Datasheet AD745 (Analog Devices) - 8

AD745 OP AMP PERFORMANCE JFET VERSUS BIPOLAR
The 0.1 Hz to 10 Hz noise is typically 0.38 µV p-p. The user The AD745 offers the low input voltage noise of an industry should pay careful attention to several design details to optimize standard bipolar opamp without its inherent input current low frequency noise performance. Random air currents can errors. This is demonstrated in Figure 3, which compares input generate varying thermocouple voltages that appear as low voltage noise vs. input source resistance of the OP37 and the frequency noise. Therefore, sensitive circuitry should be well AD745 opamps. From this figure, it is clear that at high source shielded from air flow. Keeping absolute chip temperature low impedance the low current noise of the AD745 also provides also reduces low frequency noise in two ways: first, the low lower total noise. It is also important to note that with the AD745 frequency noise is strongly dependent on the ambient tempera- this noise reduction extends all the way down to low source ture and increases above 25°C. Second, since the gradient of impedances. The lower dc current errors of the AD745 also temperature from the IC package to ambient is greater, the reduce errors due to offset and drift at high source impedances noise generated by random air currents, as previously mentioned, (Figure 4). will be larger in magnitude. Chip temperature can be reduced The internal compensation of the AD745 is optimized for higher both by operation at reduced supply voltages and by the use of a gains, providing a much higher bandwidth and a faster slew suitable clip-on heat sink, if possible. rate. This makes the AD745 especially useful as a preamplifier, Low frequency current noise can be computed from the where low-level signals require an amplifier that provides both magnitude of the dc bias current high amplification and wide bandwidth at these higher gains.  ~  I = 2 n qIB∆f  
1000 R
and increases below approximately 100 Hz with a 1/f power
SOURCE
spectral density. For the AD745 the typical value of current
OP37 AND E
noise is 6.9 fA/√Hz at 1 kHz. Using the formula:
O RESISTOR
~
nV/ Hz RSOURCE – 100
I n = 4kT/R∆f
GE A
to compute the Johnson noise of a resistor, expressed as a
T L O AD745 AND RESISTOR
current, one can see that the current noise of the AD745 is
V AD745 AND OR RESISTOR OP37 AND RESISTOR
equivalent to that of a 3.45 × 108 Ω source resistance.
10
At high frequencies, the current noise of a FET increases pro- portionately to frequency. This noise is due to the “real” part of
INPUT NOISE
the gate input impedance, which decreases with frequency. This noise component usually is not important, since the voltage
RESISTOR NOISE ONLY 1
noise of the amplifier impressed upon its input capacitance is an
100 1k 10k 100k 1M 10M
apparent current noise of approximately the same magnitude.
SOURCE RESISTANCE –
In any FET input amplifier, the current noise of the internal Figure 3. Total Input Noise Spectral Density @ 1 kHz bias circuitry can be coupled externally via the gate-to-source vs. Source Resistance capacitances and appears as input current noise. This noise is
100
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.
mV – OP37G 10 GE A T LOW NOISE CHARGE AMPLIFIERS L O V
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
1.0
capacitive accelerometers and hydrophones. When dealing with
AD745 KN INPUT OFFSET
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
0.1 100 1k 10k 100k 1M 10M
fundamental relationships:
SOURCE RESISTANCE –
Figure 4. Input Offset Voltage vs. Source Resistance Q = CV and I = dQ dt As shown, voltage, current and charge noise can all be directly
DESIGNING CIRCUITS FOR LOW NOISE
related. The change in open circuit voltage (∆V) on a capacitor An opamp’s input voltage noise performance is typically divided will equal the combination of the change in charge (∆Q/C) and into two regions: flatband and low frequency noise. The AD745 the change in capacitance with a built-in charge (Q/∆C). offers excellent performance with respect to both. The figure of 2.9 nV/冑Hz @ 10 kHz is excellent for a JFET input amplifier. REV. D –7–