Smart Digital Thermometer design 2 -- Temperature independent voltage reference

In the last blog we completed the offset circuit for our temperature sensor. We saw that for offsetting the voltage we need a precise temperature reference. This temperature reference should have fairly low temperature co-efficient (dV/dT) thus should not change its reference voltage output considerably as the thermometer is subjected to varying temperature.

Now as we are using a intelligent controller the ATMEGAGA168 it is capable of outputting an temp independent analog voltage as a PWM output. But in order to use that we need to use the PWM output pin and further filter it to eliminated ripples. At least we need a second order filter for smoothing out the voltage considerably. However the main disadvantage of using this scheme is the fixed quantization level for the PWM output. For the ATMEGA 168 controller it has 256 levels i.e. the duty cycle for the PWM output can have only certain fixed value. This may be sufficient but for this case we are opting for ultra precise accuracy so we will use a separate voltage reference IC.

When it comes to voltage reference the most common is a zener diodes. Zener diodes work in reverse bias condition and the reverse breakdown voltage is actually its voltage reference. We are going to use the LM336 band gap reference zener.

Before we continue let us characterize the features for an ideal voltage reference :

• It should be accurate in its voltage output.
• NOT temperature dependent. Voltage should not change with temperature.
• Cannot deliver high current.
• Need not have low output impedance.  

This is a simple scheme for biasing a zener reference. R1 is the biasing resistor.

For a generic zener dV/dT = 2.2mV/deg C

For a generic diode dV/dT = -2.2mV/deg C

So using them in combination will eliminate the temperature dependence.

Let us implement such a scheme

The base emitter junction of the transistor Q4 acts as a diode drop which has negative temperature coefficient so this scheme incorporates both positive and negative temperature coefficient thereby greatly reducing temperature dependence of the circuit. R2 should be sufficiently large to prevent high current drain from the circuit which would change the biasing voltage V2.

This type of scenario is highly unlikely but if it does happens current through R1 will change thus reference voltage of D1 will deviate.

Solution

Here we are using the Wilson's Current mirror concept any change in the load current will get reflected in the biasing current keeping the reference voltage more or less constant. With this current configuration we get a value of 3.1788V across the 1 MEG resistor. By using a proper voltage divider circuit we can then convert this voltage into 2.73 reference. So let us add this circuit in our design.