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AD8571/AD8572/AD8574

Data Sheet

Rev. F | Page 18 of 28

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 AD8571/AD8572/AD8574 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 AD8571/AD8572/AD8574 to have lower noise near

dc than standard low noise amplifiers that are susceptible to 1/f

noise.

RANDOM AUTO-ZERO CORRECTION ELIMINATES

INTERMODULATION DISTORTION

The AD8571/AD8572/AD8574 can be used as conventional

op amps 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.

–20

–40

–80

–60

–120

–140

–100

–160

FREQUENCY (kHz)

VS = 5V

AV = 0dB

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

–20

–40

–60

–80

–100

–120

FREQUENCY (kHz)

VS = 5V

AV = 60dB

Figure 57. Spectral Analysis of AD8571/AD8572/AD8574 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%.

–20

–40

–60

–80

–100

–120

VS = 5V

AV = 60dB

FREQUENCY (kHz)

Figure 58. Spectral Analysis of AD8572 in High Gain with an Input Signal

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