AD8551/AD8552/AD8554
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 equiva-
lent 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:
Input Overvoltage Protection
Although the AD855x is a rail-to-rail input amplifier, care should
be taken to ensure that the potential difference between the in-
puts does not exceed +5 V. Under normal operating conditions,
the amplifier will correct its output to ensure the two inputs are at
the same voltage. However, if the device is configured as a com-
parator, 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 AD855x
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 will 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 will become forward-biased. Without current limit-
ing, excessive amounts of current could 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 maximum.
1
2
)
2
en, TOTAL = en2 + 4kTr + i r
(15)
(
S
n S
Where, en = The input voltage noise 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 ϫ 10-23 J/K)
T = Ambient temperature in Kelvin (K = 273.15 + °C)
Output Phase Reversal
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 will be domi-
nated by 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 will be dominated by the Johnson
noise of the resistor itself.
Output phase reversal occurs in some amplifiers when the input
common-mode voltage range is exceeded. As common-mode volt-
age is moved outside of the common-mode range, the outputs of
these amplifiers will 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 which results in
the erratic output behavior.
Because the input current noise of the AD855x 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 AD855x 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. This will ensure
the output will not reverse its phase.
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:
(16)
en = en, TOTAL × BW
Capacitive Load Drive
Where BW is the bandwidth of interest in Hertz.
The AD855x has excellent capacitive load driving capabilities
and can safely drive up to 10 nF from a single +5 V supply.
Although the device is stable, capacitive loading will limit the
bandwidth of the amplifier. Capacitive loads will also increase
the amount of overshoot and ringing at the output. An R-C
snubber network, Figure 54, can be used to compensate the
amplifier against capacitive load ringing and overshoot.
For a complete treatise on circuit noise analysis, please refer to the
1995 Linear Design Seminar book available from Analog Devices.
Output Overdrive Recovery
The AD855x amplifiers have an excellent overdrive recovery of
only 200 µs from either supply rail. This characteristic is particu-
larly difficult for autocorrection amplifiers, as the nulling amplifier
requires a nontrivial amount of time to error correct the main am-
plifier back to a valid output. Figure 23 and Figure 24 show the
positive and negative overdrive recovery time for the AD855x.
+5V
V
AD855x
OUT
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 half-way 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. Most competitors’ auto-
correction amplifiers require a number of autozero clock cycles
to recover from output overdrive and some can take several
milliseconds for the output to settle properly.
R
60⍀
V
X
IN
200mV p-p
C
4.7nF
L
C
X
0.47F
Figure 54. Snubber Network Configuration for Driving
Capacitive Loads
Although the snubber will not recover the loss of amplifier band-
width from the load capacitance, it will allow the amplifier to drive
larger values of capacitance while maintaining a minimum of
overshoot and ringing. Figure 55 shows the output of an AD855x
driving a 1 nF capacitor with and without a snubber network.
–14–
REV. 0
ADI [ ADI ]
