AD5543/AD5553

Data Sheet

Rev. F | Page 12 of 20

PROGRAMMABLE CURRENT SOURCE

Figure 24 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 bidirectional

current flow and high voltage compliance. This circuit can be used

in 4 mA to 20 mA current transmitters with up to 500 Ω of load. In

Figure 24, it can be shown that if the resistor network is matched,

the load current is

(

)

D

V

R3

R1

R3

R2

I

REF

L

×

×

+

=

/

(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 directions

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

(

)

(

)

(

)

R3

R2

R1

R3

R2

R1

R2

R1

R3

R1

+

−

+

+

=

'

'

'

'

(6)

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

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 a few picofarads (pF).

U2

U1

AD5543/AD5553

GND

AD8628

AD8510

V+

V–

LOAD

U3

C1

10pF

R2'

15kΩ

R3'

50Ω

R3

50Ω

R1'

150kΩ

R2

15kΩ

R1

150kΩ

Figure 24. Programmable Current Source with Bidirectional Current Control

and High Voltage Compliance Capabilities

REFERENCE SELECTION

When selecting a reference for use with the AD55xx series of

current output DACs, pay attention to the output voltage,

temperature coefficient specification of the reference. Choosing

a precision reference with a low output temperature coefficient

minimizes error sources. Table 7 lists some of the references

available from Analog Devices, Inc., that are suitable for use

with this range of current output DACs.

AMPLIFIER SELECTION

The primary requirement for the current-steering mode is an

amplifier with low input bias currents and low input offset

voltage. Because of the code-dependent output resistance of the

DAC, the input offset voltage of an op amp is multiplied by the

variable gain of the circuit. A change in this noise gain between

two adjacent digital fractions produces a step change in the

output voltage due to the amplifier’s input offset voltage. This

output voltage change is superimposed upon the desired change

in output between the two codes and gives rise to a differential

linearity error, which, if large enough, can cause the DAC to be

nonmonotonic.

The input bias current of an op amp also generates an offset at

the voltage output because of the bias current flowing in the

Common-mode rejection of the op amp is important in voltage-

switching circuits because it produces a code-dependent error

at the voltage output of the circuit.

Provided that the DAC switches are driven from true wideband

Consequently, the slew rate and settling time of a voltage-

switching DAC circuit is determined largely by the output op

amp. To obtain minimum settling time in this configuration,

in this application) of the DAC. This is done by using low input

capacitance buffer amplifiers and careful board design.

Analog Devices offers a wide range of amplifiers for both

precision dc and ac applications, as listed in Table 8 and Table 9.