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current limiting resistor on adc

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vicky29

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adc input current limiting

hi,

i have designed a very simple circuit. in which a small voltage from a sensor is givcen to OPAMP--->current limiting resistor (with a zener diode)---->ADC---->uC----->LCD, as shown in figure. But as i chnage the value of current limiting resistor the voltage changes on the adc input pin (i.e. across the resistor) why s it happening so, and what value should i choose for this resistor.

Please help me.
 

If the ADC input can work with the full opamp's output range then you don't need any resistor between the opamp and ADC ..
If you need to protect the ADC input against to high voltage from the opamp a 1kΩ resistor and 5.1V zener diode should do the job ..

Regards,
IanP
 

thanks for the advise. but i tried using 1K resistor. but voltage dropped from 4.94volts to 4.37volts across the resistor. actually i'm running opamp on +12v and -12v and adc on +5v so i want to protect the adc in case high voltage comesup from opamp. do i need to use one more zener with +rail.
 

If you have voltage drop across 1kΩ resistor of 0.57V this means that the ADC input sinks 0.57mA - that's quite unusual .. check out your ADC's data sheet ..

One 5.1V zener diode provides sufficient protection of any 0-5V input ..

Regards,
IanP
 

I'm using MCP3202 12-bit ADC. and if i use 10K i get a drop of 1.01 volts i'm unable to understand. on net i found one application of which i'm giving the link **broken link removed** can this provide any help. and how is the 2 zeners attached i mean the purpose?
 

Single zener diode provides protection of +Vz and -0.7V and that is exactly what you need to do ..
I can't see any logic in connecting two zener diode, what in fact limits the voltage to +Vz+0.7 and -Vz-0.7, while protecting 0-5V circuits ..

Regards,
IanP
 

ok.....but......what to do about the sinking current.
 

One option is a unity gain buffer (voltage follower) between voltage limiting zener diode and the ADC's input ..
This buffer may be build around a rail-to-rail opamp fed of the same voltage as the ADC, so its output is within 0-5V range, and no resistor will be required between its output and the ADC's input ..

It looks like the input impedance of your ADC is not very high ..

Regards,
IanP
 

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