ADA4522-4ARZ-R7 datasheet(24/33 Pages) AD

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Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

THEORY OF OPERATION

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Rev. G | 24 of 33

Case 1

If the external source resistance is low (for example, 100 Ω in

Figure 75) or if the input step function is large, the maximum

amplifier output current is limited to the output short-circuit current

as specified in the Specifications section. The maximum differential

voltage between the input signal and the amplifier output is then

limited by the maximum amplifier output current multiplied by the

total input resistance (internal and external) and the turn-on voltage

of the back to back diode (see Figure 72 for the input EMI filter and

clamp circuit architecture).

When the noninverting input voltage changes with a step signal, the

inverting input voltage (and, therefore, the output voltage) follows

the change quickly until it reaches the maximum differential voltage

between the input signal and amplifier output possible. The invert-

ing input voltage then starts slewing with the slew rate specified

in the Specifications section until it reaches its desired output.

Therefore, as seen in Figure 74, there are two distinctive sections

of the rising and falling edge of the output waveform. With this test

condition, the amount and duration of the input/output current is

limited and, therefore, does not damage the amplifier.

Figure 74. Large Signal Transient Response Example

Figure 75. Circuit Diagram for Large Signal Transient Response

Case 2

If the external source resistance is high or if the input step function

is small, the maximum output current is limited to the instantaneous

difference between the input signal and amplifier output voltage

(which is the change in the step function) divided by the source

resistance. This maximum output current is less than the amplifier

output short-circuit current. The maximum differential voltage be-

tween the input signal and the amplifier output is then equal to the

step function. The output voltage slews until it reaches its desired

output.

Therefore, if desired, reduce the input current by adding a larger ex-

ternal resistor between the signal source and the noninverting input.

Similarly, to reduce output current, add an external resistor to the

feedback loop between the inverting input and output. This large

signal transient response issue is typically not a problem when the

amplifier is configured in closed-loop gain, where the input signal

source is usually much smaller and the gain and feedback resistors

limit the current.

Back to back diodes are also implemented in many other amplifiers;

these amplifiers show similar slewing behavior.

NOISE CONSIDERATIONS

1/f Noise

1/f noise, also known as pink noise or flicker noise, is inherent in

semiconductor devices and increases as frequency decreases. At a

low frequency, 1/f noise is a major noise contributor and causes a

significant output voltage offset when amplified by the noise gain of

the circuit. However, because the low frequency 1/f noise appears

as a slow varying offset to the ADA4522-1/ADA4522-2/ADA4522-4,

it is effectively reduced by the chopping technique. This technique

allows the ADA4522-1/ADA4522-2/ADA4522-4 to have a much

lower noise at dc and low frequency in comparison to standard low

noise amplifiers that are susceptible to 1/f noise. Figure 64 shows

the 0.1 Hz to 10 Hz noise to be only 117 nV p-p of noise.

Source Resistance

The ADA4522-1/ADA4522-2/ADA4522-4 are some of the lowest

noise high voltage zero drift amplifiers with 5.8 nV/√Hz of voltage

noise density at 1 kHz (AV = 100). Therefore, it is important to

consider the input source resistance of choice to maintain a total

low noise. The total input referred broadband noise (eN total) from

any amplifier is primarily a function of three types of noise: input

voltage noise, input current noise, and thermal (Johnson) noise

from the external resistors.

These uncorrelated noise sources can be summed up in a root sum

squared (rss) manner by using the following equation:

eN total = (eN2 + 4 kTRS + (iN × RS)2)1/2

where:

eN is the input voltage noise density of the amplifier (V/√Hz).

k is Boltzmann’s constant (1.38 × 10−23 J/K).

T is the temperature in Kelvin (K).

RS is the total input source resistance (Ω).

iN is the input current noise density of the amplifier (A/√Hz).

The total equivalent rms noise over a specific bandwidth is ex-

pressed as

eN RMS = eN total

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