Datasheet
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
DV
R3
R1R3)(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
V
IN
V
OUT
GND
TEMP
TRIM
U1 = ADR01/ADR02/ADR03/ADR06, REF01
U2 = AD5543/AD5544/AD5554
U3, U4 = AD8512
U2
5V
10V
+15V
–15V
V
DD
V
REF
GND
RF
IO
IO
AD5544
DIGITAL INPUT
CODE 20%–100% FULL SCALE
U3
V
X
0V TO –10V
R1
150kΩ
R2
15kΩ
U4
C1
10pF
VP
R3
50Ω
AD8512
R3'
50Ω
V
L
R1'
150kΩ
LOAD
500Ω
4mA TO 20mA
VN
V
O
R2'
15kΩ
02747-042
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
resistors are perfectly matched, Z
O
is infinite. Alternatively, if
they are not matched, Z
O
is either positive or negative. 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
R2R1
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 655368192 16384 24576 32768 40960 49152 57344
4
3
2
1
0
CODE (Decimal)
INL (LSB)
R
L
= 500Ω
I
L
= 0mA TO 20mA
25°C
70°C
02747-043
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,
U2 forces V
O
to be equal to V
REF
by regulating the turn-on of
N1, thereby making the load current furnished by V
IN
. In this
configuration, a 50 mA load is achievable at V
IN
of 15.0 V.
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
V
IN
V
OUT
TEMP
TRIM
GND
V–
V+
OP1177
2N7002
V
IN
V
O
R
L
1µF
C
L
02747-044
C
1
1000pF
R
2
100Ω
R
1
100Ω
Figure 42. Precision Boosted Output Regulator