Electronic – Best design practice for relying on IC input diode current clamping

Putting the zener diode across the lower resistor in the divider is always going to disturb the voltage divider if the zener conducts any current at all. And apparently the zener that you have chosen conducts a fair amount of current and is far from an ideal unit.

If I was designing a protection for the A/D inputs I would use one of two approaches.

1. Use the zener diode on the signal voltage source before the voltage divider.

2. Use an actual diode clamp on the A/D input made of a dual diode component such as a BAT54S. The upper diode points into a clamp current sink made of a shunt supply that is designed such that the clamp works at approximately the 3.3V level. Something like this:

When advanced low Vdd CMOS uses RdsOn of 25 Ohms at 3.3V with 0.2 to 2 ns rise times, designers often are unaware of need to have controlled impedance tracks and results in ringing at half wavelength of track length.

• low power Schottky diodes with small capacitance are sometimes used to provide a dynamic load impedance of around this driver impedance range to at least provide some matched load to the overshoot.

• when the transmission line is mostly inductive from poor coupling to gnd tracks or gnd plane near same gap as track width, and with CMOS input Z can be x pF and 1GOhm, ringing and UNDERSHOOT can cause glitches.

analogy

The clamp diodes are like a stiffer rubber end stop to a spring (inductive track) with a weak shock absorber in zero gravity going over a pot hole and the car still bottoms out and then jumps up but with less bouncing shock waves. A properly tuned shock absorber is like a properly tuned transmission line to give controlled step response with minimal overshoot/undershoot and ringing.

No overshoot is a theoretical perfect transmission line matched to between source and terminator which is as I said above can be 25 ohms -/+50% with temp and 10% supply variation. so often terminated with 50 Ohms. with tracks matching the load. But skin effects and dynamic dielectric constant reduction >1Ghz causes some imperfections .

A diode is a poor man's choice for lack of impedance controlled on track layout and design,pick any; coplanar, stripline, microstrip! for 50 Ohm single ended and 100 Ohm differential.