Other Parts Discussed in Thread: LM9076
Hi,
we are having an internal debate whether additional external protection needs to be added to the analog input pins when measuring 24Vdc, if the micro is powered from 3.3V and we probe through a 10:1 high impedance divider. More details:
-the device is entirely self contained, no external inputs except external 24Vdc from a SMPS brick adapter. All data exchange is wireless.
-this 24V input voltage goes into a linear regulator with LM9076BMA which powers the micro with 3.3V
-the 24V input voltage is also sampled using a 10:1 series divider 44.2k with 4.99k to ground. Another option is to sample through 162k series and 18k to ground.
-Both options give a 10:1 ratio, so the output voltage is 2.4V. Equivalent circuit is a 2.4V supply with an internal resistance of 4.5k in the first case (44k2||4.99) and 16.2k in the second case (162k||18k).
-Datasheet specifies at page 19 Absolute maximum ratings "Voltage applied to any pin (2) –0.3 V to V CC + 0.3 V" and "Diode current at any device pin ±2 mA".
Under these circumstances, do we need to add clamping diodes to Vcc and GND at the analog inputs? What is the worst scenario if we do not have them? Looking forward for your opinions, thank you for looking into this.