AD8571ARM View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8571ARM Datasheet PDF : 19 Pages
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AD8571/AD8572/AD8574
0
؊20
؊40
VS = 5V
AV = 0dB
؊60
؊80
؊100
؊120
؊140
؊160
0 1 2 3 4 5 6 7 8 9 10
FREQUENCY – kHz
Figure 50a. Spectral Analysis of AD857x Output in
Unity Gain Configuration
0
؊20
VS = 5V
AV = 60dB
؊40
؊60
؊80
؊100
؊120
0 1 2 3 4 5 6 7 8 9 10
FREQUENCY – kHz
Figure 50b. Spectral Analysis of AD857x Output with
60 dB Gain
Figure 51 shows the spectral output of an AD8572 configured
in a high gain (60 dB) with a 1 mV input signal applied. Note
the absence of any IMD products in the spectrum. The signal-
to-noise (SNR) ratio of the output signal is better than 60 dB,
or 0.1%.
0
؊20
VS = 5V
AV = 60dB
؊40
؊60
؊80
؊100
؊120
0 1 2 3 4 5 6 7 8 9 10
FREQUENCY – kHz
Figure 51. Spectral Analysis of AD857x in High Gain with
an Input Signal
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:
1
( ) en, TOTAL
= en2 + 4kTrs
+
inrs
2
2
(15)
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)
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 will be domi-
nated 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 will be 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 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:
en = en, TOTAL × BW
(16)
Where BW is the bandwidth of interest in Hertz.
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 AD857x 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 ampli-
fier requires a substantial amount of time to error correct the
main amplifier back to a valid output. Figure 23 and Figure 24
show the positive and negative overdrive recovery time for the
AD857x.
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.
REV. 0
–13–
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