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AD743JR-16-REEL Datasheet(PDF) 8 Page - Analog Devices |
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AD743JR-16-REEL Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AD743 REV. C –8– Figures 26 and 27 show two ways to buffer and amplify the output of a charge output transducer. Both require using an amplifier which has a very high input impedance, such as the AD743. Figure 26 shows a model of a charge amplifier circuit. Here, amplification depends on the principle of conservation of charge at the input of amplifier A1, which requires that the charge on capacitor CS be transferred to capacitor CF, thus yielding an output voltage of ∆Q/C F. The amplifiers input voltage noise will appear at the output amplified by the noise gain (1 + (CS/CF)) of the circuit. Figure 26. A Charge Amplifier Circuit Figure 27. Model for a High Z Follower with Gain The second circuit, Figure 27, is simply a high impedance follower with gain. Here the noise gain (1 + (R1/R2)) is the same as the gain from the transducer to the output. Resistor RB, in both circuits, is required as a dc bias current return. There are three important sources of noise in these circuits. Amplifiers A1 and A2 contribute both voltage and current noise, while resistor RB contributes a current noise of: ~ N = 4k T RB ∆f where: k = Boltzman’s Constant = 1.381 x 10 –23 Joules/Kelvin T = Absolute Temperature, Kelvin (0 °C = +273.2 Kelvin) ∆f = Bandwidth – in Hz (Assuming an Ideal “Brick Wall” Filter) This must be root-sum-squared with the amplifier’s own current noise. Figure 28 shows that these two circuits have an identical frequency response and the same noise performance (provided that CS/CF = R1/ R2). One feature of the first circuit is that a “T” network is used to increase the effective resistance of RB and improve the low frequency cutoff point by the same factor. –100 –110 –120 –130 –140 –150 –160 –170 –180 –190 –200 –210 –220 10M 100M 1 10 100 1k 10k 100k FREQUENCY – Hz TOTAL OUTPUT NOISE NOISE DUE TO R ALONE B NOISE DUE TO I ALONE B Figure 28. Noise at the Outputs of the Circuits of Figures 26 and 27. Gain = 10, CS = 3000 pF, RB = 22 MΩ However, this does not change the noise contribution of RB which, in this example, dominates at low frequencies. The graph of Figure 29 shows how to select an RB large enough to minimize this resistor’s contribution to overall circuit noise. When the equivalent current noise of RB (( √4kT)/R) equals the noise of I B ( 2qI B ), there is diminishing return in making RB larger. 1pA 10pA 100pA 1nA 10nA 5.2 x 10 10 5.2 x 10 9 5.2 x 10 8 5.2 x 10 7 5.2 x 10 6 INPUT BIAS CURRENT Figure 29. Graph of Resistance vs. Input Bias Current where the Equivalent Noise √4kT/R, Equals the Noise of the Bias Current 2qIB To maximize dc performance over temperature, the source resistances should be balanced on each input of the amplifier. This is represented by the optional resistor RB in Figures 26 and 27. As previously mentioned, for best noise performance care should be taken to also balance the source capacitance designated by CB. The value for CB in Figure 26 would be equal to CS, in Figure 27. At values of CB over 300 pF, there is a diminishing impact on noise; capacitor CB can then be simply a large bypass of 0.01 µF or greater. |
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