ADADC71
Rev. C | Page 7 of 12
DESCRIPTION OF OPERATION
On receipt of a CONVERT START command, the ADADC71
converts the voltage at its analog input into an equivalent 16-bit
binary number. This conversion is accomplished as follows: the
16-bit successive-approximation register (SAR) has its 16-bit
outputs connected both to the device bit output pins and to the
corresponding bit inputs of the feedback DAC. The analog
input is successively compared to the feedback DAC output, one
bit at a time (MSB first, LSB last). The decision to keep or reject
each bit is then made at the completion of each bit comparison
period, depending on the state of the comparator at that time.
GAIN ADJUSTMENT
The gain adjustment circuit consists of a 100 ppm/
o
C poten-
tiometer connected across ±V
S
with its slider connected
through a 510 kΩ resistor to Pin 29 (GAIN ADJUST), as shown
in
Figure 5.
If no external trim adjustment is desired, Pin 27
(COMPARATOR IN) and Pin 29 may be left open.
03537-005
ADADC71
29
0.01μF
270kΩ
+15V
10kΩ
TO
100kΩ
100ppm/°C
–15V
Figure 5. Gain Adjustment Circuit
ZERO OFFSET ADJUSTMENT
The zero offset adjustment circuit consists of a 100 ppm/
o
C
potentiometer connected across ±V
S
with its slider connected
through a 1.8 MΩ resistor to Pin 27 for all ranges. As shown in
Figure 6, the tolerance of this fixed resistor is not critical; a
carbon composition type is generally adequate. Using a carbon
composition resistor with a −1200 ppm/
o
C temperature
coefficient contributes a worst-case offset temperature
coefficient of 32 LSB
B
14
× 61 ppm/ LSB
14
× 1200 ppm/ C =
2.3 ppm/ C of FSR, if the offset adjustment potentiometer is set
at either end of its adjustment range. Since the maximum offset
adjustment required is typically no more than ±16 LSB
o
o
14
, use of
a carbon composition offset summing resistor typically
contributes no more than 1 ppm/ C of FSR offset temperature
coefficient.
o
03537-006
ADADC71
27
1.8MΩ
+15V
10kΩ
TO
100kΩ
–15V
Figure 6. Zero Offset Adjustment Circuit
An alternate offset adjustment circuit, which contributes
negligible offset temperature coefficient if metal film resistors
(temperature coefficient < 100 ppm/
o
C) are used, is shown in
Figure 7.
In either adjustment circuit, the fixed resistor connected to
Pin 27 should be located close to this pin to keep the pin
connection runs short. Pin 27 is quite sensitive to external noise
pick-up.
03537-007
ADADC71
27
22kΩ M.F.
180kΩ M.F.
180kΩ M.F.
+15V
10kΩ
TO
100kΩ
OFFSET
ADJ
–15V
Figure 7. Low Temperature Coefficient Zero Adjustment Circuit
TIMING
The timing diagram is shown in Figure 8. Receipt of a
CONVERT START signal sets the STATUS flag, indicating
conversion in progress. This in turn removes the inhibit applied
to the gated clock, permitting it to run through 17 cycles. All
the SAR parallel bits, STATUS flip-flops, and the gated clock
inhibit signal are initialized on the trailing edge of the
CONVERT START signal. At time t
0
, B
1
is reset and B
2
to B
16
are
set unconditionally. At t
1
the Bit 1 decision is made (keep) and
Bit 2 is reset unconditionally. This sequence continues until the
Bit 16 (LSB) decision (keep) is made at t
16
. The STATUS flag is
reset, indicating that the conversion is complete and that the
parallel output data is valid. Resetting the STATUS flag restores
the gated clock inhibit signal, forcing the clock output to the
low Logic 0 state. Note that the clock remains low until the next
conversion.
Corresponding parallel data bits become valid on the same
positive-going clock edge.
03537-008
t
0
t
1
t
2
t
3
t
4
t
5
t
6
t
7
t
8
t
9
t
10
t
11
t
12
t
13
t
14
t
15
t
16
t
17
(4)
(3)
(1)
01 1 0 01 110 1 11 1 0 10
0
1
1
0
0
1
1
1
0
1
1
1
1
0
1
0
MSB
STATUS
INTERNAL
CLOCK
CONVERT
START
BIT 2
BIT 3
BIT 4
BIT 5
BIT 6
BIT 7
BIT 8
BIT 9
BIT 10
BIT 11
BIT 12
BIT 13
BIT 14
BIT 15
LSB
LSBMSB
MAXIMUM THROUGHPUT TIME
CONVERSION TIME (2)
NOTES:
1. THE CONVERT START PULSEWIDTH IS 50ns MIN AND MUST REMAIN LOW DURING A
CONVERSION. THE CONVERSION IS INITIATED BY THE TRAILING EDGE OF THE
CONVERT COMMAND.
2. 50μs FOR 14 BITS AND 45μs FOR 13 BITS (MAX).
2. MSB DECISION.
3. CLOCK REMAINS LOW AFTER LAST BIT DECISION.
Figure 8. Timing Diagram (Binary Code 0110011101111010)
