Dear Kai,

Thanks for your reply,

I have used this resistor with several applications for example with ICL7135 and PCF8591 ADCs. With MCP3201 I didnt use it as all appnotes from microchip dont show this resistor.

I did a couple of designs with AVR Atmega16 and PIC 12F675 without this resistor and all worked fine.

Now I have designed a new board that has 8 inputs 4-20mA. 2 RTD inputs and 2 outputs 4-20mA. The board uses ARM7 LPC2138 micro and there is no external ADC. I am using micros internal 10 bit ADCs.

Since I needed 4-20mA output that could even drive 600 Ohms loads I used 15V supply derived fron 7815 as analog section supply and 3.3V derived from LM1117-3.3 as digital section supply. To protect analog inputs of micro from accidental high voltage from analog section I used BAT54C schottkey diodes [ Clamp diodes to 3.3V]. I didnt use any series resistor from opamp output to Micro ADC inputs.

When I tested the proto boards I was surprised to see that the digital supply was measuring 5.5V instead of 3.3V. Investigating in detail revealed that the RTD wasnt connected across the input so the RTD AMP output was full swing output 13V. Since the only thing in between this 13V output from Amp amd my 3.3V was the schottkey which could hardly resist current flow the micro Vcc got risen.

This the reason why I brought up the question here so that I can learn more about when to use and when not to use this resistor??

Thanks & Regards,
Prahlad Purohit