First of all, thank you for your replies, as this is my first post in this Comunity.
I have an Analog Input to measure the output signal (in current) of a HV transducer. The transducer is suplied to +- 24V
The Analog Input consists on:
- TVS of Vbr=15V
- An array of resistors to become the current into voltage.
- A buffer with Op. Amp. OP4177 in Sallen Key configuation (1 stage). The supply of the Operational is bipolar and suplied to +- 12V
My problem is that, when a transient overvoltage hapens in HV section (this, in theory should not happen, but it does), the voltage in the AI goes avobe de 15V of the TVS; and after many transient overvoltages, the TVS goes into continious low impedance. This is fine, as it protects the OA, but I need to reduce the number of permanent failures of this stage.
So, my question here is:
Can I increase the Vbr level of the TVS to 24V without risk of breaking down the OA? If not, I need to add some alternative protection to reduce the permanent failure rates. Any Ideas?
I attach you the AI stage and de Absolut Maximum Ratings of the OP4177.
Thank you for your help.
Absolute maximium rating specified in the datasheet should be follow to avoid permanent damage to the device. it also stated that "exposure to absolute maximum rating conditions for extended periods may affect reliability". The input voltage range of OP4177 is from Vs- to Vs+, which in your case is -12V to +12V. In an event where an input transient above 12V is experience at the input of the OP4177, the device may suffer permanent damage. As a rule of thumb, the input voltage present at the input pin should not exceed (Vs+) +.3V, (Vs-) -.3V. It should also take into consideration the amount of current that will be present at the input pins. It should be keep to 5mA and below.
I think, it would be better to keep the input voltage below your supply rails, (below 12V). Your TVS Vbr should be around 12V. You can consider using the protection circuit below and further reading about this at this mini tutorial. http://www.analog.com/static/imported-files/tutorials/MT-036.pdf
Hope it helps,
That's fine, really helpful to me.
You could also consider more modern version of OP4177, the just-released ADA4177-4,
ADA4177-4 datasheet and product info | OVP and EMI Protected, Precision, Low Noise and Bias Current Op Amp | High Supply…
which includes over voltage protection. This should simplify your input protection requirements.
Thank you Gustavo, this is great for fixing some problems I had in previous versions of my board.