AD588
Data Sheet
SUBSTITUTING FOR INTERNAL REFERENCES
RESISTANCE TEMPERATURE DETECTOR (RTD)
EXCITATION
Many converters include built-in references. Unfortunately,
such references are the major source of drift in these converters.
By using a more stable external reference like the AD588, drift
performance can be improved dramatically.
The RTD is a circuit element whose resistance is characterized
by a positive temperature coefficient. A measurement of
resistance indicates the measured temperature. Unfortunately, the
resistance of the wires leading to the RTD often adds error to this
measurement. The 4-wire ohms measurement overcomes this
problem. This method uses two wires to bring an excitation
current to the RTD and two additional wires to tap off the resulting
RTD voltage. If these additional two wires go to a high input
impedance measurement circuit, the effect of their resistance is
negligible. They therefore transmit the true RTD voltage.
AD574A 12-BIT ANALOG-TO-DIGITAL CONVERTER
The AD574A is specified for gain drift from 10 ppm/°C to
50 ppm/°C (depending on grade), using its on-chip reference.
The reference contributes typically 75% of this drift. Using an
AD588 as a reference source can improve the total drift by a
factor of 3 to 4.
I = 0
Using this combination can result in apparent increases in
full-scale error due to the difference between the on-board
reference, by which the device is laser-trimmed, and the
external reference, with which the device is actually applied.
The on-board reference is specified to be 10 V ± 100 mV, while
the external reference is specified to be 10 V ± 1 mV. This may
result in up to 101 mV of apparent full-scale error beyond the
±±5 mV specified AD574A gain error. External Resistor R± and
Resistor R3 allow this error to be nulled. Their contribution to
full-scale drift is negligible.
R
R
+
V
α R
RTD
I
RTD
OUT
EXC
–
R
R
I = 0
Figure 34. 4-Wire Ohms Measurement
The high output drive capability allows the AD588 to drive up
to six converters in a multiconverter system. All converters have
gain errors that track to better than ±5 ppm/°C.
28
2
3
4
12√8
CS
STS
27
24
HIGH
BITS
AO
5
6
R/C
CE
23
20
MIDDLE
BITS
7
6
4
3
R1
50Ω
AD574A
A3
A4
1
10 REF IN
19
16
LOW
BITS
R
R3
500Ω
20 TURN
B
8
REF OUT
A1
R1
AD588
14
15
1
7
12 BIPP OFF
+5V
+15V
–15V
R4
R5
R2
61.9Ω
13
14
9
10V
20V
IN
IN
R2
V
IN
10V
11
+V
2
S
R6
ANA COM
DIG 15
COM
R3
A2
16 –V
S
5
9
10
8
12
11
13
Figure 35. AD588/AD574A Connections
Rev. M | Page 16 of 20
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