Datasheet

Data Sheet AD5545/AD5555
Rev. G | Page 13 of 24
PROGRAMMABLE CURRENT SOURCE
Figure 24 shows a versatile V-to-I conversion circuit using
improved Howland Current Pump. In addition to the precision
current conversion it provides, this circuit enables a bidirec-
tional current flow and high voltage compliance. This circuit
can be used in a 4 mA to 20 mA current transmitter with up to
a 500 Ω of load. In Figure 24, it shows that if the resistor
network is matched, the load current is
( )
D
V
R3
R1
R3
R2
I
REFL
×
×
+
=
(5)
R3, in theory, can be made small to achieve the current needed
within the U3 output current driving capability. This circuit is
versatile such that the AD8510 can deliver ±20 mA in both
directions, and the voltage compliance approaches 15 V, which
is mainly limited by the supply voltages of U3. However, users
must pay attention to the compensation. Without C1, it can be
shown that the output impedance becomes
(
)
( )
( )
R3R2
1R3
R2RR1
R2R1R3
1R
Z
O
+
′′
+
′
+
′
=
–
(6)
If the resistors are perfectly matched, Z
O
is infinite, which is
desirable, and the resistors behave as an ideal current source.
On the other hand, if they are not matched, Z
O
can be either
positive or negative. The latter can cause oscillation. As a result,
C1 is needed to prevent the oscillation. For critical applications,
C1 could be found empirically but typically falls in the range of
a few picofarads.
AD5545/AD5555
AD8628
AD8510
V
REF
V
REF
I
OUT
V
DD
V
DD
V
DD
C1
10pF
V
SS
LOAD
GND
0
291
8-0-023
V
L
I
L
R
FB
U2
U3
U1
V+
V–
R3'
50Ω
R1'
150kΩ
R2'
15k
Ω
R1
150kΩ
R2
15k
Ω
R3
50
Ω
Figure 24. Programmable Current Source with Bidirectional
Current Control and High Voltage Compliance Capabilities