AD8657

Rev. A | Page 20 of 24

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 instance, wires and PCB traces can act as antennas and

pick up high frequency EMI signals.

Precision op amps, such as the AD8657, 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 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:

20

40

60

80

100

120

140

10M

100M

1G

10G

FREQUENCY (Hz)

Figure 72. EMIRR vs. Frequency

4 mA TO 20 mA PROCESS CONTROL CURRENT

LOOP TRANSMITTER

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 73

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

which the circuit must operate. Using the AD8657 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

at the receiver side.

R. This creates a current flowing through the sense resistor,

details):

With a full-scale input voltage, current flowing through R is

increase in the current flowing through the sense resistor.

Therefore

Figure 73 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.

100

Ω

18V

C2

10µF

C3

0.1µF

C1

390pF

C4

0.1µF

R4

3.3k

Ω

Q1

D1

4mA

TO

20mA

R3

1.2k

Ω

1M

Ω

1%

200k

Ω

1%

0V TO 5V

R1

68k

Ω

1%

R2

2k

Ω

1%

NOTES

1. R1 + R2 = R´.

1/2

AD8657

C5

10µF

100

Ω

1%

GND

ADR125

Figure 73. 4 mA to 20 mA Current Loop Transmitter