I'm working on a project where I need a 3.3V reference for a PIC A/D. I tried using a BZX55B3V3 which is a 3.3V Zener using a circuit like below:
**broken link removed**
Initially I tried it with R=10K and Uin = 4.5-5V. In theory, Uout should be 3.3V under no/minimal load, right? I got closer to 2.2V. I tried reducing R to 1K and that got me somewhere in the 2.8-3 volt range. When I used a 9V battery for Uin and R=470Ω I got around 3.5V, with about 10 mA draw.
So what gives? Why would a 3.3V zener conduct at 2.2V at low current levels? The only thing connected at the output was my DVM, so it isn't an output impedance issue loading the circuit. Since this is a battery-operated circuit, Uin can vary depending on battery strength, and I don't want my reference to be drawing several mA of current to get the correct voltage, but I need my reference voltage to be reasonably stable. I couldn't find a precision shunt reference @ 3.3V that was in stock, which is why I went with the zener.
Don't use a zener, use a precision voltage reference - check my PIC analogue tutorial for an example (you can add a preset to make it adjustable if you really need 3,3V? - or just add an attenuator to drop the input voltage).
Zeners, especially the lower voltage ones don't have as sharp a knee as the pictures show. Check the "test current" in the data sheet. My bet is it is 5ma. -- So 240 ohms for your 4.5 volts would do it.
Some Pics have an internal Fixed Voltage Reference (FVR) that can be assigned to the a-d module, like the 18f14k50 for instance. Looks like the new enhanced midrange (16F1xxx) devices have this too