How to protect an ADC input

[Frankeman]

Hi,

I'am developping a Current to Voltage convertor so that i can read a sensor current from 0-20mA.
The design is standard, a 250 resistor, an OpAmp and then the ADC, so far so good.
At 20mA the Voltage over the resistor is 5V and this is also the maximum voltage the ADC can measure.
But what happens when the current becomes 27mA because of a faulty sensor, then the voltage over the resistor is 6,75V.
This is killing the ADC, how can i protect this ?

Frank.

 

[Styx]

The ADC should have an internal diode to the 5V rail (if it doesn't add on).
THIS will clamp the input to the ADC at 4.4V, However, it wont limit the current so put a resistor in series to the ADC

 

[Roff]

The ADC should have an internal diode to the 5V rail (if it doesn't add on).
THIS will clamp the input to the ADC at 4.4V, However, it wont limit the current so put a resistor in series to the ADC

Errmm, I think that should read 5.6V, not 4.4V.

 

[Styx]

err umm yer... diode-maths wrong way round, good catch

 

[Nigel Goodwin]

Check my PIC tutorial, the analogue tutorial hardware uses a series resistor to feed the analogue input from the opamp, this allows the protection diodes to pass a safe amount of current - if you don't have a resistor, then too much current could flow.

 

[Frankeman]

I have seen the Trick with the diode to the positive 5V rail which feed the complete circuit.
Under normal conditions the positive rail is at 5V but what happens with this voltage as the input gets above the the 5.6 volt.
Does'nt you blow op the whole circuit ?, or is the 5V rail staying at 5 volt.

I like the idea of an opamp which is feed with 5V so the output can not be higher, so it is safe, and that this opamp can have an input voltage of let say 30V, are they in the market.