AD8657ARMZ-RL View Datasheet(PDF) - Analog Devices
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AD8657ARMZ-RL Datasheet PDF : 24 Pages
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AD8657/AD8659
EMI REJECTION RATIO
Circuit performance is often adversely affected by high frequency
electromagnetic interference (EMI). In the event where signal
strength is low and transmission lines are long, an op amp must
accurately amplify the input signals. However, all op amp pins—
the noninverting input, inverting input, positive supply, negative
supply, and output pins—are susceptible to EMI signals. These
high frequency signals are coupled into an op amp by various
means such as conduction, near field radiation, or far field radi-
ation. For example, wires and PCB traces can act as antennas and
pick up high frequency EMI signals.
Precision op amps, such as the AD8657 and AD8659, do not
amplify EMI or RF signals because of their relatively low
bandwidth. However, due to the nonlinearities of the input
devices, op amps can rectify these out-of-band signals. When
these high frequency signals are rectified, they appear as a dc
offset at the output.
To describe the ability of the AD8657/AD8659 to perform as
intended in the presence of an electromagnetic energy, the
electromagnetic interference rejection ratio (EMIRR) of the
noninverting pin is specified in Table 2, Table 3, and Table 4 of
the Specifications section. A mathematical method of
measuring EMIRR is defined as follows:
EMIRR = 20 log (VIN_PEAK/ΔVOS)
140
120
100
80
60
40
VIN = 100mVPEAK
VSY = 2.7V TO 18V
20
10M
100M
1G
10G
FREQUENCY (Hz)
Figure 77. EMIRR vs. Frequency
4 mA TO 20 mA PROCESS CONTROL CURRENT
LOOP TRANSMITTER—AD8657
The 2-wire current transmitters are often used in distributed
control systems and process control applications to transmit
analog signals between sensors and process controllers. Figure 78
shows a 4 mA to 20 mA current loop transmitter.
The transmitter powers 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
Data Sheet
which the circuit must operate. Using the AD8657 is an excellent
choice due to its low supply current of 34 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, while signal ground is in the
receiver. The loop current is measured at the load resistor, RL,
at the receiver side.
At a zero-scale input, a current of VREF/RNULL flows through Rïž´.
This creates a current flowing through the sense resistor, ISENSE,
determined by the following equation (see Figure 78 for details):
ISENSE, MIN = (VREF × R)/(RNULL × RSENSE)
With a full-scale input voltage, current flowing through Rïž´ 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
I = I + I SENSE, MAX SENSE, MIN SENSE, DELTA
When Rïž´ >> RSENSE, the current through the load resistor at the
receiver side is almost equivalent to ISENSE.
Figure 78 is designed for a full-scale input voltage of 5 V. At 0 V
of input, loop current is 3.5 mA; and at a full scale 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.
The AD8657 and ADR125 both consume only 160 µA quiescent
current, making 3.34 mA current available to power additional
signal conditioning circuitry or to power a bridge circuit.
VREF
RNULL
1MΩ
1%
C2 C3
10µF 0.1µF
ADR125
VOUT VIN
GND
C4 C5
0.1µF 10µF
VIN
0V TO 5V
RSPAN
200kΩ
1%
R1
68kΩ
1%
R2
2kΩ
1%
1/2
AD8657
R3
1.2kΩ
C1
390pF
Q1
R4
3.3kΩ
D1
RSENSE
100Ω
1%
VDD
18V
4mA
TO
20mA
RL
100Ω
NOTES
1. R1 + R2 = R´.
Figure 78. 4 mA to 20 mA Current Loop Transmitter
Rev. B | Page 22 of 24
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