ADA4522-4ARZ-R7 datasheet(26/33 Pages) AD

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Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

THEORY OF OPERATION

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Rev. G | 26 of 33

Figure 78. Current Noise Density at Gain = 1

EMI REJECTION RATIO

Circuit performance is often adversely affected by high frequency

EMI. When the signal strength is low and transmission lines are

long, an op amp must accurately amplify the input signals. Howev-

er, 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 example, wires and printed circuit board (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.

The ADA4522-1/ADA4522-2/ADA4522-4 have integrated EMI filters

at their input stage. To describe the ability of the ADA4522-1/

ADA4522-2/ADA4522-4 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 = 20log(VIN_PEAK/ΔVOS)

Figure 79. EMIRR vs. Frequency

CAPACITIVE LOAD STABILITY

The ADA4522-1/ADA4522-2/ADA4522-4 can safely drive capacitive

loads of up to 250 pF in any configuration. As with most amplifiers,

driving larger capacitive loads than specified may cause excessive

overshoot and ringing, or even oscillation. A heavy capacitive load

reduces the phase margin and causes the amplifier frequency

response to peak. Peaking corresponds to overshooting or ringing

in the time domain. Therefore, it is recommended that external

compensation be used if the ADA4522-1/ADA4522-2/ADA4522-4

must drive a load exceeding 250 pF. This compensation is particu-

larly important in the unity-gain configuration, which is the worst

case for stability.

A quick and easy way to stabilize the op amp for capacitive load

drive is by adding a series resistor, RISO, between the amplifier

output terminal and the load capacitance, as shown in Figure

80. RISO isolates the amplifier output and feedback network from

the capacitive load. However, with this compensation scheme, the

output impedance as seen by the load increases, and this reduces

gain accuracy.

Figure 80. Stability Compensation with Isolating Resistor, RISO

Figure 81 shows the effect on overshoot with different values of

RISO.

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