ADA4898-2: Maximum voltage in an unpowered state

Document created by analog-archivist Employee on Feb 23, 2016
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This circuit is part of a magnetic field analyser. A new application requires
us to double the sensor range.  

The worst case scenario is that the user exposes our analyser without power to
fields greater that its measurement range... that is why I placed the
protection diodes.   

Timing does not allow us to modify the board and use another protection
Sensor coil is not exposed to ESD events as it is not accessible from
the case.

I have attached a schematic of the circuity involved in this issue. ADA4898   
outputs are connected to a ADG1633 mux that does have protection diodes to
power rails.  

Supply lines are high impedance when the ADA4898-2 is unpowered.  

We want to know our safety margin if we remove protection diodes? 


Each input of the ADA4898-2 pin has two ESD diodes. One of these diodes is
connected to VCC (+Ve supply), the other to VEE (-Ve supply).  

Since the supply lines are left at high impedance when the ADA4898-2 is
unpowered the maximum voltage at the “H+” and “H-“ input, that the ADA4898-2 is
not damaged, is dependent on the duration and the frequency of the overvoltage
and partially depends on the ADG1633BCPZ switch. 

The bypass capacitors connected to VCC and VEE will have an impedance depending
on the signal frequency. The impedance is Xc=1/(2ðfC), where C is the total of
all bypass capacitors connected to VCC and VEE. Kindly add all of them to
determine the value of C. The current into these capacitors should not exceed 5
mA for extended periods of time. The resistance in series with each input of
the ADA4898-2 in the schematic is 2.6 kΩ .Therefore the safe voltage at the “H
+” and “H-“ inputs would be 5 mA*SQRT(Xc^2+2.6k^2), that the ADA4898-2 won’t be
damaged in an unpowered state. 

The ADG1633 will also have ESD diodes. The safe voltage here would be 5mA*(2.6kΩ

If the ADG1633 S1A and/or S2A are closed this over-voltage will also be present
at pins 1 and 4  of the ADG1633 and can potentially harm the components
following the ADG1633.