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AD743 Datasheet(PDF) 7 Page - Analog Devices |
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AD743 Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page REV. E AD743 –7– 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 2, which compares input voltage noise versus input source resis- tance of the OP27 and 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 3). 100 1k 10k 100k 1 10 100 1000 1M 10M SOURCE RESISTANCE ( ) OP27 AND RESISTOR AD743 AND RESISTOR RESISTOR NOISE ONLY AD743 AND RESISTOR OR OP27 AND RESISTOR ( ) (– – –) ( — ) RSOURCE RSOURCE O E Figure 2. Total Input Noise Spectral Density @ 1 kHz vs. Source Resistance SOURCE RESISTANCE ( ) OP27 AD743 100 10 1 0.1 100 1k 10k 100k 1M 10M Figure 3. 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 AD743 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 gen- erate 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. 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 magni- tude of the dc bias current ˜IqI f nB = 2 ∆ 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 ˜ / IkT R f n = 4 ∆ 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 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 appar- ent 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 match- ing 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 QCV I dQ dt == and 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). |
Similar Part No. - AD743_15 |
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Similar Description - AD743_15 |
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