Datasheet

AD8551/AD8552/AD8554 Data Sheet
Rev. F | Page 18 of 24
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 re-
sistance 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
(
) [
]
2
1
2
2
_
4
S
n
S
n
TOTAL
n
Ri
kTre
e
++
=
(15)
Where:
e
n
= the input voltage noise density of the amplifier.
i
n
= the input current noise of the amplifier.
R
S
= 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 (e
n
) of the AD8551/AD8552/
AD8554 is 42 nV/√Hz, and the input noise, i
n
, is 2 fA/√Hz. The
e
n, 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.
Because the input current noise of the AD8551/AD8552/
AD8554 is very small, it does not become a dominant term
unless R
S
is greater than 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
BWee
TOTALn
n
×=
,
(16)
where BW is the bandwidth of interest in Hertz.
OUTPUT OVERDRIVE RECOVERY
The AD8551/AD8552/AD8554 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 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 AD8551/AD8552/
AD8554.
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.
INPUT OVERVOLTAGE PROTECTION
Although the AD8551/AD8552/AD8554 are rail-to-rail input
amplifiers, 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 AD8551/AD8552/AD8554 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 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.
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 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 and causing a radical shifting of internal
voltages, resulting in the erratic output behavior.
The AD8551/AD8552/AD8554 amplifiers have been carefully
designed to prevent any output phase reversal, provided both
inputs are maintained within the supply voltages. If there is the
potential of one or both inputs exceeding either supply voltage,
place a resistor in series with the input to limit the current to
less than 2 mA to ensure the output does not reverse its phase.