The circuit below is basically taken from CN-0099 and is part of a weight cell :
Our problem is that on one of our weight cell units has failed because the ADA4096-2 op amp was broken.
( We use ADA4096-2 instead of the op amp AD8603 suggested in CN-0099 to allow +24V circuit operation).
Measurements with the broken ADA4096-2 (with a 500ohm Rload attached between 4-20mA and 0V):
+IN @ U3.3 was measured to 496mV
SENSE @ U3.5 was measured to 5.996V
REF @ U3.1 was measured to 5.5V
Also the 4-20mA output @ U7.3 was measured to 5.5V across 500ohm Rload.
So, the output 4-20mA output current was actually as much as 11mA (5.5V / 500ohm) even though the current through the 124ohm shunt resistor was only 4mA. Calculation: (5.996-5.5V) / 124ohm. This meant that U7.3 was sourcing 11mA - 4mA = 7mA to the 500ohm Rload.
Measurements after replacing the broken ADA4096-2:
SENSE @ U3.5 was measured to 2.496V
REF @ U3.1 was measured to 2V
The 4-20mA output @ U7.3 was now measured to 2V across 500 ohm which is correct for 4mA.
We need help to understand why the ADA4096-2 op amp in this case have failed in our weight cell application.
Before we proceed posting a formal FA:
1. Do you think ADA4096-2 is a suitable op amp for replacing AD8603 in our +24V single supply application ?
2. Do you think lack of ESD transient protection…
I am referring this to one of our op amp applications engineers who will assist you in the failure analysis.
2. Do you think lack of ESD transient protection in the circuit above is a "probable cause" for ADA4096-2 failure ?
According to datasheet ADA4096-2 input pins have internal overvoltage protection up to 32 V above and below the supply rails. But I guess this still make them vulnerable to ESD transients.
If your is yes on my questions 1 and 2 above, we simply need to add some transient protection to our circuit.
Rgds, Tore Kallevik