Datasheet
AD588
Rev. L | Page 16 of 20
SUBSTITUTING FOR INTERNAL REFERENCES
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.
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.
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
±25 mV specified AD574 gain error. External Resistor R2 and
Resistor R3 allow this error to be nulled. Their contribution to
full-scale drift is negligible.
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.
RESISTANCE TEMPERATURE DETECTOR (RTD)
EXCITATION
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.
I
EXC
R
R
I = 0
RTD
V
OUT
α R
RTD
+
–
I = 0
R
R
00531-034
Figure 34. 4-Wire Ohms Measurement
12√8STS
00531-035
R3
R
B
R1
R2
R4
R5
R6
A1
A4
AD588
A3
AO
CE
REF IN
REF OUT
BIPP OFF
10V
IN
20V
IN
ANA COM
HIGH
BITS
MIDDLE
BITS
LOW
BITS
+5V
+15V
–15V
DIG
COM
AD574A
–V
S
+V
S
R2
61.9Ω
R1
50Ω
R3
500Ω
20 TURN
V
IN
10V
CS
R/C
15
11
7
1
16
19
20
23
24
27
9
14
13
12
8
10
6
5
4
3
16
2
15
14
1
13111281095
7643
28
2
A2
Figure 35. AD588/AD574A Connections