Bandgap Voltage Reference

In couple of my designs I have used voltage references. I thought to write up a blog on Bandgap voltage reference in such a way that it may be effectual for professionals and understandable to novice designers.

Voltage reference is an electronic device (circuit or component) that produces a fixed (constant) voltage irrespective of the loading on the device, power supply variation and temperature.

Reference voltages and/or currents with little dependence to temperature prove useful in many analog circuits.

As many process parameters vary with temperature, if a reference is temperature-independent, it is usually process independent, as well.

If two quantities with opposite temperature coefficient are added with proper weighting, the resultant quantity theoretically exhibits zero temperature coefficient.

For V1 and V2 with opposite temperature dependence, the coefficients c1 and c2 can be chosen in such a way that

Thus, the reference voltage Vref=c1V1+c2V2 exhibits zero temperature coefficient.

Among various devices in the semiconductor technology, the characteristics of the bipolar transistors have proven the most reproducible and well-defined quantities that provide positive and negative temperature coefficients.

Bandgap Voltage Reference

• The most popular technique for both Bipolar and CMOS technologies.

• Generates a fixed dc reference voltage that does not change with temperature.

• Cancels the negative temperature dependence of a PN junction with positive temperature dependence from a PTAT (proportional-to-absolute-temperature) circuit.

The term with negative temperature dependence is the forward-biased voltage of a diode (usually base-emitter junction). The PTAT term is realized by amplifying the voltage difference of two forward-biased diodes ( i.e., base-emitter junctions).

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Sanjay Chawla

10/23/2008 - 5:27pm

# of views: 13345

# 1

Ajay Kumar Yadav

October 20,2008 - 3:15pm

Always bear in mind that voltage references do not power up instantly (this is true of references inside ADCs and DACs as well as discrete designs). Thus, it is rarely possible to turn on an ADC and reference, whether internal or external, make a reading, and turn off again within a few microseconds, however attractive such a procedure might be in terms of energy saving.

# 2

Art Gray

October 20,2008 - 3:45pm

Great information.....!

# 3

Mark Thomson

October 20,2008 - 3:59pm

Can you please provide some information on the drift of the Voltage references?

# 4

Sanjay Chawla

October 20,2008 - 4:35pm

The XFETand buried zener reference families have the best long term drift and TC performance. The XFETseries achieve long terms drifts of 0.2 ppm/1000 hours, while the buried zener types come in at 25ppm/1000 hours. Note that where a figure is given for long term drift, it is usually drift expressed in ppm/1000 hours. There are 8766 hours in a year and many engineers multiply the 1000 hour figure by 8.77 to find the annual drift - this is not correct, and can in fact be quite pessimistic. Long term drift in precision analog circuits is a "random walk" phenomenon and increases with the square root of the elapsed time (this supposes that drift is due to random micro-effects in the chip and not some over-riding cause such as contamination). The 1 year figure will therefore be about 8.766 3 times the 1000 hour figure, and the ten year value will be roughly 9 times the 1000 hour value. In practice, things are a little better even than this, as devices tend to stabilize with age. The accuracy of an ADC or DAC can be no better than that of its reference. Reference temperature drift affects full-scale accuracy