ADA4522-4ARUZ-R7 datasheet(24/32 Pages) AD

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ADA4522-1/ADA4522-2/ADA4522-4

Data Sheet

Rev. E | Page 24 of 32

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

expressed as

eN RMS = eN total BW

where BW is the bandwidth in hertz.

This analysis is valid for broadband noise calculation up to a

decade before the switching frequency. If the bandwidth of

concern includes the switching frequency, more complicated

calculations must be made to include the effect of the increase

in noise at the switching frequency.

With a low source resistance of RS < 1 kΩ, the voltage noise of

the amplifier dominates. As the source resistance increases, the

thermal noise of RS dominates. As the source resistance further

increases, where RS > 50 kΩ, the current noise becomes the

main contributor of the total input noise.

Residual Ripple

As shown in Figure 60, Figure 61, and Figure 62, the ADA4522-1/

ADA4522-2/ADA4522-4 have a flat noise spectrum density at

lower frequencies and exhibits spectrum density bumps and peaks

at higher frequencies.

The largest noise bump is centered at 6 MHz; this bump is due

to the decrease in the input gain at higher frequencies. This

decrease is a typical phenomenon and can also be seen in other

amplifiers. In addition to the noise bump, a sharp peak due to the

chopping networks is seen at 4.8 MHz. However, this magnitude is

significantly reduced by the offset and ripple correction loop. Its

magnitude may be different with different amplifier units or with

different circuitries around the amplifier. This peak can potentially

be hidden by the noise bump and, therefore, may not be detected.

The offset and ripple correction loop, designed to reduce the

4.8 MHz switching artifact, also creates a noise bump centered

at 800 kHz and a noise peak on top of this noise bump. Although

the magnitude of the bump is mostly constant, the magnitude of

the 800 kHz peak is different from unit to unit. Some units may

not exhibit the 800 kHz noise peak; however, for other units,

peaks occur at multiple integrals of 800 kHz, such as 1.6 MHz or

2.4 MHz.

These noise peaks, albeit small in magnitude, can be significant

when the amplifier has a closed-loop frequency that is higher

than the chopping frequency. To suppress the noise spike to a

desired level, either configure the amplifier in a high gain

configuration or apply a post filter at the output of the amplifier.

Figure 76 shows the voltage noise density of the ADA4522-1/

ADA4522-2/ADA4522-4 in various gain configurations. Note that

the higher the gain, the lower the available bandwidth is. The

earlier bandwidth roll-off effectively filters out the higher noise

spectrum.

100

10k

100k

10M

100M

FREQUENCY (Hz)

VSY = ±15V

AV = 1

AV = 10

AV = 100

Figure 76. Voltage Noise Density with Various Gains

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