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
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 ꢀV of its final
value is the overdrive recovery time. Many competitors’ auto-
correction 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 is a rail-to-rail input amplifier, care
should be taken to ensure that the potential difference between
the inputs does not exceed 5 V. Under normal operating condi-
tions, 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 could
cause excessive current to flow through internal diodes in the
AD857x used to protect the input stage against overvoltage.
en,TOTAL = [en + 4kTrs + (inrs )2 ]1/2
(15)
2
where:
en = input voltage noise of the amplifier.
in = 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 AD857x is
51 nV/√Hz, and the input noise, in, is 2 fA/√Hz. The en, TOTAL is
dominated by input voltage noise provided 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.
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.
Because the input current noise of the AD857x is very small, in
does not become a dominant term unless rS is greater than
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
OUTPUT PHASE REVERSAL
en = en,TOTAL
×
BW
(16)
Output phase reversal occurs in some amplifiers when the input
common-mode voltage range is exceeded. As common-mode
voltage is moved outside of 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.
where BW is the bandwidth of interest in Hertz.
OUTPUT OVERDRIVE RECOVERY
The AD857x amplifiers have an excellent overdrive recovery of
only 200 ꢀs from either supply rail. This characteristic is par-
ticularly 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
time for the AD857x.
The AD857x amplifier has been carefully designed to prevent
any output phase reversal, provided both inputs are maintained
within the supply voltages. If one or both inputs could 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 the
output does not reverse its phase.
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
Rev. B | Page 18 of 24
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