AD8571ARMZ View Datasheet(PDF) - Analog Devices
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AD8571ARMZ Datasheet PDF : 24 Pages
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AD8571/AD8572/AD8574
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
resistance 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
en, TOTAL = [en2 + 4kTrs + (inrs)2]1/2
(15)
where:
en is the input voltage noise of the amplifier.
in is the input current noise of the amplifier.
rs is the source resistance connected to the noninverting
terminal.
k is Boltzmann’s constant (1.38 × 10−23 J/K).
T is the ambient temperature in Kelvin (K = 273.15 + °C).
The input voltage noise density, en, of the AD857x is 51 nV/√Hz,
and the input noise, in, is 2 fA/√Hz. The en, TOTAL is dominated by
the input voltage noise provided that the source resistance is less
than 172 kΩ. With source resistance greater than 172 kΩ, the
overall noise of the system is dominated by the Johnson noise of
the resistor itself.
Because the input current noise of the AD857x is very small, in
does not become a dominant term unless rs > 4 GΩ, which is an
impractical value of source resistance.
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
en = en, TOTAL × BW
(16)
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
step-down to the output settling to within 100 μV of its final
value is the overdrive recovery time. Many autocorrection
amplifiers require a number of auto-zero clock cycles to recover
from output overdrive, and some can take several milliseconds
for the output to settle properly.
INPUT OVERVOLTAGE PROTECTION
Although the AD857x are rail-to-rail input amplifiers, care
should be taken 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 that 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, which could
cause excessive current to flow through the internal diodes in the
AD857x used to protect the input stage against overvoltage.
If either input exceeds either supply rail by more than 0.3 V,
large amounts of current begin to flow through the ESD
protection diodes in the amplifier. These diodes are connected
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 subject to overvoltage, appropriate series
resistors should be inserted to limit the diode current to less
than 2 mA.
where BW is the bandwidth of interest in Hertz.
OUTPUT OVERDRIVE RECOVERY
The AD857x amplifiers have an excellent overdrive recovery
of only 200 μs from either supply rail. This characteristic is
particularly difficult for autocorrection amplifiers because the
nulling amplifier requires a substantial 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 AD857x.
OUTPUT PHASE REVERSAL
Output phase reversal occurs in some amplifiers when the input
common-mode voltage range is exceeded. As common-mode
voltage moves outside the common-mode range, the outputs of
these amplifiers suddenly jump in the opposite direction to
the supply rail. This is the result of the differential input pair
shutting down, causing a radical shifting of internal voltages
that results in the erratic output behavior.
The AD857x amplifier has been carefully designed to prevent
any output phase reversal, provided that both inputs are
maintained within the supply voltages. If one or both inputs
exceed either supply voltage, a resistor should be placed in
series with the input to limit the current to less than 2 mA to
ensure that the output does not reverse its phase.
Rev. E | Page 18 of 24
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