AD8571ARMZ View Datasheet(PDF) - Analog Devices
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AD8571ARMZ Datasheet PDF : 24 Pages
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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 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.
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.
0
–20
VS = 5V
AV = 0dB
–40
–60
–80
–100
–120
–140
–160
1
2
3
4
5
6
7
8
9 10
FREQUENCY (kHz)
Figure 56. Spectral Analysis of AD8572 Output in Unity Gain Configuration
AD8571/AD8572/AD8574
0
VS = 5V
–20
AV = 60dB
–40
–60
–80
–100
–120
0 1 2 3 4 5 6 7 8 9 10
FREQUENCY (kHz)
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%.
0
VS = 5V
–20
AV = 60dB
–40
–60
–80
–100
–120
0 1 2 3 4 5 6 7 8 9 10
FREQUENCY (kHz)
Figure 58. Spectral Analysis of AD8572 in High Gain with an Input Signal
Rev. E | Page 17 of 24
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