REV. A

AD5543/AD5553

–9–

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:

I

RR

R

R

VD

L

REF

=

+

()

××

23

1

3

/

(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:

Z

RR R

R

RR

R

R

R

R

+

()

+

()

+

()

13

1

2

12

3

1

2

3

'

'' –

'

(6)

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

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.

U2

U1

AD5543/AD5553

GND

AD8628

AD8510

V+

V–

LOAD

U3

R3

50

C1

10pF

R3'

50

R2

15k

R1

150k

R2'

15k

R1'

150k

Figure 10. Programmable Current Source with Bidirec-

tional Current Control and High Voltage Compliance

Capabilities