ADR01/ADR02/ADR03/ADR06

Rev. F | Page 17 of 20

To optimize the resolution of this circuit, dual-supply op amps

should be used because the ground potential of ADR02 can

ometer setting.

PROGRAMMABLE 4 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 to 20 mA current

transmitter in the industrial control market (see Figure 42). This

circuit employs a Howland current pump at the output, which

yields 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

R

R

)

R

(R

I

2

3

1

3

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. On the

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

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% FULL SCALE

U3

0V TO –10V

R1

150k

Ω

R2

15k

Ω

U4

C1

10pF

VP

R3

50

Ω

AD8512

R3'

50

Ω

R1'

150k

Ω

LOAD

500

Ω

4–20mA

VN

R2'

15k

Ω

Figure 42. Programmable 4 to 20 mA Transmitter

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

current, and the voltage compliance approaches 15 V.

The Howland current pump yields a potentially infinite output

impedance, which is highly desirable, but resistance matching is

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

mined using Equation 3. As can be seen by this equation, if the

latter is true, oscillation may occur. For this reason, a capacitor, C1,

in the range of 1 pF to 10 pF should be connected 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, and the total worst-case INL error is

merely 4 LSB. 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 43 with measurement taken at 25°C

and 70°C; total system error of 4 LSB 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

70

Figure 43. Result of Programmable 4 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 44. In this circuit,

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

200

Ω

U1

ADR01/

ADR02/

ADR03/

ADR06

TEMP TRIM

GND

V–

V+

OP1177

2N7002

1

µF

Figure 44. Precision Boosted Output Regulator