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AD8500 Datasheet(PDF) 20 Page - Analog Devices

Part # AD8500
Description  22 關A, RRIO, CMOS, 18 V
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD8500 Datasheet(HTML) 20 Page - Analog Devices

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AD8546/AD8548
Data Sheet
Rev. B | Page 20 of 24
Note that 100 kΩ resistors are used in series with the input of
the op amp. If smaller resistor values are used, the supply current
of the system increases much more. For more information about
using op amps as comparators, see the AN-849 Application Note,
Using Op Amps as Comparators.
4 mA TO 20 mA PROCESS CONTROL CURRENT
LOOP TRANSMITTER
A 2-wire current transmitter is often used in distributed control
systems and process control applications to transmit analog signals
between sensors and process controllers. Figure 70 shows a 4 mA
to 20 mA current loop transmitter.
RL
100Ω
VDD
18V
C2
10µF
C3
0.1µF
C1
390pF
C4
0.1µF
R4
3.3kΩ
Q1
D1
4mA
TO
20mA
R3
1.2kΩ
RNULL
1MΩ
1%
VREF
RSPAN
200kΩ
1%
VIN
0V TO 5V
R1
68kΩ
1%
R2
2kΩ
1%
NOTES
1. R1 + R2 = R´.
1/2
AD8546
C5
10µF
RSENSE
100Ω
1%
VOUT
GND
ADR125
VIN
Figure 70. 4 mA to 20 mA Current Loop Transmitter
The transmitter is powered directly from the control loop
power supply, and the current in the loop carries signal from
4 mA to 20 mA. Thus, 4 mA establishes the baseline current
budget within which the circuit must operate.
The AD8546 is an excellent choice due to its low supply current
of 33 μA per amplifier over temperature and supply voltage. The
current transmitter controls the current flowing in the loop, where
a zero-scale input signal is represented by 4 mA of current and a
full-scale input signal is represented by 20 mA. The transmitter
also floats from the control loop power supply, VDD, whereas signal
ground is in the receiver. The loop current is measured at the load
resistor, RL, at the receiver side.
With a zero-scale input, a current of VREF/RNULL flows through
R. This creates a current, ISENSE, that flows through the sense
resistor, as determined by the following equation:
ISENSE,MIN = (VREF × R)/(RNULL × RSENSE)
With a full-scale input voltage, current flowing through R is
increased by the full-scale change in VIN/RSPAN. This creates an
increase in the current flowing through the sense resistor.
ISENSE,DELTA = (Full-Scale Change in VIN × R)/(RSPAN × RSENSE)
Therefore,
ISENSE,MAX = ISENSE,MIN + ISENSE,DELTA
When R >> RSENSE, the current through the load resistor at the
receiver side is almost equivalent to ISENSE.
Figure 70 shows a design for a full-scale input voltage of 5 V. At
0 V of input, the loop current is 3.5 mA, and at a full-scale input
of 5 V, the loop current is 21 mA. This allows software calibration
to fine-tune the current loop to the 4 mA to 20 mA range.
Together, the AD8546 and the ADR125 consume quiescent
current of only 160 µA, making 3.34 mA current available to
power additional signal conditioning circuitry or to power a
bridge circuit.


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