Analog and mixed signal design: Some useful identities for MOS IC design

Powerful simulation programs for circuit simulation and IC simulation can be used today to simulate IC circuits to the nth degree. However when the initial design is done it is usually done using some identities for the various DC and AC parameters of the MOSFET. This allows fast hand calculations and possibly a sanity check of the results obtained from the more complex models built into the simulator. A set of these identities can be found on the SPG website under engineer’s corner. Interested readers can access these at > engineer’s corner.

The MOS Varactor: An introduction

In many IC designs frequency based trimming or control is required. For instance a filter may need to to be trimmed for corner frequencies. A PLL VCO needs to be controlled by changing the frequency based on its feedback signal. An adaptive equalizer needs to shift its pole-zero configuration. These and many other related applications need a device to be voltage controllable, and offer a change of reactance. The varactor is a useful component that is used frequently to do this. In general varactors are assumed to be junction type devices where the depletion capacitance can be changed to vary the reactance. In CMOS or BiCMOS processes another type of varactor is available, almost as a byproduct of the MOSFET structure. This is the MOS varactor It seems that every CMOS process has the capability to produce a MOS varactor. However, although the varactor is available it may have some limitations of Q and sensitivity. In addition CMOS technology vendors do not characterize or optimize their MOS varactors. This is left to those specialized technology vendors who offer high performance or RF type processes. A recent report on the MOS Varactor is available as an introduction at > engineer’s corner for interested parties.

First order filter parameter calculation algorithms

A recurring problem in ac filter circuit design, is the calculation of attenuation at a particular frequency or conversely the calculation of a frequency given the attenuation. In addition related calculations deal with estimations of time constants and filter parameters such as resistance and capacitance. These calculations play a crucial role in the design of anti-aliasing filters, low pass filters, phase lock loops etc. A paper published recently by the techteam at Signal Processing Group Inc, documents these calculations and provides examples for interested readers, cookbook fashion. The paper is located at >engineer’s corner.

An analog frontend for multiple applications

Signal Processing Group Inc, has released an interesting device ( silicon proven and volume production proven ) for use as a mixed signal controller device. At its input is a two/three wire interface ( a clock, data and latching pins) which is used to communicate with a micro-controller and memory of the users choice. The protocol is much like a I2C protocol. The inputs are digital words which drive currents multiplexed into a set of six outputs. These outputs can be used to drive LEDs ( 50 mA each) or other transducers such as pressure sensors, motors, etc . A feedback TIA ( trans-impedance amplifier) is used to capture an analog feedback signal. This feedback signal is converted to a 10 bit digital word ( conversion time is approximately 100us) and sent via the serial interface to a micro-controller for processing. Looking at these functional blocks it appears that the device would be well suited for feedback control of various micro-systems including automatic lighting control, toys, sensor interface, etc. For further information and a detailed datasheet please go to the SPG website at and use the link to proven IP.