Datasheet AD743 (Analog Devices) - 7

AD743 OP AMP PERFORMANCE: JFET VS. BIPOLAR
low frequency noise performance. Random air currents can gen- The AD743 is the first monolithic JFET op amp to offer the low erate varying thermocouple voltages that appear as low frequency input voltage noise of an industry-standard bipolar op amp without noise; therefore, sensitive circuitry should be well shielded from its inherent input current errors. This is demonstrated in Figure 2, air flow. Keeping absolute chip temperature low also reduces low which compares input voltage noise versus input source resis- frequency noise in two ways. First, the low frequency noise is tance of the OP27 and AD743 op amps. From this figure, it is strongly dependent on the ambient temperature and increases clear that at high source impedance the low current noise of the above +25°C. Second, since the gradient of temperature from the AD743 also provides lower total noise. It is also important to IC package to ambient is greater, the noise generated by random note that with the AD743 this noise reduction extends all the air currents, as previously mentioned, will be larger in magnitude. way down to low source impedances. The lower dc current errors Chip temperature can be reduced both by operation at reduced of the AD743 also reduce errors due to offset and drift at high supply voltages and by the use of a suitable clip-on heat sink, source impedances (Figure 3). if possible. Low frequency current noise can be computed from the magni-
1000
tude of the dc bias current
R OP27 AND SOURCE RESISTOR
˜
E ( — )
I = 2qI f ∆
O
n B
z) H /
and increases below approximately 100 Hz with a 1/f power spectral
R 100 SOURCE (nV
density. For the AD743, the typical value of current noise is
E IS
6.9 fA/√Hz at 1 kHz. Using the formula
O AD743 AND RESISTOR N AD743 AND OR E RESISTOR
˜
G OP27 AND RESISTOR
I = 4kT / R f ∆
( )
n
LTA O 10
to compute the Johnson noise of a resistor, expressed as a current,
T V U
one can see that the current noise of the AD743 is equivalent to
P IN
that of a 3.45 108 Ω source resistance.
RESISTOR NOISE ONLY (– – –)
At high frequencies, the current noise of a FET increases pro- portionately to frequency. This noise is due to the “real” part of
1 100 1k 10k 100k 1M 10M
the gate input impedance, which decreases with frequency. This
SOURCE RESISTANCE (
⍀
)
noise component usually is not important, since the voltage noise Figure 2. Total Input Noise Spectral Density @ 1 kHz of the amplifier impressed upon its input capacitance is an appar- vs. Source Resistance ent current noise of approximately the same magnitude. In any FET input amplifier, the current noise of the internal
100
bias circuitry can be coupled externally via the gate-to-source capacitances and appears as input current noise. This noise is
OP27
totally correlated at the inputs, so source impedance match- ing will tend to cancel out its effect. Both input resistance and
10
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
1
noise. This combination makes this device particularly suitable
INPUT OFFSET VOLTAGE (mV) AD743
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
0.1
charge uncertainty as a measure of system noise.
100 1k 10k 100k 1M 10M SOURCE RESISTANCE (
⍀
)
Charge (Q) is related to voltage and current by the simply stated Figure 3. Input Offset Voltage vs. Source Resistance fundamental relationships dQ
DESIGNING CIRCUITS FOR LOW NOISE
Q = CV and I = dt An op amp’s input voltage noise performance is typically divided into two regions: flatband and low frequency noise. The AD743 As shown, voltage, current, and charge noise can all be directly offers excellent performance with respect to both. The figure of related. The change in open circuit voltage (∆V) on a capacitor 2.9 nV/√Hz @ 10 kHz is excellent for a JFET input amplifier. The will equal the combination of the change in charge (∆Q/C) and 0.1 Hz to 10 Hz noise is typically 0.38 µV p-p. The user should the change in capacitance with a built in charge (Q/∆C). pay careful attention to several design details in order to optimize REV. E –7– Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS CONNECTION DIAGRAMS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD SUSCEPTIBILITY ORDERING GUIDE Typical Performance Characteristics OP AMP PERFORMANCE: JFET VS. BIPOLAR DESIGNING CIRCUITS FOR LOW NOISE LOW NOISE CHARGE AMPLIFIERS HOW CHIP PACKAGE TYPE AND POWER DISSIPATION AFFECT INPUT BIAS CURRENT REDUCED POWER SUPPLY OPERATION FOR LOWER IB AN INPUT IMPEDANCE COMPENSATED, SALLEN-KEY FILTER TWO HIGH PERFORMANCE ACCELEROMETER AMPLIFIERS LOW NOISE HYDROPHONE AMPLIFIER BALANCING SOURCE IMPEDANCES OUTLINE DIMENSIONS Revision History