2. Assume two logic families have the following D.C. characteristics: Logic Fami
ID: 2248121 • Letter: 2
Question
2. Assume two logic families have the following D.C. characteristics: Logic Family "A” VOH 4.2 V | VOL-0.45 V | VIH3.60 V | VIL-1.60 V Logic Family "B" VOH = 3.7 V | Vol. = 0.37 V | VIH = 2.45 V | VIL-1.45 V (a) Calculate the following (show work): · (LO 1-14) DCNM A-B · (LO 1-14) DCNM B-A . (LO 1-20) Practical Fanout AB LO 1-20) Practical Fanout B-+A (b) Draw the circuit and calculate the value of the current limiting resistor for a Type “A" gate driving an LED to the maximum brightness possible in a current sourcing configuration. Assume Vled is 1.5V. (LO 1-21) (c) Draw the circuit and calculate the value of the current limiting resistor for a Type “B” gate driving an LED to the maximum brightness possible in a current sinking configuration. Assume Vled is 1.5V. (LO 1-21)Explanation / Answer
DCNM - Direct Current Noise Margin:
DC margin is a measure of noise that is needed to corrupt a worst-case output voltage into a value that can not be recognized properly by the input.
ie.The maximum DC voltage amplitude that shall be added algebraically to the noise-free worst-case input without causing the output voltage to deviate from the allowable voltage level.
the formula for calculating the noise margin is given by,
DCNM = min { Vohmin - VIhmin , VILmax - V0Lmax }
A.1) calculate DCNM A-B
from the given data,
Vohmin = Voh ( in family A) = 4.2 v
VIhmin = VIh ( in family B) = 2.45 v
VILmax = VIL ( in family B) = 1.45 v
V0Lmax = V0L (in family A) = 0.45 v
ie when DCNM A-B, the output voltages in formula (Vohmin,V0Lmax) are taken from family A and the input voltages in formula (VIhmin,VILmax) are taken from family B.
DCNM A-B = min { 4.2 - 2.45 , 1.45-0.45 }
= min { 1.75 , 1.0 }
= 1.0
A.2) calculate DCNM B-A
from the given data,
Vohmin = Voh ( in family B) = 3.7 v
VIhmin = VIh ( in family A) = 3.60 v
VILmax = VIL ( in family A) = 1.60 v
V0Lmax = V0L (in family B) = 0.37 v
ie when DCNM B-A , the output voltages in formula (Vohmin,V0Lmax) are taken from family B and the input voltages in formula (VIhmin,VILmax) are taken from family A.
DCNM A-B = min { 3.70 - 3.60 , 1.60 - 0.37 }
= min {0.10 , 1.23 }
= 0.10
Fanout:
The Fan out is defined as the number of gate inputs that a gate output can drive without exceeding the worstcase loading specifications.
Fan-out depends on the characteristic of both the output devices and the inputs driven.
a practical fan-out is the minimum of the HIGH and LOW state fan-outs.
Practical Fan-out = min { IOHmax / IIH , IOLmax / IIL }
A.3) calculate Practical Fan-out A-B
from the given data,
IOHmax = IOH ( in family A) = -8 mA
IIH = IIH ( in family B) = 42 uA = 0.042 mA
IOLmax = IOL ( in family A) = 8 mA
IIL = IIL (in family B) = -0.42 mA
ie when DCNM A-B, the output currents in formula (IOHmax , IOLmax ) are taken from family A and the input currents in formula (IIH , IIL) are taken from family B.
Practical Fan-out = min { IOHmax / IIH , IOLmax / IIL }
= min { -8 / 0.042 ,8 / -0.42 }
= min { -190.476 ,-19.047 }
= -19.047
~= 20 // fan out is a absolute number without sign
A.4) calculate Practical Fan-out B-A
from the given data,
IOHmax = IOH ( in family B) = -0.550 mA
IIH = IIH ( in family A) = 0.3 uA =0.0003 mA
IOLmax = IOL ( in family B) = 8.5 mA
IIL = IIL (in family A) = -0.25uA = -0.00025 mA
ie when DCNM A-B, the output currents in formula (IOHmax , IOLmax ) are taken from family A and the input currents in formula (IIH , IIL) are taken from family B.
Practical Fan-out = min { IOHmax / IIH , IOLmax / IIL }
= min { -0.550 / 0.0003 ,8.5 / -0.00025 }
= min { -1833.33 ,-34000 }
= -1833.33
~= 1833 // fan out is a absolute number without sign
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