ADR02 AD [Analog Devices], ADR02 Datasheet - Page 17

• Manufacturer Part Number: ADR02
• Description: Ultracompact Precision10 V/5 V/2.5 V/3.0 V Voltage References
• Manufacturer: AD [Analog Devices]

To optimize the resolution of this circuit, dual-supply op amps
should be used because the ground potential of ADR02 can
swing from –5 V at zero scale to V
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
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.
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
resistors are perfectly matched, Z
if they are not matched, Z
15V
U1 = ADR01/ADR02/ADR03/ADR06, REF01
U2 = AD5543/AD5544/AD5554
U3, U4 = AD8512
I
TEMP TRIM
V
L
IN
=
GND
U1
(R
V
OUT
CODE 20%–100% FULL SCALE
Figure 42. Programmable 4 to 20 mA Transmitter
2
+
R
R
5V
3
10V
3
′
)
V
V
GND
R
DD
REF
DIGITAL INPUT
1
U2
×
AD5544
V
O
REF
RF
is either positive or negative. If the
2
N
IO
IO
×
D
O
L
is infinite. On the other hand,
0V TO –10V
at full scale of the potenti-
U3
AD8512
+15V
–15V
150kΩ
V
R1'
X
150kΩ
R1
VN
VP
4–20mA
U4
15kΩ
15kΩ
10pF
LOAD
R2'
500Ω
R2
C1
R3
50Ω
R3'
50Ω
V
V
Rev. F | Page 17 of 20
O
L
(2)
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.
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.
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,
U2 forces V
N1, thereby making the load current furnished by V
configuration, a 50 mA load is achievable at V
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.
V
IN
Figure 43. Result of Programmable 4 to 20 mA Current Transmitter
Z
–1
O
5
4
3
2
1
0
0
=
R
I
L
L
V
= 0mA TO 20mA
V
I
TEMP TRIM
= 500Ω
O
t
t
ADR01/
ADR02/
ADR03/
ADR06
IN
8192
Figure 44. Precision Boosted Output Regulator
to be equal to V
=
GND
70
U1
ADR01/ADR02/ADR03/ADR06
⎛
⎜
⎝
o
V
C
R1
R1R2
16384 24576 32768 40960 49152 57344
OUT
′
R1
R2
′
′
−
1
CODE (Decimal)
⎞
⎟
⎠
OP1177
REF
15V
2N7002
V+
V–
by regulating the turn-on of
U2
25
o
C
N1
IN
of 15 V. Moderate
R
200Ω
L
IN
65536
. In this
1µF
C
L
(3)
V
O