AD674B/AD774B
Circuit layout should attempt to locate the ADC, associated
analog input circuitry, and interconnections as far as possible
from logic circuitry. For this reason, the use of wire-wrap circuit
construction is not recommended. Careful printed-circuit layout
and manufacturing is preferred.
UNIPOLAR CALIBRATION
The connections for unipolar ranges are shown in Figure 7. The
AD674B or AD774B is trimmed to a nominal 1/2 LSB offset so
that the exact analog input for a given code will be in the middle
of that code (halfway between the transitions to the codes above
and below it). Thus, when properly calibrated, the first transition
(from 0000 0000 0000 to 0000 0000 0001) will occur for an input
level of +1/2 LSB (1.22 mV for 10 V range).
UNIPOLAR RANGE CONNECTIONS FOR THE AD674B
AND AD774B
The AD674B and AD774B contain all the active components
required to perform a complete 12-bit A/D conversion. Thus,
for most situations, all that is necessary is connection of the
power supplies (+5 V, +12/+15 V, and –12/–15 V), the analog
input, and the conversion initiation command, as discussed on
the next page.
If Pin 12 is connected to Pin 9, the unit will behave in this manner,
within specifications. If the offset trim (R1) is used, it should be
trimmed as above, although a different offset can be set for a
particular system requirement. This circuit will give approximately
15 mV of offset trim range.
The full-scale trim is done by applying a signal 1 1/2 LSB below
the nominal full scale (9.9963 for a 10 V range). Trim R2 to
give the last transition (1111 1111 1110 to 1111 1111 1111).
AD674B/AD774B
OFFSET
R1
100kꢀ
2
3
4
5
STS 28
12/8
CS
A
0
–12V/
–15V
+12V/
+15V
HIGH BITS
BIPOLAR OPERATION
24–27
R/C
GAIN
The connections for bipolar ranges are shown in Figure 8.
Again, as for the unipolar ranges, if the offset and gain specifica-
tions are sufficient, one or both of the trimmers shown can be
replaced by a 50 Ω 1% fixed resistor. The analog input is
applied as for the unipolar ranges. Bipolar calibration is similar
to unipolar calibration. First, a signal 1/2 LSB above negative
full scale (–4.9988 V for the 5 V range) is applied and R1 is
trimmed to give the first transition (0000 0000 0000 to 0000
0000 0001). Then a signal 1 1/2 LSB below positive full scale
(+4.9963 V for the 5 V range) is applied and R2 trimmed to
give the last transition (1111 1111 1110 to 1111 1111 1111).
MIDDLE BITS
CE
6
20–23
R2
100ꢀ
100kꢀ
10 REF IN
LOW BITS
8
REF OUT
16–19
100ꢀ
12 BIP OFF
+5V
1
7
0TO 10V
10V
13
IN
+15V
ANALOG
INPUTS
14 20V
IN
–15V 11
0TO 20V
9
ANA COM
DIG COM 15
Figure 7. Unipolar Input Connections
AD674B/AD774B
All of the thin-film application resistors of the AD674B and
AD774B are factory trimmed for absolute calibration. Therefore,
in many applications, no calibration trimming will be required.
The absolute accuracy for each grade is given in the specification
tables. For example, if no trims are used, 2 LSB max zero offset
error and 0.25% (10 LSB) max full-scale error are guaranteed.
If the offset trim is not required, Pin 12 can be connected directly
to Pin 9; the two resistors and trimmer for Pin 12 are then not
needed. If the full-scale trim is not required, a 50 Ω 1% metal
film resistor should be connected between Pin 8 and Pin 10.
2
3
4
5
STS 28
12/8
CS
HIGH BITS
A
0
24–27
R/C
CE
MIDDLE BITS
6
R2
20–23
GAIN
10 REF IN
100ꢀ
LOW BITS
8
REF OUT
16–19
100ꢀ
R1
ꢂ5V
OFFSET
12 BIP OFF
+5V
1
7
+15V
10V
13
IN
ANALOG
INPUTS
14 20V
–15V 11
IN
The analog input is connected between Pins 13 and 9 for a 0 V
to 10 V input range, between Pins 14 and 9 for a 0 V to 20 V
input range. Input signals beyond the supplies are easily accommo-
dated. For the 10 V span input, the LSB has a nominal value of
2.44 mV; for the 20 V span, 4.88 mV. If a 10.24 V range is
desired (nominal 2.5 mV/bit), the gain trimmer (R2) should be
replaced by a 50 Ω resistor and a 200 Ω trimmer inserted in
series with the analog input to Pin 13 (for a full-scale range of
20.48 V [5 mV/bit] use a 500 Ω trimmer into Pin 14). The
gain trim described below is now done with these trimmers.
The nominal input impedance into Pin 13 is 5 kΩ, and into Pin
14 is 10 kΩ.
ꢂ10V
9
ANA COM
DIG COM 15
Figure 8. Bipolar Input Connections
GROUNDING CONSIDERATIONS
The analog common at Pin 9 is the ground reference point for
the internal reference and is thus the “high quality” ground for
the ADC; it should be connected directly to the analog reference
point of the system. To achieve the high-accuracy performance
available from the ADC in an environment of high digital noise
content, the analog and digital commons must be connected
together at the package. In some situations, the digital common
at Pin 15 can be connected to the most convenient ground ref-
erence point; digital power return is preferred.
–8–
REV. C
ADI [ ADI ]
