AD588TQ Analog Devices, AD588TQ Datasheet - Page 12

• Manufacturer Part Number: AD588TQ
• Description: High Precision Voltage Reference
• Manufacturer: Analog Devices

AD588
12-Bit Analog-to-Digital Converter—AD574A
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. Therefore,
the total drift using an AD588 to supply the reference can be
improved by a factor of 3 to 4.
Using this combination may 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 resistors R2 and 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 will
have gain errors that track to better than ± 5 ppm/°C.
RTD EXCITATION
The resistance temperature detector (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
R3
R
B
R2
A2
A1
R1
R4
R5
Figure 22. AD588/AD574A Connections
R6
AD588
A3
A4
–12–
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. Therefore, they
transmit the true RTD voltage.
A practical consideration when using the 4-wire ohms technique
with an RTD is the self-heating effect that the excitation current
has on the temperature of the RTD. The designer must choose
the smallest practical excitation current that still gives the desired
resolution. RTD manufacturers usually specify the self-heating
effect of each of their models or types of RTDs.
Figure 24 shows an AD588 providing the precision excitation
current for a 100 Ω RTD. The small excitation current of 1 mA
dissipates a mere 0.1 mW of power in the RTD.
–V
+V
S
V
10V
S
IN
20 TURN
500
R3
61.9
R2
I
EXC
50
R1
Figure 23. 4-Wire Ohms Measurement
AO
R/C
CE
REF IN
REF OUT
BIPP OFF
10V
20V
ANA COM
12 8
CS
R
R
IN
IN
AD574A
MIDDLE
RTD
HIGH
+15V
BITS
BITS
LOW
BITS
–15V
COM
STS
+5V
DIG
I = 0
I = 0
R
R
V
OUT
+
–
R
RTD
REV. D