AD7490: Range Bit

Document created by analog-archivist Employee on Feb 23, 2016
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I want to use the 16 Channel of the AD7490 to convert some 2,5V Vin and 5V Vin
by setting Range Bit of the Control Register.

        e.g.    Channel       Analog Input Range
                0               0 to REFin            
                1               0 to 2 x REFin          Wating time for Range
switch ?
                2               0 to 2 x REFin
                3               0 to 2 x REFin
                4               0 to REFin              Wating time for Range
switch ?
                5               0 to 2 x REFin          Wating time for Range
switch ?
                6               0 to REFin              Wating time for Range
switch ?
                7               0 to 2 x REFin          Wating time for Range
switch ?
                8               0 to REFin              Wating time for Range
switch ?
                9               0 to 2 x REFin          Wating time for Range
switch ?
                10              0 to REFin              Wating time for Range
switch ?
                .....

*       Is it possible to switch the Analog Input Range form 0 - REFin (0-2,5V),
to 0 - 2 x REFin (0-5V) for each conversion ?
*       Is there a minimum Time to wait, while switching the Input Range form
0-2,5V to 0-5V or 0-5V to 0-2,5V ?
*       Is it better to convert all the 0-2,5V channel in sequence and then
switch the RANGE bit to 0-5V to convert all 0-5V channels ?
*       The overvoltage protection ist the same for the 0-2,5V and 0-5V Range ?
VDD+200mV or limiting the Input current below 10mA

 

A write to the control register sets up the operating conditions (output
coding, range and power down bits) for the next conversion.

· Is it possible to switch the Analog Input Range form 0 - REFin (0-2,5V), to 0
- 2 x REFin (0-5V) for each conversion ?
Yes. Assuming you are operating the part as a normal multi-channel ADC (you are
not making use of the on-chip channel sequencer), a write to the control
register will set up the Range for the next conversion.


· Is there a minimum Time to wait, while switching the Input Range form 0-2,5V
to 0-5V or 0-5V to 0-2,5V ?
No, a write to the control register sets up the operating conditions for the
next conversion. On the same data transfer you will read out the current
conversion result. Provided you complete the data transfer which consists of: 
falling edge on /CS, followed 16 SCLK cycles following by a rising edge on /CS,
the only requirement is to respect the t_Quiet_Time before initiating the next
conversion and data transfer. Same requirement for all conversions. 

· Is it better to convert all the 0-2,5V channel in sequence and then switch
the RANGE bit to 0-5V to convert all 0-5V channels ?
There is no requirement to do things this way. You can simply write to the
control register specifying the required range and the next channel for
conversion. Of course, you may wish to use the sequencer function and convert
all the 2.5V channels then change the range, re-load the shadow register and
convert all the 5V channels.

· The overvoltage protection ist the same for the 0-2,5V and 0-5V Range ?
VDD+200mV or limiting the Input current below 10mA
The absolute maximum ratings are not a function of the input range. As you
correctly state, the input Overvoltage / protection circuitry should clamp the
input voltage to within 200mV of VDD and limit the current to less than 10mA (
a 5mA current limit would be a good conservative design choice). Of course, if
you are in the 2.5V range, any input which exceeds 2.5V will be converter as a
full-scale output code.

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