Page 128 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 7: Standard Integrated Circuits (ICs)

• CMOS logic has advantage of having smaller dimensions with new coming (usually smaller) Notes
technologies. Thus, the RC constant (RC defines transition time, where R = resistance is
coming from R-ON resistance and C = capacitance is coming from gate capacitance) is
smaller. Smaller RC constant means shorter transition time. Logic is faster and can do more
in same time.
• CMOS devices do not produce as much waste heat as transistor-transistor logic (TTL).
CMOS also allows a high density of logic functions on a chip. It has the characteristics of
high noise immunity.
• One of the benefits of using TTL circuits over CMOS circuits is the fact that they are not
easily damaged by static unlike CMOS devices.

• Due to the output structure of TTL devices, the output impedance is asymmetrical between
the high and low state, making them unsuitable for driving transmission lines. This is usually
solved by buffering the outputs with special line driver devices where signals need to be
sent through cables.
Conventional CMOS devices work over a range of −55°C to +125°C. There
were theoretical indications as early as August 2008 that silicon CMOS will
work down to −233°C (40K).

MEMSIC

t is hard enough to design mixed-signal processing onto the same chip as a Micro-
Electro-Mechanical Systems (MEMS) device, but MEMSIC has managed to integrate these
Itechnologies on the same silicon and sell hundreds of thousands of accelerometers in a
variety of industries.
The company has also overcome two other hurdles: keeping production costs low by sticking to
a standard CMOS IC process, and standardizing development on a single, lean set of EDA tools.
Detecting Acceleration and Motion

Most accelerometers depend on moving mass to determine motion, but MEMSIC differentiates
itself from its competitors through its use of a thermo-mechanical sensor in silicon.

In the centre of the 1mm-square sensor is a heater operating at 100°C above ambient temperature.
Around the heater are symmetrically placed thermopiles for reporting temperature in different
locations. (A thermopile is a series of thermocouples, or temperature-sensing elements,
connected in a series to boost voltage.) The entire sensor is hermetically sealed in an air/
gas cavity, outside of which is analog circuitry for amplification, control, analog-to-digital
conversion and, in the 3-axis models, digital compensation/calibration circuitry.
In the absence of motion, the thermal profile is balanced among the thermopiles, but any motion
or acceleration modifies the convection pattern around the heater, such that the thermopiles in
the direction of the acceleration become hotter than the others. The analog circuitry interprets
the resulting signal changes from the thermopiles as motion and acceleration.
With no moving parts, MEMSIC’s accelerometers are longer-lasting, more reliable, and as
much as 25 times more shock-resistant (up to 100,000 g) than their mechanical counterparts
for measuring tilt, inclination, shock, or vibration. The chips appear in such products as car

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