input protection circuit

[lynch]

I currently have to design a analogue input circuit that measures 0-10v for an industrial envronment. The 0-10V will be buffered (op-amp) and then low pass filtered (RC) then used as a input to a ADC.

What circuits do people use to protect their inputs from over voltage, voltage spikes, shorts ...etc?

I mean apart from current limiting resistors how can i avoid damaging my op-amp input circuit.

I cannot find any useful literature on input protection circuitry and would appreciate a point in the right direction.

Many Thanks in advance for anyones help

 

[A.Anand Srinivasan]

generally SCRs are used for overvoltage protection and current diodes or other current limiting mechanisms are used for protection from excess current... keeping the glitches due to SCR in mind, diode are better in input protection compared to SCR...

 

[lynch]

Thanks Anand, but are there any usefull books on the subject with examples designs etc?

 

[IanP]

One of the most common input protection circuit consists of a resistor followed by 2 diodes: one connected between the ADC's input and +Vcc and one connected between the ADC's input and -Vdd (or 0V, if it is the lowest voltage in the circuit) ..
Here is an example:
**broken link removed**

Regards,
IanP

 

[the_edge]

D1, D2 Shotky diodes
DZ Zener diode

If your input already have buffer, you dont have to use this from shematic...
This one I usualy use... It works just fine for me...

 

[rowokii]

Adding a bit to Ianp's suggestion:

If the input goes above VCC + Vforward-diode, the diode between input to VCC gets forward biased and clamp the input. The diode to GND prevents input going below GND by more the Vforward.

Schottky diode is often used since they are fast acting and have lower Vf.

One drawback with this is your power supply now takes the ESD hit, so it might have to have a clamp circuit prevent the VCC from going up.

 

[lynch]

Thanks for the suggestion guys i will use these ideas and look around for some more material.

Question for the edge:
What wattage of zener diode do you use for your circut?

The pullup resistor before the RC is this a very high value? (100k)