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AD5280/AD5282

Rev. C | Page 22 of

PROGRAMMABLE CURRENT SOURCE

+5V

102Ω

V+

REF191

GND

AD5280

1µF

0 TO (2.048 + VL)

VOUT

VIN

100Ω

V–

OP8510

–2.048V TO VL

SLEEP

OP8510

Figure 61. Programmable Current Source

A programmable current source can be implemented with the

circuit shown in Figure 61. REF191 is a unique, low supply

headroom and high current handling precision reference that

can deliver 20 mA at 2.048 V. The load current is simply the

voltage across Terminal B to Terminal W of the digital

potentiometer divided by RS.

REF

(8)

The circuit is simple, but attention must be paid to two things.

First, dual-supply op amps are ideal because the ground

potential of REF191 can swing from −2.048 V at zero scale to VL

at full scale of the potentiometer setting. Although the circuit

works under single supply, the programmable resolution of the

system is reduced.

For applications that demand higher current capabilities, a

few changes to the circuit in Figure 61 produce an adjustable

current in the range of hundreds of milliamps. First, the voltage

reference needs to be replaced with a high current, low dropout

regulator, such as the ADP3333, and the op amp needs to be

swapped with a high current dual-supply model, such as the

AD8532. Depending on the desired range of current, an

appropriate value for RS must be calculated. Because of the high

current flowing to the load, the user must pay attention to the

load impedance so as not to drive the op amp beyond the

positive rail.

PROGRAMMABLE BIDIRECTIONAL CURRENT

SOURCE

AD5280

+5V

150kΩ

R2A

14.95kΩ

R2B

50kΩ

R2I

15kΩ

10pF

R1I

150kΩ

500kΩ

+15V

V+

OP2177

V–

V+

V–

–5V

–15V

Figure 62. Programmable Bidirectional Current Source

For applications that require bidirectional current control or

higher voltage compliance, a Howland current pump can be a

solution (see Figure 62). If the resistors are matched, the load

current is

(

)

(9)

In theory, R2B can be made as small as needed to achieve the

current needed within the A2 output current driving capability.

In this circuit, the OP2177 can deliver ±5 mA in either direction,

and the voltage compliance approaches 15 V. It can be shown

that the output impedance is

(

)

()

R1'

R2'

−

(10)

This output impedance can be infinite if Resistor R1' and

Resistor R2' match precisely with R1 and R2A + R2B,

respectively. On the other hand, it can be negative if the

resistors are not matched. As a result, C1 must be in the range

of 1 pF to 10 pF to prevent the oscillation.

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