AD8571/AD8572/AD8574

Rev. B | Page 17 of 24

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1/f NOISE CHARACTERISTICS

Another advantage of auto-zero amplifiers is their ability to

cancel flicker noise. Flicker noise, also known as 1/f noise, is

noise inherent in the physics of semiconductor devices and

increases 3 dB for every octave decrease in frequency. The 1/f

corner frequency of an amplifier is the frequency at which the

flicker noise is equal to the broadband noise of the amplifier. At

lower frequencies, flicker noise dominates, causing higher

degrees of error for sub-Hertz frequencies or dc precision

applications.

Because the AD857x amplifiers are self-correcting op amps,

they do not have increasing flicker noise at lower frequencies. In

essence, low frequency noise is treated as a slowly varying offset

error and is greatly reduced as a result of autocorrection. The

correction becomes more effective as the noise frequency

approaches dc, offsetting the tendency of the noise to increase

exponentially as frequency decreases. This allows the AD857x

to have lower noise near dc than standard low noise amplifiers

that are susceptible to 1/f noise.

Figure 57. Spectral Analysis of AD857x Output with 60 dB Gain

Figure 58 shows the spectral output of an AD8572 configured in

a high gain (60 dB) with a 1 mV input signal applied. Note the

absence of any IMD products in the spectrum. The signal-to-

noise (SNR) ratio of the output signal is better than 60 dB,

or 0.1%.

RANDOM AUTO-ZERO CORRECTION ELIMINATES

INTERMODULATION DISTORTION

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The AD857x can be used as a conventional op amp for gains up

to 1 MHz. The auto-zero correction frequency of the device

continuously varies, based on a pseudorandom generator with a

uniform distribution from 2 kHz to 4 kHz. The randomization

of the autocorrection clock creates a continuous randomization

of intermodulation distortion (IMD) products that show up as

simple broadband noise at the output of the amplifier. This

noise naturally combines with the amplifier voltage noise in a

root-squared-sum fashion, resulting in an output free of IMD.

Figure 56 shows the spectral output of an AD8572 with the

amplifier configured for unity gain and the input grounded.

Figure 57 shows the spectral output with the amplifier

configured for a gain of 60 dB.

Figure 58. Spectral Analysis of AD857x in High Gain with an Input Signal

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Figure 56. Spectral Analysis of AD857x Output in Unity Gain Configuration