Wednesday 15 May 2013
CHARACTERISTICS OF A LOGIC FAMILY
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CURRENT LEVELS
IIH – High level input current: The current that
flows into an input when a specied high-level voltage is applied to that input
IIL - Low level input current: The current that
flows into an input when a specified low-level voltage is applied to that input
IOH – High level output current: The current that
flows from an output in the logical 1 state under specified load conditions
IOL– Low level output current: The current that
flows from an output in the logical 0 state under specified load conditions
VOLTAGE LEVELS
VIH - High level input voltage : The minimum
voltage level required for a logical 1 at an input.
VIL - Low level input voltage : The maximum voltage
level required for a logical 0 at an input
VOH - High level output voltage : The minimum
voltage level at logical circuit output in logical 1 state under specified load
conditions
VOL - High level output voltage : The maximum
voltage level at logical circuit output in logical 0 state under specified load
conditions
FAN OUT
The maximum number of standard logic inputs that an output
can drive reliably. If this number is exceeded, the output logic-level voltage
cannot be guaranteed.
PROPAGATION DELAY
There are two types of propagation delay
tPLH : delay time in going from logical 0 to
logical 1 state (LOW to HIGH)
tPHL : delay time in going from logical 1 to
logical 0 state (HIGH to LOW)
POWER DISSIPATION
It is the product of input voltage and input current
PD (avg ) = ICC( avg ) * VCC
ICC( avg ) = (ICCH +ICCL)/2
ICCH = the current drain when all the gate
outputs is HIGH
ICCL = the current drain when all the gate
outputs is LOW
NOISE IMMUNITY
It is defined as the ability of the circuit to tolerate
noise without causing spurious changes in output voltage.
Stray electric and magnetic fields can induce voltages on
the connecting wires between logic circuits. These unwanted spurious signals
are called noise and sometimes cause voltage at the input to a logic circuit to
drop below VIH(min) or rise above VIL(max)
HIGH state noise margin VNH = VOH (
min) – VIH (min)
LOW state noise margin VNL = VIL(max)
– VOL (max)
CURRENT SOURCING
When output of gate 1 is at HIGH state, it supplies a
current IIH to the input of gate 2 which acts as a resistance to ground.
Therefore output of gate 1 is acting as a source of current for gate 2.
CURRENT SINKING
When
output of gate 1 is at LOW state, it supplies a current IIL to the output
of gate 1 which acts as a resistance to ground. Therefore output of gate 1 is
acting as a sink of current for gate 2. 
Speed-Power product is called figure of merit
OPERATING
TEMPERATURE
The temperature
range in which an IC functions properly is a very important parameter. The
accepted temperature ranges are: 0 to +700C for consumer and industrial
applications whereas -55 0C to +1250C for military
applications.
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