Data Sheet AD8571/AD8572/AD8574
Rev. F | Page 19 of 28
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 e
n
, 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 i
n
,
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
e
n, TOTAL
= [e
n
2
+ 4kTr
s
+ (i
n
r
s
)
2
]
1/2
(15)
where:
e
n
is the input voltage noise of the amplifier.
i
n
is the input current noise of the amplifier.
r
s
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, e
n
, of the AD8571/AD8572/
AD8574 is 51 nV/√Hz, and the input noise, i
n
, is 2 fA/√Hz. The
e
n, 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 AD8571/AD8572/
AD8574 is very small, i
n
does not become a dominant term
unless r
s
> 4 GΩ, which is an impractical value of source
resistance.
The total noise, e
n
,
TOTAL
, is expressed in volts-per-square-root
Hertz, and the equivalent rms noise over a certain bandwidth
can be found as
e
n
= e
n, TOTAL
× BW (16)
where
BW is the bandwidth of interest in Hertz.
OUTPUT OVERDRIVE RECOVERY
The AD8571/AD8572/AD8574 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 AD8571/AD8572/
AD8574.
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 AD8571/AD8572/AD8574 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 AD8571/AD8572/AD8574 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.
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 AD8571/AD8572/AD8574 amplifiers have 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.
