AD9631 datasheet(13/24 Pages) AD

13 / 24 page

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

(

)

REF

(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

(

)

(

)

(

)

′

–

(6)

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.

AD5545/AD5555

AD8628

AD8510

VREF

IOUT

VDD

10pF

VSS

LOAD

GND

02918- 0- 023

RFB

V+

V–

R3'

Ω

R1'

150k

Ω

R2'

15k

Ω

150k

Ω

15k

Ω

Figure 24. Programmable Current Source with Bidirectional

Current Control and High Voltage Compliance Capabilities

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