2
REV.G
AD7711
–27–
3-Wire RTD Configurations
One possible 3-wire configuration using the AD7711 is outlined
in Figure 20. In the 3-wire configuration, the lead resistances
will result in errors if only one current source is used because
the 200 mA will flow through R
L1
, developing a voltage error
between AIN1(+) and AIN1(–). In the scheme outlined below,
the second RTD current source is used to compensate for the
error introduced by the 200 mA flowing through R
L1
. The second
RTD current flows through R
L2
. Assuming R
L1
and R
L2
are
equal (the leads would normally be of the same material and of
equal length) and RTD1 and RTD2 match, then the error voltage
across R
L2
equals the error voltage across R
L1
and no error voltage
is developed between AIN1(+) and AIN1(–). Twice the voltage
is developed across R
L3
but because this is a common-mode
voltage, it will not introduce any errors. The circuit in Figure 20
shows the reference voltage for the AD7711 derived from the
part’s own internal reference.
V
SS
DGND
ANALOG 5V SUPPLY
AV
DD
DV
DD
REF IN(+) REF OUT
2.5V
REFERENCE
REF IN(–)
200A
RTD1
PGA
A = 1–128
AIN1(+)
AIN1(–)
INTERNAL
CIRCUITRY
200A
AD7711
AGND
R
L1
R
L2
R
L3
RTD2
RTD
Figure 20. 3-Wire RTD Application with the AD7711
The circuit in Figure 21 shows an alternate 3-wire configura-
tion. In this case, the circuit has the same benefits in terms of
eliminating lead resistance errors as outlined in Figure 20, but it
has the additional benefit that the reference voltage is derived
from one of the current sources. This gives all the benefits of
eliminating RTD tempco errors as outlined in Figure 19. The
voltage on either RTD input can go to within 2 V of the AV
DD
supply. The circuit is shown for a 2.5 V reference.
V
SS
DGND
AV
DD
DV
DD
REF IN(+)
200A
RTD1
PGA
A = 1–128
AIN1(+)
AIN1(–)
INTERNAL
CIRCUITRY
200A
AD7711
AGND
R
L1
R
L2
R
L3
RTD2
REF IN(–)
12.5k⍀
RTD
Figure 21. Alternate 3-Wire Configuration
OUTLINE DIMENSIONS
24-Lead Plastic Dual In-Line Package [PDIP]
(N-24)
Dimensions shown in inches and (millimeters)
24
1
12
13
1.185 (30.01)
1.165 (29.59)
1.145 (29.08)
0.295 (7.49)
0.285 (7.24)
0.275 (6.99)
0.150 (3.81)
0.135 (3.43)
0.120 (3.05)
0.015 (0.38)
0.010 (0.25)
0.008 (0.20)
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
SEATING
PLANE
0.015 (0.38) MIN
0.180
(4.57)
MAX
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
0.150 (3.81)
0.130 (3.30)
0.110 (2.79)
0.100
(2.54)
BSC
0.060 (1.52)
0.050 (1.27)
0.045 (1.14)
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
COMPLIANT TO JEDEC STANDARDS MO-095AG
