Data Sheet

ADR01/ADR02/ADR03/ADR06

Rev. R | Page 17 of 20

PROGRAMMABLE 4 mA TO 20 mA CURRENT

TRANSMITTER

Because of their precision, adequate current handling, and small

footprint, the devices are suitable as the reference sources for

many high performance converter circuits. One of these

applications is the multichannel 16-bit, 4 mA to 20 mA current

transmitter in the industrial control market (see Figure 40).

This circuit employs a Howland current pump at the output to

yield better efficiency, a lower component count, and a higher

voltage compliance than the conventional design with op amps

and MOSFETs. In this circuit, if the resistors are matched such

that R1 = R1′, R2 = R2′, R3 = R3′, the load current is

N

REF

L

D

V

R3

R1

R3)

(R2

I

2

×

×

′

+

=

(2)

where D is similarly the decimal equivalent of the DAC input

code and N is the number of bits of the DAC.

According to Equation 2, R3′ can be used to set the sensitivity.

R3′ can be made as small as necessary to achieve the current

needed within U4 output current driving capability. Alter-

natively, other resistors can be kept high to conserve power.

In this circuit, the AD8512 is capable of delivering 20 mA of

current, and the voltage compliance approaches 15.0 V.

U1

15V

GND

TEMP TRIM

U1 = ADR01/ADR02/ADR03/ADR06, REF01

U2 = AD5543/AD5544/AD5554

U3, U4 = AD8512

U2

5V

10V

+15V

–15V

GND

RF

IO

IO

AD5544

DIGITAL INPUT

CODE 20%–100% FUL L SCALE

U3

0V TO –10V

R1

150kΩ

R2

15kΩ

U4

C1

10pF

VP

R3

50Ω

AD8512

R3'

50Ω

R1'

150kΩ

LOAD

500Ω

4mA TO 20mA

VN

R2'

15kΩ

Figure 40. Programmable 4 mA to 20 mA Transmitter

The Howland current pump yields a potentially infinite output

impedance, that is highly desirable, but resistance matching is

critical in this application. The output impedance can be deter-

mined using Equation 3. As shown by this equation, if the

latter is true, oscillation can occur. For this reason, connect

Capacitor C1 in the range of 1 pF to 10 pF between VP and the

output terminal of U4 to filter any oscillation.













−

′

′

′

=

=

1

R1R2

R2

R1

R1

I

V

Z

t

t

O

(3)

In this circuit, an ADR01 provides the stable 10.000 V reference

for the AD5544 quad 16-bit DAC. The resolution of the adjust-

able current is 0.3 µA/step; the total worst-case INL error is

merely 4 LSBs. Such error is equivalent to 1.2 µA or a 0.006%

system error, which is well below most systems’ requirements.

The result is shown in Figure 41 with measurement taken at 25°C

and 70°C; total system error of 4 LSBs at both 25°C and 70°C.

5

–1

0

65536

8192

16384

24576

32768

40960

49152

57344

4

3

2

1

0

CODE (Decimal)

25°C

70°C

Figure 41. Result of Programmable 4 mA to 20 mA Current Transmitter

PRECISION BOOSTED OUTPUT REGULATOR

A precision voltage output with boosted current capability can

be realized with the circuit shown in Figure 42. In this circuit,

Moderate heat is generated on the MOSFET, and higher current

can be achieved with a replacement of a larger device. In

addition, for a heavy capacitive load with a fast edging input

signal, a buffer should be added at the output to enhance the

transient response.

U2

15V

N1

200Ω

U1

ADR01/

ADR02/

ADR03/

ADR06

TEMP TRIM

GND

V–

V+

OP1177

2N7002

1µF

1000pF

100Ω

100Ω

Figure 42. Precision Boosted Output Regulator