REV. D
AD745
–7–
OP AMP PERFORMANCE JFET VERSUS BIPOLAR
The AD745 offers the low input voltage noise of an industry
standard bipolar opamp without its inherent input current
errors. This is demonstrated in Figure 3, which compares input
voltage noise vs. input source resistance of the OP37 and the
AD745 opamps. 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 4).
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.
SOURCE RESISTANCE –
1000
100
INPUT NOISE VOLTAGE – nV/ Hz
100
10
1
1k 10k 100k 1M 10M
R
SOURCE
R
SOURCE
E
O
OP37 AND
RESISTOR
AD745 AND
RESISTOR
AD745 AND RESISTOR
OR
OP37 AND RESISTOR
RESISTOR NOISE ONLY
Figure 3. Total Input Noise Spectral Density @ 1 kHz
vs. Source Resistance
SOURCE RESISTANCE –
100
10
0.1
100 10M1k
INPUT OFFSET VOLTAGE – mV
10k 100k 1M
1.0
OP37G
AD745 KN
Figure 4. Input Offset Voltage vs. Source Resistance
DESIGNING CIRCUITS FOR LOW NOISE
An opamp’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/冑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 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 tempera-
ture and increases above 25°C. Second, 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
~
I
n
= 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:
I
~
n
= 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 × 10
8
Ω source resistance.
At high frequencies, the current noise of a FET increases pro-
portionately 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).
