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AD745JN Datasheet PDF : 12 Pages
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AD745
OP AMP PERFORMANCE JFET VS. BIPOLAR
The AD745 offers 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 OP37 and
the AD745 op amps. From this figure, it is clear that at high
source impedance the low current noise of the AD745 also
provides lower total noise. It is also important to note that with
the AD745 this noise reduction extends all the way down to low
source impedances. The lower dc current errors of the AD745
also reduce errors due to offset and drift at high source
impedances (Figure 25).
The internal compensation of the AD745 is optimized for
higher gains, providing a much higher bandwidth and a faster
slew rate. This makes the AD745 especially useful as a
preamplifier, where low level signals require an amplifier that
provides both high amplification and wide bandwidth at these
higher gains.
1000
R SOURCE
EO
OP37 &
RESISTOR
(—)
100
R SOURCE
AD745 & RESISTOR
OR
OP37 & RESISTOR
AD745 + RESISTOR
()
10
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
ADOP37G
10
1.0
AD745 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 typically
divided into two regions: flatband and low frequency noise. The
AD745 offers excellent performance with respect to both. The
figure of 2.9 nV/͙Hz @ 10 kHz is excellent for a 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
=
2qI B ∆f
and
increases
below approximately 100 Hz with a 1/f power spectral density.
For the AD745 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 AD745 is equivalent to that of a
3.45 × 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 AD745 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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