I have an INA823 running from +/-12V supplies, and it generally works perfectly.
I have had 3 of these PCBs fail, 2 with the ref pin shorted to +12V, and 1 with the ref pin shorted to -12V (im still waiting to receive the unit to confirm it is definitely -12V)
both the output and ref-pin have 7V bidirectional TVS protection, so the ref should not be able to over-volt in comparison to the power rails.
Any ideas what could cause this failure?
Thanks
Ian
Hi Ian,
The bidirectional SMAJ7.0CA TVS have working reverse voltage of 7V while the maximum 12V peak pulse applies ONLY for a very short duration - see below. For this reason for +/-12V supplies you should use R55 resistor to limit the current to 10mA (cannot be zero ohm shown) or use TVS with higher Vbr voltage (like 12V SMAJ12CA) to prevent damaging TVS.
Having said that, you typically need TVS on the output pin (you use R70 22ohm) to guard against inductive kickbacks but not on REF pin..
Hello Marek,
The issue is not the TVS, that is still continuing to work fine, but I will take your advice and add some additional buffer resistance in there anyway as that is a good point. (Note, the 7V is well within the normal designed operating range of the system. Peak output level = 4.5V and peak Ref = ±3V (with respect to G))
The issue is the ref protection diode internal to the INA823.
When the ref pin is pulled to GND or -12 there is a significant current current (100mA current limit) through the ref connection. When the ref pin is pulled to +12V there is negligible current through the ref pin).
When only the INA823 is replaced, there is 0.03mA through the ref pin and everything works as it should
Hi Ian,
SMAJ7.0CA TVS is rated for the max peak current of 166.5A while ESD diode on REF pin rating is 10mA - thus, no surprise that TVS still works while 3 units have damaged REF pin. The reason for TVS on REF is to protect the pin against accidental shorts BEYOND supply voltage. The fact that you have 3 REF pin failures means the TVS did not work to clamped the voltage within the rails. This may have happened by applying +/-3V reference voltage BEFORE turning on the supplies (V+=V-=0V), which would result in 3V across the internal ESD protection diodes and resulting axcessive current - see below in red circle.
Since the internal ESD diodes on all I/O pins are rated for 10mA max current, I strongly recommend to add R55 resistor to limit the current to 10mA or less.
