AD7689: Vref voltage

Document created by analog-archivist Employee on Feb 23, 2016
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I am going to use the AD7689 in order to digitize 4...20 mA signals and
temperature signals from various sensor. The reference voltage source will be
an external 2.5V (AD441BRZ). The inputs will be driven by AD8628 and AD8231. As
the positive supply rail is +5V for all amplifiers, Vref can be exceeded but
not any further than Vdd. If this happens on one or more inputs, is this going
to affect the conversion results of the other analog inputs and will the
converter return FFFF for those inputs where Vref is exceeded?


You should have no problems with the AD7689 if the inputs exceed the reference
but remain within the power lines.

The crosstalk between channels is mostly capacitive and is ≤125 dB at 100 KHz.
It will be less at lower frequencies. If the inputs are out of range but within
the crosstalk will be no worse. However there is one very important precaution
when using CMOS multiplexer circuits - any unused signal inputs MUST be
grounded or connected to a potential within the range Vdd-Vss. If unused pins
are left open-circuit there is a risk of substrate charge injection and
of crosstalk performance. This applies to most MUXes and multiplexed ADCs from
Analog Devices and any of our competitors.

The data sheet explicitly states on page 16 that over-range and under-range
return FS and zero codes respectively so you need have no concern about them.