AD7266: Timing for analog input selection to CS falling

Document created by analog-archivist Employee on Feb 23, 2016
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What is the min time from Analog input selection ( A0-A2) to CS falling Edge. I
can't find this information in the datasheet ! And I have some problems when I
do analog input scanning 1- 6. SCLK speed 24.5MHZ AVDD = VDD = 5V external VREF
= 4.096V SGL = 1 fixed VDRIVE = 3.3V

 

For the AD7266 you need to have the logic levels on the channel selection pins
setup prior to the acquisition time for the part.  The Tacq on figure 31 page
17 of the AD7266 datasheet actually refers to the Acquisition time for the
part. This time is details on page 4 of the datasheet as "Track and hold
Acquisition time". This is 90ns for Vdd =5V and 110ns for Vdd = 3V. In your
application you need to have the channel selected prior to the acquisition time
to ensure the correct conversion result. So in your case using a 5V Vdd if you
have the channel selection pins setup 95ns before you brought CS low to
initiate a conversion you would be well inside the specification.You much not
change the logic level on the channel selection pins during the conversion. You
can changes the state of these pins once the ADC returns to track mode which is
on the 13th rising SCLK edge after CS falling edge.
The CS falling edge initiates the conversion. The conversion result bits are
clocked out on the falling edge of the SCLK input and 14 SCLKs are required to
access the data. The data stream consists of two leading zeros followed by the
12 bits of conversion data. The data is provided MSB first. The output result
for the AD7265 is for the selected channel. So after the 13th SCLK rising edge
you can select the next channel for conversion and once CS is brought low the
selected channel is what is converted and the result that is simultaneously
read is for that channel. The conversion and read occur together effectively.
So if you select a channel, say channel1, allow adequate acquisition time,
apply CS falling edge, read conversion result will be for channel 1.

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