Datasheet

ADA4522-1/ADA4522-2/ADA4522-4 Data Sheet
Rev. F | Page 24 of 33
NOISE CONSIDERATIONS
1/f Noise
1/f noise, also known as pink noise or flicker noise, is inherent
in semiconductor devices and increases as frequency decreases.
At a low frequency, 1/f noise is a major noise contributor and
causes a significant output voltage offset when amplified by the
noise gain of the circuit. However, because the low frequency
1/f noise appears as a slow varying offset to the ADA4522-1/
ADA4522-2/ADA4522-4, it is effectively reduced by the chopping
technique. This technique allows the ADA4522-1/ADA4522-2/
ADA4522-4 to have a much lower noise at dc and low frequency in
comparison to standard low noise amplifiers that are susceptible
to 1/f noise. Figure 64 shows the 0.1 Hz to 10 Hz noise to be only
117 nV p-p of noise.
Source Resistance
The ADA4522-1/ADA4522-2/ADA4522-4 are some of the
lowest noise high voltage zero drift amplifiers with 5.8 nV/√Hz
of voltage noise density at 1 kHz (A
V
= 100). Therefore, it is
important to consider the input source resistance of choice to
maintain a total low noise. The total input referred broadband
noise (e
N
total) from any amplifier is primarily a function of
three types of noise: input voltage noise, input current noise,
and thermal (Johnson) noise from the external resistors.
These uncorrelated noise sources can be summed up in a root
sum squared (rss) manner by using the following equation:
e
N
total = (e
N
2
+ 4 kTR
S
+ (i
N
× R
S
)
2
)
1/2
where:
e
N
is the input voltage noise density of the amplifier (V/√Hz).
k is Boltzmann’s constant (1.38 × 10
−23
J/K).
T is the temperature in Kelvin (K).
R
S
is the total input source resistance (Ω).
i
N
is the input current noise density of the amplifier (A/√Hz).
The total equivalent rms noise over a specific bandwidth is
expressed as
e
N RMS
= e
N
total
BW
where BW is the bandwidth in hertz.
This analysis is valid for broadband noise calculation up to a
decade before the switching frequency. If the bandwidth of
concern includes the switching frequency, more complicated
calculations must be made to include the effect of the increase
in noise at the switching frequency.
With a low source resistance of R
S
< 1 kΩ, the voltage noise of
the amplifier dominates. As the source resistance increases, the
thermal noise of R
S
dominates. As the source resistance further
increases, where R
S
> 50 kΩ, the current noise becomes the
main contributor of the total input noise.
Residual Ripple
As shown in Figure 60, Figure 61, and Figure 62, the ADA4522-1/
ADA4522-2/ADA4522-4 have a flat noise spectrum density at
lower frequencies and exhibits spectrum density bumps and peaks
at higher frequencies.
The largest noise bump is centered at 6 MHz; this bump is due
to the decrease in the input gain at higher frequencies. This
decrease is a typical phenomenon and can also be seen in other
amplifiers. In addition to the noise bump, a sharp peak due to the
chopping networks is seen at 4.8 MHz. However, this magnitude is
significantly reduced by the offset and ripple correction loop. Its
magnitude may be different with different amplifier units or with
different circuitries around the amplifier. This peak can potentially
be hidden by the noise bump and, therefore, may not be detected.
The offset and ripple correction loop, designed to reduce the
4.8 MHz switching artifact, also creates a noise bump centered
at 800 kHz and a noise peak on top of this noise bump. Although
the magnitude of the bump is mostly constant, the magnitude of
the 800 kHz peak is different from unit to unit. Some units may
not exhibit the 800 kHz noise peak; however, for other units,
peaks occur at multiple integrals of 800 kHz, such as 1.6 MHz or
2.4 MHz.
These noise peaks, albeit small in magnitude, can be significant
when the amplifier has a closed-loop frequency that is higher
than the chopping frequency. To suppress the noise spike to a
desired level, either configure the amplifier in a high gain
configuration or apply a post filter at the output of the amplifier.
Figure 76 shows the voltage noise density of the ADA4522-1/
ADA4522-2/ADA4522-4 in various gain configurations. Note that
the higher the gain, the lower the available bandwidth is. The
earlier bandwidth roll-off effectively filters out the higher noise
spectrum.
100
10
1
100 1k 10k 100k 1M 10M 100M
VOLTAGE NOISE DENSITY (nV/Hz)
FREQUENCY (Hz)
V
SY
= ±15V
A
V
= 1
A
V
= 10
A
V
= 100
13168-072
Figure 76. Voltage Noise Density with Various Gains
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