AD743KN データシートの表示（PDF） - Analog Devices
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AD743KN Datasheet PDF : 12 Pages
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AD743
OP AMP PERFORMANCE: JFET VS. BIPOLAR
The AD743 is the first monolithic JFET op amp to offer the low
input voltage noise of an industry-standard bipolar op amp
without its inherent input current errors. This is demonstrated
in Figure 24, which compares input voltage noise vs. input
source resistance of the OP27 and the AD743 op amps. From
this figure, it is clear that at high source impedance the low
current noise of the AD743 also provides lower total noise. It is
also important to note that with the AD743 this noise reduction
extends all the way down to low source impedances. The lower
dc current errors of the AD743 also reduce errors due to offset
and drift at high source impedances (Figure 25).
1000
R SOURCE
EO
OP27 &
RESISTOR
(—)
100
R SOURCE
AD743 & RESISTOR
OR
OP27 & RESISTOR
10
AD743 + RESISTOR
()
RESISTOR NOISE ONLY
(– – –)
1
100
1k
10k
100k
1M
10M
SOURCE RESISTANCE – Ω
Figure 24. Total Input Noise Spectral Density @ 1 kHz vs.
Source Resistance
100
ADOP27G
10
1.0
AD743 KN
0.1
100
1k
10k
100k
1M
10M
SOURCE RESISTANCE – Ω
Figure 25. Input Offset Voltage vs. Source Resistance
DESIGNING CIRCUITS FOR LOW NOISE
An op amp’s input voltage noise performance is typicaly divided
into two regions: flatband and low frequency noise. The AD743
offers excellent performance with respect to both. The figure of
2.9 nV/√Hz @ 10 kHz is excellent for JFET input amplifier.
The 0.1 Hz to 10 Hz noise is typically 0.38 µV p-p. The user
should pay careful attention to several design details in order to
optimize low frequency noise performance. Random air currents
can generate varying thermocouple voltages that appear as low
frequency noise: therefore sensitive circuitry should be well
shielded from air flow. Keeping absolute chip temperature low
also reduces low frequency noise in two ways: first, the low
frequency noise is strongly dependent on the ambient
temperature and increases above +25°C. Secondly, since the
gradient of temperature from the IC package to ambient is
greater, the noise generated by random air currents, as
previously mentioned, will be larger in magnitude. Chip
temperature can be reduced both by operation at reduced
supply voltages and by the use of a suitable clip-on heat sink, if
possible.
Low frequency current noise can be computed from the
~
magnitude of the dc bias current (In = 2qIB∆f ) and increases
below approximately 100 Hz with a 1/f power spectral density.
For the AD743 the typical value of current noise is 6.9 fA/√Hz
~
at 1 kHz. Using the formula, In = 4kT /R∆f , to compute the
Johnson noise of a resistor, expressed as a current, one can see
that the current noise of the AD743 is equivalent to that of a
3.45 ϫ 108 Ω source resistance.
At high frequencies, the current noise of a FET increases
proportionately to frequency. This noise is due to the “real” part
of the gate input impedance, which decreases with frequency.
This noise component usually is not important, since the voltage
noise of the amplifier impressed upon its input capacitance is an
apparent current noise of approximately the same magnitude.
In any FET input amplifier, the current noise of the internal
bias circuitry can be coupled externally via the gate-to-source
capacitances and appears as input current noise. This noise is
totally correlated at the inputs, so source impedance matching
will tend to cancel out its effect. Both input resistance and input
capacitance should be balanced whenever dealing with source
capacitances of less than 300 pF in value.
LOW NOISE CHARGE AMPLIFIERS
As stated, the AD743 provides both low voltage and low current
noise. This combination makes this device particularly suitable
in applications requiring very high charge sensitivity, such as
capacitive accelerometers and hydrophones. When dealing with
a high source capacitance, it is useful to consider the total input
charge uncertainty as a measure of system noise.
Charge (Q) is related to voltage and current by the simply stated
fundamental relationships:
Q
=
CV
and
I
=
dQ
dt
As shown, voltage, current and charge noise can all be directly
related. The change in open circuit voltage (∆V) on a capacitor
will equal the combination of the change in charge (∆Q/C) and
the change in capacitance with a built in charge (Q/∆C).
REV. C
–7–
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