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TL/F/10189
DM54LS502/DM74LS502 8-Bit Successive Approximation Register
April 1992
DM54LS502/DM74LS502
8-Bit Successive Approximation Register
General Description
The LS502 is an 8-bit register with the interstage logic necessary to perform serial-to-parallel conversion and provide
an active LOW Conversion Complete (CC
) signal coincident
with storage of the eighth bit. An active LOW Start (S
) input
performs synchronous initialization which forces Q7 LOW
and all other outputs HIGH. Subsequent clocks shift this Q7
LOW signal downstream which simultaneously backfills the
register such that the first serial data (D input) bit is stored in
Q7, the second bit in Q6, the third in Q5, etc. The serial
input data is also synchronized by an auxiliary flip-flop and
brought out on Q
D
.
Designed primarily for use in the successive approximation
technique for analog-to-digital conversion, the LS502 can
also be used as a serial-to-parallel converter ring counter
and as the storage and control element in recursive digital
routines.
Features
Y
Low power Schottky version of 2502
Y
Storage and control for successive approximation A to
D conversion
Y
Performs serial-to-parallel conversion
Connection Diagram
Dual-In-Line Package
TL/F/10189– 1
Order Number DM54LS502J, DM54LS502W,
DM74LS502WM or DM74LS502N
See NS Package Number J16A, M16B, N16E or W16A
Logic Symbol
TL/F/10189– 2
V
CC
e
Pin 16
GND
e
Pin 8
Pin
Description
Names
D Serial Data Input
S
Start Input (Active LOW)
CP Clock Pulse Input (Active Rising Edge)
Q
D
Synchronized Serial Data Output
CC
Conversion Complete Output (Active LOW)
Q0–Q7 Parallel Register Outputs
Q
7 Complement of Q7 Output
C
1995 National Semiconductor Corporation RRD-B30M105/Printed in U. S. A.
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Absolute Maximum Ratings (Note)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Supply Voltage 7V
Input Voltage 7V
Operating Free Air Temperature Range
DM54LS
b
55§Ctoa125§C
DM74LS 0
§
Ctoa70§C
Storage Temperature Range
b
65§Ctoa150§C
Note:
The ‘‘Absolute Maximum Ratings’’ are those values
beyond which the safety of the device cannot be guaranteed. The device should not be operated at these limits. The
parametric values defined in the ‘‘Electrical Characteristics’’
table are not guaranteed at the absolute maximum ratings.
The ‘‘Recommended Operating Conditions’’ table will define
the conditions for actual device operation.
Recommended Operating Conditions
Symbol Parameter
DM54LS502 DM74LS502
Units
Min Nom Max Min Nom Max
V
CC
Supply Voltage 4.5 5 5.5 4.75 5 5.25 V
V
IH
High Level Input Voltage 2 2 V
V
IL
Low Level Input Voltage 0.7 0.8 V
I
OH
High Level Output Current
b
0.4
b
0.4 mA
I
OL
Low Level Output Current 4 8 mA
T
A
Free Air Operating Temperature
b
55 125 0 70
§
C
ts(H) Setup Time HIGH or LOW 5 16
ns
t
s
(L) S to CP 5 16
th(H) Hold Time HIGH or LOW 5 0
ns
th(L) S to CP 5 0
ts(H) Setup Time HIGH or LOW 5 8
ns
t
s
(L) DtoCP 5 8
th(H) Hold Time HIGH or LOW 5 10
ns
t
h
(L) D to CP 5 10
tw(H) CP Pulse Width HIGH or LOW 20 46
ns
t
w
(L) 20 46
Electrical Characteristics over recommended operating free air temperature range (unless otherwise noted)
Symbol Parameter Conditions Min
Typ
Max Units
(Note 1)
V
I
Input Clamp Voltage V
CC
e
Min, I
I
eb
18 mA
b
1.5 V
V
OH
High Level Output V
CC
e
Min, I
OH
e
Max, DM54 2.5
V
Voltage V
IL
e
Max
DM74 2.7
V
OL
Low Level Output VCCMin, I
OL
e
Max, DM54 0.4
Voltage V
IH
e
Min
DM74 0.5 V
I
OL
e
4 mA, V
CC
e
Min DM74 0.4
I
I
Input Current@Max V
CC
e
Max, V
I
e
7V DM74
0.1 mA
Input Voltage V
I
e
10V DM54
I
IH
High Level Input Current V
CC
e
Max, V
I
e
2.7V 20 mA
I
IL
Low Level Input Current V
CC
e
Max, V
I
e
0.4V
b
0.8 mA
I
OS
Short Circuit V
CC
e
Max DM54
b
20
b
100
mA
Output Current (Note 2)
DM74
b
20
b
100
I
CC
Supply Current V
CC
e
Max 65 mA
Note 1: All typicals are at V
CC
e
5V, T
A
e
25§C.
Note 2: Note more than one output should be shorted at a time, and the duration should not exceed one second.
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Switching Characteristics V
CC
ea
5.0V, T
A
ea
25§C
Symbol Parameter
DM54LS502 DM74LS502
Units
R
L
e
2kX,C
L
e
15 pF
Min Max Min Max
f
max
Maximum Clock Frequency 25 15 MHz
t
PLH
Propagation Delay 35 35
ns
t
PHL
CP to Qnor CC 25 25
Functional Description
The register stages are composed of transparent RS latches arranged in master/slave pairs. The master and slave
latches are enabled separately by non-overlapping complementary signals w1 and w2 derived internally from the CP
input. Master latches are enabled when CP is LOW and
slave latches are enabled when CP is HIGH. Information is
transferred from master to slave, and thus to the outputs, by
the LOW-to-HIGH transition of CP.
Initializing the register requires a LOW signal on S
while
exercising CP. With S
and CP LOW, all master latches are
SET (Q side HIGH). A LOW-to-HIGH CP transition, with S
remaining LOW, then forces the slave latches to the condition wherein Q7 is LOW and all other register outputs, including CC
, are HIGH. This condition will prevail as long as
S
remains LOW, regardless of subsequent CP rising edge.
To start the conversion process, S
must return to the HIGH
state. On the next CP rising edge, the information stored in
the serial data input latch is transferred to Q
D
and Q7, while
Q6 is forced to the LOW state. On the rising edge of the
next seven clocks, this LOW signal is shifted downstream,
one bit at a time, while the serial data enters the register
position one bit behind this LOW signal, as shown in the
Truth Table. Note that after a serial data bit appears at a
particular output, that register position undergoes no further
changes. After the shifted LOW signal reaches CC
, the register is locked up and no further changes can occur until the
register is initialized for the next conversion process.
Figure a
shows a simplified hook-up of a LS502, a D/A converter and a comparator arranged to convert an analog input voltage into an 8-bit binary number by the successive
approximation technique.
Figure b
is an idealized graph
showing the various values that the D/A converter output
voltage can assume in the course of the conversion. The
vertical axis is calibrated in fractions of the full-scale output
capability of the D/A converter and the horizontal axis represents the successive states of the Truth Table. At time t1,
Q7 is LOW and Q6 – Q0 are HIGH, causing the D/A output
to be one-half of full scale. If the analog input voltage is
greater than this voltage the comparator output (hence the
D input of the LS502) will be LOW, and at times t2 the D/A
output will rise to three-fourths of full scale because Q7 will
remain LOW and contribute 50% while Q6 is forced LOW
and contributes another 25%. On the other hand, if the analog input voltage is less than one-half of full scale, the comparator output will be HIGH and Q7 will go HIGH at t2. Q6
will still be forced LOW at t2, and the D/A output will decrease to 25% of full scale. Thus with each successive
clock, the D/A output will change by smaller increments.
When the conversion is completed at t9, the binary number
represented by the register outputs will be the numerator of
the fraction n/256, representing the analog input voltage as
a fraction of the full scale output D/A converter.
TL/F/10189– 4
FIGURE a.
TL/F/10189– 5
FIGURE b.
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Truth Table
Time
Inputs Outputs
t
n
DSQ
D
Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q0 CC
0XLXXXXXXXXXX
1D7HX LHHHHHHHH
2D6HD7D7LHHHHHHH
3D6HD6D7D6LHHHHHH
4D4HD5D7D6D5LHHHHH
5D3HD4D7D6D5D4LHHHH
6 D2 H D3 D7 D6 D5 D4 D3 L H H H
7 D1 H D2 D7 D6 D5 D4 D3 D2 L H H
8 D0 H D1 D7 D6 D5 D4 D3 D2 D1 L H
9 X H D0D7D6D5D4D3D2D1D0 L
10 X H X D7 D6 D5 D4 D3 D2 D1 D0 L
HeHIGH Voltage Level
L
e
LOW Voltage Level
X
e
Immaterial
Logic Diagram
TL/F/10189– 3
Note: Cell logic is repeated for register stages Q5 to Q1.
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Physical Dimensions inches (millimeters)
16-Lead Ceramic Dual-In-Line Package (J)
Order Number DM54LS502J
NS Package Number J16A
16-Lead Wide Small Outline Molded Package (M)
Order Number DM74LS502WM
NS Package Number M16B
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DM54LS502/DM74LS502 8-Bit Successive Approximation Register
Physical Dimensions inches (millimeters) (Continued)
16-Lead Molded Dual-In-Line Package (N)
Order Number DM74LS502N
NS Package Number N16E
16-Lead Ceramic Flat Package (W)
Order Number DM54LS502W
NS Package Number W16A
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