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AD8557 - cannot get the gain or offset to write

Question asked by dfansler on Jul 21, 2016
Latest reply on Sep 14, 2017 by vps

I am using (or trying too) an AD8557 connected to a pressure sensor (wheatstone bridge).  All is being controlled by an Atmel AT32 processor (3.3v).  With a tube attached to the pressure sensor, I can get 4mV across the + and - of the bridge.  With no pressure, the output of the + and - leg of the bridge is 1.654v, with respect to ground. The output of the AD8557 shows 13mV with no pressure to 38mV with the 4mV.  Obviously I would like a little higher output voltage from the AD8557 - and therein lies the problem - I cannot get the output to change, regardless of what I program the 2nd Stage, Offset, and 1st Stage to be.  So far I have only tried this in the Simulate Mode.

 

I have used my oscilloscope to verify that the pulses going to the Digin of the AD8557 are correct - and they are.

I am using 100us for a one, 5us for a zero and 20us between pulses.  I am waiting 100ms between the 3 different transmissions.

 

I am sending the 2nd Stage gain, followed by the offset and then the 1st Stage gain.  I have tried these in different order, with different values for the gains and offset, with no luck.

 

An example of what I am sending would be the offset with a value of 0x80 (1000000b)  The three  pictures show the 38 bit wave form in two sections - the first section goes from the start of the 38 bits through the 1st two bits of the last 12 bit ending packet.  The second section shows the last 10 bits of the 12 bit ending packet.

 

The third picture is a smaller time scale of the beginning of the 38 bits showing a 1 and three 0's so that the timing of the 1, 0 and between pulses is clear.

Any suggestions would be appreciated!

Kind regards,

David

AD8557 1st section.jpgAD8557 2nd section.jpgAD8557 Timing.jpg

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