a voltage on the analog
inputs.

Regarding your request for another ADC recommendation, I hope the reset
procedure above will solve the problem. However, we can say that the AD7812 is
a very old part and I would recommend newer designs from ADI since they will
prove cheaper and possibly easier to use. A good alternative to the AD7812
would be the AD7939.

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