AD674B/AD774B
CIRCUIT OPERATION
DRIVING THE ANALOG INPUT
The AD674B and AD774B are complete 12-bit monolithic A/D
converters that require no external components to provide the
complete successive-approximation analog-to-digital conversion
function. A block diagram is shown in Figure 5.
The AD674B and AD774B are successive-approximation analog-
to-digital converters. During the conversion cycle, the ADC input
current is modulated by the DAC test current at approximately
a 1 MHz rate. Thus it is important to recognize that the signal
source driving the ADC must be capable of holding a constant
output voltage under dynamically changing load conditions.
5V SUPPLY
STATUS
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
V
LOGIC
STS
DATA MODE SELECT
12/8
CHIP SELECT
CS
BYTE ADDRESS/
SHORT CYCLE A
MSB
N
DB11 (MSB)
FEEDBACKTO AMPLIFIER
V+
Y
B
B
L
CONTROL
3
DB10
DB9
3
4
S
T
A
T
E
0
E
READ/CONVERT R/C
5
DB8
12
CLOCK
SAR
COMP
A
CHIP ENABLE
CE
12V/15V SUPPLY
6
DB7
N
Y
B
B
L
–
ADC
O
U
T
P
U
T
7
DB6
+
V
DIGITAL
DATA
OUTPUTS
CURRENT
LIMITING
RESISTORS
CC
10V
I DAC
10V REFERENCE
REF
8
DB5
REF OUT
E
I
R
IN
IN
CURRENT
OUTPUT
DAC
ANALOG COMMON
AC
9
DB4
B
I
I
REFERENCE INPUT
N
Y
B
B
L
DIFF
B
U
F
TEST
10
11
12
13
14
DB3
IIN IS MODULATED BY
REF IN
I REF
COMPARATOR
–12V/–15V SUPPLY
CHANGES IN TEST CURRENT.
AMPLIFIER PULSE LOAD
199.95
kꢀ
+
DB2
F
V
EE
–
E
R
S
BIPOLAR OFFSET
RESPONSE LIMITED BY
E
DB1
BIPOFF
OPEN-LOOP OUTPUT IMPEDANCE.
DAC
V–
10V SPAN INPUT
VEE
C
N
DB0 (LSB)
10V
IN
LSB
SAR
ANALOG COMMON
20V SPAN INPUT
DIGITAL
COMMON DC
VOLTAGE
DIVIDER
20V
IN
AD674B/AD774B
Figure 6. Op Amp—ADC Interface
Figure 5. Block Diagram of AD674B and AD774B
The closed-loop output impedance of an op amp is equal to the
open-loop output impedance (usually a few hundred ohms)
divided by the loop gain at the frequency of interest. It is often
assumed that the loop gain of a follower-connected op amp is
sufficiently high to reduce the closed-loop output impedance to
a negligibly small value, particularly if the signal is low fre-
quency. However, the amplifier driving the ADC must either
have sufficient loop gain at 1 MHz to reduce the closed-loop
output impedance to a low value or have low open-loop output
impedance. This can be accomplished by using a wideband op
amp, such as the AD711.
When the control section is commanded to initiate a conversion
(as described later) it enables the clock and resets the
successive-approximation register (SAR) to all zeroes. Once a
conversion cycle has begun, it cannot be stopped or restarted
and data is not available from the output buffers. The SAR,
timed by the clock, will sequence through the conversion cycle
and return an end-of-convert flag to the control section. The
control section will then disable the clock, bring the output
status flag low, and enable control functions to allow data read
by external command.
If a sample-hold amplifier is required, the monolithic AD585 or
AD781 is recommended, with the output buffer driving the
AD674B or AD774B input directly. A better alternative is the
AD1674, which is a 10 µs sampling ADC in the same pinout as the
AD574A, AD674A, or AD774B and is functionally equivalent.
During the conversion cycle, the internal 12-bit current output
DAC is sequenced by the SAR from the most significant bit
(MSB) to least significant bit (LSB) to provide an output cur-
rent that accurately balances the input signal current through
the divider network. The comparator determines whether the
addition of each successively weighted bit current causes the
DAC current sum to be greater or less than the input current; if
the sum is less, the bit is left on; if more, the bit is turned off.
After testing all the bits, the SAR contains a 12-bit binary code
that accurately represents the input signal to within 1/2 LSB.
SUPPLY DECOUPLING AND LAYOUT
CONSIDERATION
It is critical that the power supplies be filtered, well regulated,
and free from high-frequency noise. Use of noisy supplies will
cause unstable output codes. Switching power supplies is not
recommended for circuits attempting to achieve 12-bit accuracy
unless great care is used in filtering any switching spikes present
in the output. Few millivolts of noise represent several counts of
error in a 12-bit ADC.
The temperature-compensated reference provides the primary
voltage reference to the DAC and guarantees excellent stability
with both time and temperature. The reference is trimmed to
10.00 V 1%; it can supply up to 2.0 mA to an external load in
addition to the requirements of the reference input resistor
(0.5 mA) and bipolar offset resistor (0.5 mA). Any external load
on the reference must remain constant during conversion. The
thin-film application resistors are trimmed to match the full-
scale output current of the DAC. The input divider network
provides a 10 V or 20 V input range. The bipolar offset resistor
is grounded for unipolar operation and connected to the 10 V
reference for bipolar operation.
Decoupling capacitors should be used on all power supply pins;
the 5 V supply decoupling capacitor should be connected directly
from Pin 1 to Pin 15 (digital common) and the +VCC and –VEE
pins should be decoupled directly to analog common (Pin 9). A
suitable decoupling capacitor is a 4.7 µF tantalum type in paral-
lel with a 0.1 µF ceramic disc type.
REV. C
–7–
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