Comments on ESD Protection - Differential Amplifier
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ESD Protection - Differential Amplifier
More about the circuit
The circuit is sensing (low side sensing for many reasons) current used to power up the lights with 28V(part of the schematics not shown here -> relay with LC filter on the output side to limit the inrush current). The nominal current flowing through R6 is around 0.2A but could be up to 1.5A in peaks(peaks last around 100ms).
First opamp "A1A" is used to convert the differential signal seen on R6(around 200mV) to a single-ended signal with a ~300Hz.
R14, C9, R16, C8, and C7 are here for diff and common mode filtering(cut-off frequencies shown in the bottom right corner of the picture).
Using R12 and R22 around "A1B" and "A1C" I will trim the precise voltage values so that open circuit and overcurrent events flowing through R6 are detectable.
The information whether the circuit is open or there is overcurrent will be reported through Q5 to another device(not on this PCB - most likely industrial computer 5V Input).
Question
How would you position the TVS diodes around the "A1A" inputs so that circuit stays the same but become as immune to ESD events as possible?
ESD that we are talking about here is +-15kV with 330R and 150pF model(IEC6100-4-2 Class 4).
EDIT
Per Andy Aka's suggestion, I am adding more details and maybe further explaining my question.
The load is floating while the bus voltage(28V on the picture) could be between 24V and 32V.
I am worried that +-15kV air discharge will kill "A1A" so I want to protect it with clever positioning of TVS Diodes across the opamp inputs.
Why do I think TVS diodes are needed?
If I would use this circuit as-is and expose it to ESD testing I would blow R14 and R16 because they are 125mW rated. Adding TVS in front of it would dump the usual ESD voltage to a level 200-500V where RC filter on the input could do the job easier.
Are TVS diodes needed in order to keep A1 opamp safe during Class 4 ESD strike? If they are not, how to place them?
Post
[In addition to the answers which have already been posted.]
A current sensing circuit like this lends itself to another scheme for ESD protection, which uses only one TVS. Only the line in which you're sensing the current is exposed to ESD. The small value current-sense resistor will keep the differential inputs at approximately the same potential[1]. So, one TVS on the CURRENT_IN (or the CURRENT_OUT net) will protect both of the differential amp inputs. That TVS will also protect other components which are powered with that current.
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Compared to ESD voltages ↩︎
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