Analog and mixed signal design: Temperature independent resistors

In IC technology all resistor materials have an associated temperature coefficient. Most commonly, resistors are made from polysilicon, diffusion of various kinds and metal. The most common of these resistors is poly and diffusion. In certain applications a temperature independent resistor may be required. In order to do this one has to search the technology properties to see if there are resistor materials in the technology that can provide (1) An appropriate sheet resistance and (2) opposite temperature coefficients. Almost all semiconductor technologies provide this. Once the materials are established a first order temperature independent resistor may be synthesized as shown in a recent report released by Signal Processing Group Inc. This report may be found at:>engineer’s corner.

Lumped and distributed elements

How does one determine whether to treat a component as a lumped or distributed one? The answer is, that if the the element size is greater than lambda/20, where lambda is the effective wavelength of the signal associated with the element, then it should be treated as a distributed component or element. This means that for typical discrete designs, the lumped approximations are valid for frequencies in the 500 to 1000 Mhz range. For ICs the frequency range is much larger because of the small size of the elements encountered there. This range may be up to 10 Ghz. One has to ask, where did the 5% of lambda come from? It is like most other things in practical engineering an approximation and a thumb rule. It should be considered a guideline. A distributed model is usually more accurate for any frequency above DC but experience says that the 5% guideline is a good transition value. Note: The effective lambda is the lambda in free space divided by the square root of the effective dielectric constant. The effective dielectric constant in homogeneous media is simply the relative permittivity. For non-homogeneous media is not. Usually for non-homogeneous systems such as microstrip the effective dielectric constant is less than the relative permittivity.

Analog and mixed signal design: A peaking current source

Most of us are very familiar with the Widlar current source which uses a resistor in series with a diode connected bipolar to act as a source for a current. It is probably the most popular current source in existence. However, this source does have its problems such as variations with resistance,
and the low input resistance of the bipolar. There is another lesser known current source known as a “peaking” current source that at times can be used with advantages beyond those offered by the time honored Widlar source. It is also useful when the supply voltages are low. A white paper on this source is available now by courtesy of the Techteam at Signal Processing Group Inc. For interested readers it is located at> engineer’s corner.

Analog and mixed signal design:Why 50 Ohm?

Has anyone wondered why we use 50 Ohms as the the reference resistance in so many of our designs. Why 50 Ohm seems to be a defacto standard. We normalize to 50 Ohm; we use 50 Ohm in our oscilloscopes; we pick 50 ohms as a good convenient reference resistor. But how did this happen. Where did this 50 Ohm factor come from. We ran across a explanation which sounds reasonable enough and decided to post it to this blog. Standard coaxial lines in England in the 1930’s used a commonly available center conductor which turned out to be 50 Ohms! Others say that for minimum signal attenuation, the transmission line characteristic impedance is 77 Ohm. For maximum power handling it is around 30 Ohm. A good compromise is 50 Ohm for both performance parameters. So this is how 50 Ohm became a convenient impedance level!?

The eye diagram – a practical aid to the design of systems

The eye diagram is a very useful and practical tool for analyzing, evaluating, diagnosing and correcting errors in digital communication systems or indeed any digital/wireless system. The premise is fairly simple. Using the eye diagram a number of valuable parameters may be extracted at a glance. These parameters play a critical role in the transmission and reception of data. An intuitive understanding of the eye diagram is essential for good design technique and analysis of systems. Simulation of the eye diagram and its measurement can be better understood if one knows the underlying technique of eye diagram construction. A brief expose of this tool can be found at > engineer’s corner.