AD7655: Driven significantly below ground

Document created by analog-archivist Employee on Feb 23, 2016
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I am using an AD7655 in a data aquusition sytem in which the input sriving
(aquasition) voltage can go more than 0.3V negative on a regular basis. Page 18
of the AD7655 states that the input clamp diodes can handle up to 120mA. Could
you please tell me what the maximum current that can be handled on an
occasional basis when the device is powered up is and what the maximum current
is on a regular (once every 4mS for 2ms is. It could be that you do not
recommend exceeding -0.3V on a regular basis and that the device should not be
made to sink regular negative current, if this is the case it makes my
interface circuit more complicated.

 

120mA is the maximum current that can be applied on an occasional basis. This
allows you to choose a protection resistor that is not too high, or to rely on
the maximum short cct current limitation of a driver op amp to protect the ADC
input.

What you are referring to is 50%duty cycle at 250Hz going below -0.3V. In this
case there is a considerable thermal effect. How serious depends on the
amplitude of the signal - how far negative it goes.

I would recommend some kind of external limiting which avoids driving the input
negative on a regular basis e.g. an inverting op amp with a diode in parallel
with the feedback resistor.

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