AD574A
+V
S
TO A1
V
REF
AGND
+V
S
14 13 12 11 10
9
8
R2
100Ω
GAIN
+15V
R1
100k
OFFSET
C1
A
+
+
+
C3
+15V
+5V
–15V
page. Analog input connections and calibration are easily ac-
complished; the unipolar operating mode is shown in Figure 4.
2 12/8
STS 28
HIGH 27
BIT 24
MIDDLE 23
BITS 20
–V
S
TO A1
C2
11 1
8
10
13
R4
100k
12
9
3
4
15
A
2 6 7
OFFSET
R1
100k
–12V/–15V
+12V/+15V
3 CS
4 A
O
5 R/C
6 CE
GAIN
10k 10k
100pF
A1
AD585
A2
27
16
12-BIT
3-STATE
DATA
AD574A
100k
AD574A
10 REF IN
8 REF OUT
12 BIP OFF
1
2
3
4
5
6
7
R3
–15V 100Ω
R2
100Ω
100Ω
LOW 19
BITS 16
–V
S
ANALOG
INPUT
0V TO +10V
CONVERT
7404 OR EQ.
28
5
0 TO +10V
+5V
13 10V
IN
ANALOG
INPUTS
0 TO +20V
9 ANA COM
DIG COM 15
+15V
14 20V
IN
–15V
7
11
1
STATUS
NOTE
1. C1, C2, C3 ARE 47
µ
F TANTALUM, BYPASSED BY
0.1
µ
F CERAMIC. LOCATE AT ASSOCIATED A2 PINS.
Figure 3. AD574A with AD585 Sample and Hold
SUPPLY DECOUPLING AND LAYOUT
CONSIDERATIONS
Figure 4. Unipolar Input Connections
It is critically important that the AD574A power supplies be fil-
tered, well regulated, and free from high frequency noise. Use of
noisy supplies will cause unstable output codes. Switching
power supplies are not recommended for circuits attempting to
achieve 12-bit accuracy unless great care is used in filtering any
switching spikes present in the output. Remember that a few
millivolts of noise represents several counts of error in a 12-bit
ADC.
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 +V
CC
and –V
EE
pins should be decoupled directly to analog common
(Pin 9). A suitable decoupling capacitor is a 4.7
µF
tantalum
type in parallel with a 0.1
µF
disc ceramic type.
Circuit layout should attempt to locate the AD574A, 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 con-
struction is preferred.
GROUNDING CONSIDERATIONS
All of the thin-film application resistors of the AD574A are
trimmed for absolute calibration. Therefore, in many applica-
tions, 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, the AD574AK guarantees
±
1 LSB max zero offset error and
±
0.25% (10 LSB) max
full-scale error. (Typical full-scale error is
±
2 LSB.) 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 needed, a 50
Ω ±
1% metal film resistor
should be connected between Pin 8 and Pin 10.
The analog input is connected between Pin 13 and Pin 9 for a
0 V to +10 V input range, between 14 and Pin 9 for a 0 V to
+20 V input range. The AD574A easily accommodates an input
signal beyond the supplies. For the 10 volt span input, the LSB
has a nominal value of 2.44 mV; for the 20 volt 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
trim-
mer 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 10 kΩ into Pin 14.
UNIPOLAR CALIBRATION
The analog common at Pin 9 is the ground reference point for
the internal reference and is thus the “high quality” ground for
the AD574A; it should be connected directly to the analog refer-
ence point of the system. In order to achieve all of the high
accuracy performance available from the AD574A in an envi-
ronment of high digital noise content, the analog and digital
commons should be connected together at the package. In some
situations, the digital common at Pin 15 can be connected to
the most convenient ground reference point; analog power
return is preferred.
UNIPOLAR RANGE CONNECTIONS FOR THE AD574A
The AD574A is intended to have 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, 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).
If Pin 12 is connected to Pin 9, the unit will behave in this man-
ner, 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 ap-
proximately
±
15 mV of offset trim range.
The AD574A contains all the active components required to
perform a complete 12-bit A/D conversion. Thus, for most situ-
ations, all that is necessary is connection of the power supplies
(+5 V, +12 V/+15 V and –12 V/–15 V), the analog input, and
the conversion initiation command, as discussed on the next
REV. B
–7–
AD [ ANALOG DEVICES ]
