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AD5545 Datasheet

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PROGRAMMABLE CURRENT SOURCE
Figure 23 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 23, it shows that if the resistor
network is matched, the load current is
(R2 + R3)
IL =
R1
R3
×VREF × D
(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
ZO
=
R1R3(R1+ R2)
R1(R2+ R3)R1(R2
+
R3)
(6)
AD5545/AD5555
If the resistors are perfectly matched, ZO is infinite, which is
desirable, and the resistors behave as an ideal current source.
On the other hand, if they are not matched, ZO 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.
VDD
VREF
U1
VDD RFB
VREF
GND
IOUT
AD5545/AD5555
AD8628
U2
R1' R2'
150k15k
C1
10pF
R1
150k
VDD
U3
V+
AD8510
V–
VSS
R3'
50
R3
50
VL
R2
15k
LOAD IL
02918-0-023
Figure 23. Programmable Current Source with Bidirectional
Current Control and High Voltage Compliance Capabilities
Rev. I | Page 13 of 23
Analog Devices
Analog Devices


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