Suggestions for ADC input voltage protection

[Thangadurai]

Hi,

I need to interface an ADC with my microcontroller for measuring the RPM. to measure the RPM i am using one module that will give 4-20ma output corresponding to RPM. I am using 250 ohm resisitor to convert that 4-20ma current to 1 to 5v for adc input. i want to protect the adc input from excessive voltage drop across that 250ohm resistor due to any fault. Please give me some suggestion to protect the ADC if more than 5v is appeared in ADC input channel.

 

[cameo_2007]

Re: ADC Input Protection

put a 5V Zener across the ADC pin, and probably you can reduce the upper limit from 5V to a little less.

 

[ucah]

Re: ADC Input Protection

As cameo_2007 suggested, you can use a zener for clamping, but you need to factor capacitance which will limit your bandwidth, tolerance which for some zener types -but not all- can be poor and temperature dependence.
Another solution is to clamp with a schottky diode at a 5V supply, which should give you a clamping threshold of approximately 5.25V, and will be a bit more stable.

 

[Thangadurai]

Re: ADC Input Protection

i checked with zener by connecting across the ADC pin, upto 3.5v input it gives without any voltage drop from input. signal voltage is dropped if input voltage exceeds 3.5v. i think this is due to the tolerance level and the leakage current of zener diode(1N5221B). i think it will work with the schokky diode construstion as ucah said,at the same time schottky diode is also used for surge protection.i didnt tried yet just i read the theory of schottky clamping circuit. Is there any other solution to protect the input than using zener or schottky?