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AD7606 analog range appears to be only -0.5 to +0.5V?!?

Question asked by tglawrence on Jun 2, 2014
Latest reply on Jun 5, 2014 by jcolao


Having a strange issue with the AD7606 analog chip.  In older designs we have used the AD7657 without any issue so was suprizing that having a fit with the AD7606.

 

There is a delay between the +5V and +3.3V on power up

 

All signals on the digital side look good.

 

!Stdby, REF_SELECT are tied to +3.3V

RANGE, FIRS_DATA, OS_0, OS_1, OS_3 are tied to ground

FIRST_DATA is tied to +3.3V via a 10K pull up resistor.

 

On REFIN/REFOUT pin measure +2.49Volts

 

AN_TRIGGER is high for 14.48 useconds (just enough to read all 8 analog channels)

 

BUSY is high for 3.8 useconds when AN_TRIGGER goes high

 

Did not measure the time between the end of Busy and the first downward pulse of !CS and !RD, but there was enough time

 

Between read pulses there is 0.8 useconds.

 

CS, RD pulse is over 50 nseconds

 

V1 is measured to range between 0 to +5V.

 

When V1 is between 0.5 V and +5V the data for channel 1 is 7FFFh

From 0 to 0.5 V the data ranges from around 50h to 7FFFh.

 

During this test the voltage on REFIN/REFOUT does not change from +2.49 Volts

 

This same results occurs when attemt to read all 8 channels and when only read channel 1.

 

All the digitial I/O look like clean 0 to +3.3V digital signals.

 

This does explain why all my analog reading are very high.

 

All analog inputs should be restriced to the -5V to +5V range.  But will have testing wires in place to verify that do not have an issue with one of the analog inputs being out of range.

 

What could be causing this behavour?

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