ADR01(RevF) データシートの表示（PDF） - Analog Devices
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ADR01 Datasheet PDF : 20 Pages
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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 VL at full scale of the potenti-
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
IL
=
(R2 + R3)
R3′
R1
×
VREF ×
2N
D
(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.
5V
U2
U1
15V
VIN VOUT
TEMP TRIM
VDD
RF
IO
10V VREF AD5544
GND
IO
GND
0V TO –10V
+15V R1
R2
150kΩ 15kΩ
U3
VX
VP
R3
–15V
C1 50Ω
10pF
DIGITAL INPUT
CODE 20%–100% FULL SCALE AD8512
U4
VO
U1 = ADR01/ADR02/ADR03/ADR06, REF01
U2 = AD5543/AD5544/AD5554
U3, U4 = AD8512
R1'
150kΩ
R2'
15kΩ
VN
R3'
50Ω
VL
LOAD
500Ω
4–20mA
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
resistors are perfectly matched, ZO is infinite. On the other hand,
if they are not matched, ZO is either positive or negative. If the
ADR01/ADR02/ADR03/ADR06
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.
ZO
= Vt
It
=
R1′
⎜⎛ R1′R2 −1⎟⎞
(3)
⎝ R1R2′ ⎠
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
RL = 500Ω
IL = 0mA TO 20mA
4
3
2
70oC
1
25oC
0
–1
0 8192 16384 24576 32768 40960 49152 57344 65536
CODE (Decimal)
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,
U2 forces VO to be equal to VREF by regulating the turn-on of
N1, thereby making the load current furnished by VIN. In this
configuration, a 50 mA load is achievable at VIN of 15 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.
N1
VIN
U1
ADR01/
ADR02/
ADR03/
ADR06
2N7002
15V
VIN
VOUT
TEMP TRIM
GND
V+
OP1177
V–
U2
RL
200Ω
CL VO
1µF
Figure 44. Precision Boosted Output Regulator
Rev. F | Page 17 of 20
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