ADA4661-2ARMZ-R7 datasheet(25/32 Pages) AD

25 / 32 page

Data Sheet

ADA4661-2

Rev. 0 | Page 25 of 32

EMI REJECTION RATIO

Circuit performance is often adversely affected by high frequency

electromagnetic interference (EMI). When 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

radiation. For instance, wires and PCB traces can act as antennas

and pick up high frequency EMI signals.

Amplifiers do not amplify EMI or RF signals due to 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 ADA4661-2 to perform as

intended in the presence of 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)

Figure 78. EMIRR vs. Frequency

CURRENT SHUNT MONITOR

Many applications require the sensing of signals near the

positive or negative rail. Current shunt monitors are one such

application and are mostly used for feedback control systems.

They are also used in a variety of other applications, including

power metering, battery fuel gauging, and feedback controls in

electrical power steering. In such applications, it is desirable to

use a shunt with very low resistance to minimize the series

voltage drop. This not only minimizes wasted power but also

allows the measurement of high currents while saving power.

The low input bias current, low offset voltage, and rail-to-rail

feature of the ADA4661-2 makes the amplifier an excellent

choice for precision current monitoring.

Figure 79 shows a low-side current sensing circuit, and Figure 80

shows a high-side current sensing circuit. Current flowing

through the shunt resistor creates a voltage drop. The ADA4661-2,

configured as a difference amplifier, amplifies the voltage drop

by a factor of R2/R1. Note that for true difference amplification,

matching of the resistor ratio is very important, where R2/R1 =

R4/R3. The rail-to-rail output feature of the ADA4661-2 allows

the output of the op amp to almost reach its positive supply.

This allows the current shunt monitor to sense up to approxi-

mately VSY/(R2/R1 × RS) amperes of current. For example, with

VSY = 18 V, R2/R1 = 100, and RS = 100 mΩ, this current is

approximately 1.8 A.

Figure 79. Low-Side Current Sensing Circuit

Figure 80. High-Side Current Sensing Circuit

ACTIVE FILTERS

Active filters are used to separate signals, passing those of

interest and attenuating signals at unwanted frequencies. For

example, low-pass filters are often used as antialiasing filters

in data acquisition systems or as noise filters to limit high

frequency noise.

The high input impedance, high bandwidth, low input bias

current, and dc precision of the ADA4661-2 make it a good fit

for active filter applications. Figure 81 shows the ADA4661-2 in

a four-pole Sallen-Key Butterworth low-pass filter configuration.

The four-pole low-pass filter has two complex conjugate pole

pairs and is implemented by cascading two two-pole low-pass

filters. Section A and Section B are configured as two-pole low-

pass filters in unity gain. Table 8 shows the Q requirement and

pole position associated with each stage of the Butterworth

filter. Refer to Chapter 8, “Analog Filters,” in Linear Circuit

Design Handbook, available at www.analog.com/AnalogDialogue,

for pole locations on the S plane and Q requirements for filters

of a different order.

100

120

140

10M

100M

10G

FREQUENCY (Hz)

VSY = 3V TO 18V

VIN = 100mV PEAK

VIN = 50mV PEAK

SUPPLY

VSY

VOUT*

1/2

ADA4661-2

*VOUT = AMPLIFIER GAIN × VOLTAGE ACROSS RS = R2/R1 × RS × I

1/2

ADA4661-2

SUPPLY

VSY

VOUT*

*VOUT = AMPLIFIER GAIN × VOLTAGE ACROSS RS = R2/R1 × RS × I

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