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AD745JN Datasheet(PDF) 7 Page - Analog Devices |
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AD745JN Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page 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 100 100 10 1 1k 10k 100k 1M 10M RSOURCE RSOURCE EO 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 10M 1k 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/ 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 ~ In = 2qIB∆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 × 108 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). |
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