AD8571ARMZ datasheet(17/24 Pages) AD

17 / 24 page

AD8571/AD8572/AD8574

Rev. E | Page 17 of 24

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FREQUENCY (kHz)

VS = 5V

AV = 60dB

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 with autocorrection. The correction

becomes more effective as the noise frequency approaches dc,

offsetting the tendency of the noise to increase exponentially as

frequency decreases, which 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 ratio (SNR) of the output signal is better than 60 dB, or 0.1%.

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VS = 5V

AV = 60dB

FREQUENCY (kHz)

RANDOM AUTO-ZERO CORRECTION ELIMINATES

INTERMODULATION DISTORTION

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 IMD products that show up as simple broadband noise at the

output of the amplifier. This broadband noise naturally combines

with the amplifier voltage noise in a root-squared-sum fashion,

resulting in an output free 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 AD8572 in High Gain with an Input Signal

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FREQUENCY (kHz)

VS = 5V

AV = 0dB

Figure 56. Spectral Analysis of AD8572 Output in Unity Gain Configuration

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