Hope I can ask this question here. Last time, Mr. Kai had guided me regarding 7135.

Is it necessary to limit the input current of 7135 ADC using a series resistor? The data book says that "if input is more than Vcc, then input current must be limited to 50 microamps." In all the test circuits, they have shown 100K series resistor. I have removed the 100k series resistor, and it works fine without heating up.

The problem with a 100 K series resistor is that, if the output impedence of the circuit that drives the ADC input is high, then zero-count-at-zero-input doesn't happen. I manually have to reduce the input impedence of the driving circuitry by adding parellel resistor, and this creates measurement errors since output levels change from specification and software has to be tuned. But, I am able to get zero reading at zero input. If I can remove the series resistor, my problem gets solved since I can connect 200k or 300k in parellel with the source without the series resistor dividing the output voltage of the source.

So, is it necessary to limit the input current of 7135 ADC using a series resistor if input is less than or equal to Vcc?

Raj Nambiar