Datasheet

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.
01104-056
0
–20
–40
–80
–60
–120
–140
–100
–160
1 2 3 4 5
6 7 8 9 10
OUTPUT SIGNAL
FREQUENCY (kHz)
V
S
= 5V
A
V
= 0dB
Figure 56. Spectral Analysis of AD8572 Output in Unity Gain Configuration
0
–20
–40
–60
–80
–100
–120
0 1 2 3 4 5 6 7 8
9 10
01104-057
OUTPUT SIGNAL
FREQUENCY (kHz)
V
S
= 5V
A
V
= 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%.
0
–20
–40
–60
–80
–100
–120
0 1
2 3 4 5 6 7 8 9
10
01104-058
V
S
= 5V
A
V
= 60dB
FREQUENCY (kHz)
OUTPUT SIGNAL
Figure 58. Spectral Analysis of AD8572 in High Gain with an Input Signal