Voltage Transfer Characteristic (VTC) for logic inverters have been standardized. VTC is
the graph between Vout and VIN.
On vertical axis, VOH and VOL correspond to output
high and output low voltage levels respectively. On the horizontal axis, VIL is
input low voltage and VIH is the input high voltage.
As the input voltage is increased from 0V, VIL is the
maximum input voltage that provides a high output voltage (logical 1 output).
VIH is the minimum input voltage that provides a low
output voltage (logical 0 output). VOH, VOL, VIL and VIH are referred to as the
critical voltages of the VTC.
VOH > VIH
VOL < VIL
Midpoint
Voltage:
Sometimes referred as Threshold voltage (Vth). The
voltage at which Vout = VIN on VTC is referred as Midpoint voltage. Midpoint
voltage can be found graphically by superimposing (the unity slope) Vout = VIN
and finding its intersection with the
VTC.
Logic
Swing and Transition Width
Logic
Swing
The magnitude of voltage difference between the
output high and low voltage levels.
VLS = VOH – VOL
Transition
Width
The amount of voltage change that is required of the
input voltage to cause a change in the output voltage from the high to the low
level (and vice versa).
VTW = VIH - VIL
Noise
in Digital Circuits
Noise
Variations in the steady-state voltage levels of
digital circuits (i.e. Logical 1 and logical 0 states) are undesirable and
cause logic errors. This variation is termed as Noise.
Noise
Margins
Voltage Noise Margin represents the safety margin
for the high and low voltage levels.
Noise voltages must have magnitudes less than the
voltage noise margins.
VNMH = VOH – VIH
VNML = VIL – VOL
Noise
Sensitivities
The effects of input variations are quantified in
terms
of the noise sensitivities. The high noise sensitivity
is defined as the difference
between input and midpoint voltage for VIN at VOH.
The low noise sensitivity is defined as the
difference between input and midpoint voltage for VIN at VOL.
VNSH = VOH – VM
VNSL = VM - VOL
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