AD8551/AD8552/AD8554
3.3nF
Because the input current noise of the AD855x is very small,
it does not become a dominant term unless RS is greater than
4 GΩ, which is an impractical value of source resistance.
100kΩ
100Ω
The total noise (en, TOTAL) is expressed in volts per square root
Hertz, and the equivalent rms noise over a certain bandwidth
can be found as
V
= 1mV rms
IN
@ 200Hz
Figure 59. Reducing Autocorrection Clock Noise Using a Feedback Capacitor
en = en,TOTAL × BW
(16)
0
V
A
= 5V
= 60dB
SY
where BW is the bandwidth of interest in Hertz.
V
–20
–40
OUTPUT OVERDRIVE RECOVERY
The AD855x amplifiers have an excellent overdrive recovery of
only 200 μs from either supply rail. This characteristic is par-
ticularly difficult for autocorrection amplifiers because the
nulling amplifier requires a nontrivial amount of time to error
correct the main amplifier back to a valid output. Figure 29 and
Figure 30 show the positive and negative overdrive recovery
times for the AD855x.
–60
–80
–100
–120
The output overdrive recovery for an autocorrection amplifier is
defined as the time it takes for the output to correct to its final
voltage from an overload state. It is measured by placing the
amplifier in a high gain configuration with an input signal that
forces the output voltage to the supply rail. The input voltage is
then stepped down to the linear region of the amplifier, usually
to halfway between the supplies. The time from the input signal
stepdown to the output settling to within 100 μV of its final
value is the overdrive recovery time.
0
1
2
3
4
5
6
7
8
9
10
FREQUENCY (kHz)
Figure 60. Spectral Analysis Using a Feedback Capacitor
BROADBAND AND EXTERNAL RESISTOR NOISE
CONSIDERATIONS
The total broadband noise output from any amplifier is primarily
a function of three types of noise: input voltage noise from the
amplifier, input current noise from the amplifier, and Johnson
noise from the external resistors used around the amplifier.
Input voltage noise, or en, is strictly a function of the amplifier
used. The Johnson noise from a resistor is a function of the re-
sistance and the temperature. Input current noise, or in, creates
an equivalent voltage noise proportional to the resistors used
around the amplifier. These noise sources are not correlated
with each other and their combined noise sums in a root-
squared-sum fashion. The full equation is given as
1
INPUT OVERVOLTAGE PROTECTION
Although the AD855x is a rail-to-rail input amplifier, exercise
care to ensure that the potential difference between the inputs
does not exceed 5 V. Under normal operating conditions, the
amplifier corrects its output to ensure the two inputs are at the
same voltage. However, if the device is configured as a comparator,
or is under some unusual operating condition, the input voltages
may be forced to different potentials. This can cause excessive
current to flow through internal diodes in the AD855x used to
protect the input stage against overvoltage.
2
2
en _ TOTAL
=
[
en2 + 4kTrS +
(
inRS
)
]
(15)
Where:
If either input exceeds either supply rail by more than 0.3 V,
large amounts of current begin to flow through the ESD pro-
tection diodes in the amplifier. These diodes connect between
the inputs and each supply rail to protect the input transistors
against an electrostatic discharge event and are normally
reverse-biased. However, if the input voltage exceeds the supply
voltage, these ESD diodes become forward-biased. Without
current limiting, excessive amounts of current can flow through
these diodes, causing permanent damage to the device. If inputs
are subjected to overvoltage, appropriate series resistors should
be inserted to limit the diode current to less than 2 mA maximum.
en = the input voltage noise density of the amplifier.
in = the input current noise of the amplifier.
RS = source resistance connected to the noninverting terminal.
k = Boltzmann’s constant (1.38 × 10−23 J/K).
T = ambient temperature in Kelvin (K = 273.15 + °C).
The input voltage noise density (en) of the AD855x is 42 nV/√Hz,
and the input noise, in, is 2 fA/√Hz. The en, TOTAL is dominated by
the input voltage noise, provided the source resistance is less
than 106 kΩ. With source resistance greater than 106 kΩ, the
overall noise of the system is dominated by the Johnson noise of
the resistor itself.
Rev. C | Page 18 of 24
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