AD5543(RevA) 데이터 시트보기 (PDF) - Analog Devices

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AD5543
(Rev.:RevA)

Current Output/Serial Input, 16-/14-Bit DAC

Analog Devices 

AD5543/AD5553

Programmable Current Source

Figure 10 shows a versatile V-I conversion circuit using an

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 4 to 20 mA current transmitters with up to 500 Ω

of load. In Figure 10, it can be shown that if the resistor network is

matched, the load current is:

(R2 + R3) / R1

IL =

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 AD8510 can deliver ± 20 mA in both direc-

tions and the voltage compliance approaches 15 V, which is

limited mainly 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:

R1' R3(R1+ R2)

ZO = R1(R2' +R3') – R1'(R2 + R3)

(6)

If the resistors are perfectly matched, ZO is infinite, which is

desirable, and behaves as an ideal current source. On the other

hand, if they are not matched, ZO can be either positive or nega-

tive. Negative 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 few pF.

VDD

VREF

U1

VDD

VREF

RFB

IOUT

GND

AD8628

AD5543/AD5553 U2

R1' R2'

150k⍀ 15k⍀

C1

10pF

R1

150k⍀

VDD

U3

V+

AD8510

V–

VSS

R3'

50⍀

R3

50⍀

VL

R2

15k⍀

LOAD IL

Figure 10. Programmable Current Source with Bidirec-

tional Current Control and High Voltage Compliance

Capabilities

REV. A

–9–

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