AD707
PRECISION CURRENT TRANSMITTER
R3
100kΩ
R4
100kΩ
R
SCALE
= 10 mV/10 mA = 1
Ω
I
ERROR
due to the AD707C:
Maximum I
ERROR
= 2(V
OS
)/R
SCALE
+ 2(V
OS
Drift)/R
SCALE
+
I
OS
(100 k/R
SCALE
)
6
+V
S
2
7
0.1µF
V
IN
3
AD707
4
0.1µF
R
SCALE
= 2 (15
µV)/l Ω
+2 (0.1
µV/°C)/l Ω
+ 1 nA (100 k)/l
Ω
(1.5 nA @ 125°C)
= 30
µA
+ 0.2
µA/°C
+ 100
µA
(150
µA
@ 125°C)
= 130
µA/10
mA = 1.3% @ 25°C
R
L
I
L
–V
S
R1
100kΩ
R2
100kΩ
t
L
= –––––––
R
SCALE
V
IN
R2
(
–––
)
R1
= 180
µA/10
mA = 1.8% @ 125°C
Low drift, high accuracy resistors are required to achieve high
precision.
Figure 26. Precision Current Source/Sink
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Pin Metal Can
(H-08A)
REFERENCE PLANE
0.185 (4.70)
0.165 (4.19)
0.050
(1.27)
MAX
0.750 (19.05)
0.500 (12.70)
0.250 (6.35)
MIN
8-Pin Plastic DIP
(N-8)
0.430 (10.92)
0.348 (8.84)
8
0.100
(2.54)
BSC
5
1
0.160 (4.06)
0.110 (2.79)
5
4
0.280 (7.11)
0.240 (6.10)
0.325 (8.25)
0.300 (7.62)
0.195 (4.95)
0.115 (2.93)
PIN 1
6
7
0.045 (1.14)
0.027 (0.69)
0.335 (8.51)
0.305 (7.75)
0.370 (9.40)
0.335 (8.51)
4
0.200
(5.08)
BSC
3
2
0.100
(2.54)
BSC
1
0.210 (5.33)
MAX
0.060 (1.52)
0.015 (0.38)
8
0.160 (4.06)
0.115 (2.93)
0.040 (1.02) MAX
0.045 (1.14)
0.010 (0.25)
0.019 (0.48)
0.016 (0.41)
0.021 (0.53)
0.016 (0.41)
0.034 (0.86)
0.027 (0.69)
45
°
BSC
0.130
(3.30)
MIN
0.022 (0.558)
0.070 (1.77) SEATING
PLANE
0.100
0.014 (0.356) (2.54) 0.045 (1.15)
BSC
0.015 (0.381)
0.008 (0.204)
BASE & SEATING PLANE
8-Pin Cerdip
(Q-8)
0.005 (0.13) MIN
0.055 (1.4) MAX
0.1968 (5.00)
0.1890 (4.80)
8
1
5
4
8
PIN 1
1
0.405 (10.29) MAX
0.200
(5.08)
MAX
0.200 (5.08)
0.125 (3.18)
5
0.310 (7.87)
0.220 (5.59)
4
0.320 (8.13)
0.290 (7.37)
0.060 (1.52)
0.015 (0.38)
0.1574 (4.00)
0.1497 (3.80)
0.2440 (6.20)
0.2284 (5.80)
PIN 1
0.0098 (0.25)
0.0040 (0.10)
0.0688 (1.75)
0.0532 (1.35)
0.0196 (0.50)
x 45°
0.0099 (0.25)
SEATING
PLANE
0.150
(3.81)
MIN
0.015 (0.38)
0.008 (0.20)
15
°
0.0500 0.0192 (0.49)
(1.27) 0.0138 (0.35)
BSC
0.0098 (0.25)
0.0075 (0.19)
8°
0°
0.0500 (1.27)
0.0160 (0.41)
0.023 (0.58)
0.014 (0.36)
0.100
(2.54)
BSC
0.070 (1.78)
0.030 (0.76)
0
°
SEATING
PLANE
–8–
REV. B
PRINTED IN U.S.A.
8-Lead SOIC
(SO-8)
C1164a–2–12/95
The AD707’s excellent dc performance, especially the low offset
voltage, low offset voltage drift and high CMRR, makes it
possible to make a high precision voltage-controlled current
transmitter using a variation of the Howland Current Source
circuit (Figure 26). This circuit provides a bidirectional load
current which is derived from a differential input voltage.
The performance and accuracy of this circuit will depend almost
entirely on the tolerance and selection of the resistors. The scale
resistor (R
SCALE
) and the four feedback resistors directly affect
the accuracy of the load current and should be chosen carefully
or trimmed.
As an example of the accuracy achievable, assume I
L
must be
10 mA, and the available V
IN
is only 10 mV.
AD [ ANALOG DEVICES ]
