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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 REV. E –8– AD743 Figures 4 and 5 show two ways to buffer and amplify the output of a charge output transducer. Both require using an amplifier that has a very high input impedance, such as the AD743. Figure 4 shows a model of a charge amplifier circuit. Here, amplifica- tion depends on the principle of conservation of charge at the input of amplifier A1, which requires that the charge on capaci- tor CS be transferred to capacitor CF, thus yielding an output voltage of ∆Q/C F. The amplifier’s input voltage noise will appear at the output amplified by the noise gain (1 + (CS/CF)) of the circuit. A1 *OPTIONAL, SEE TEXT CS CF CB* RB* R1 = CS CF R1 R2 R2 RB* Figure 4. Charge Amplifier Circuit A2 *OPTIONAL, SEE TEXT CS CB* RB* R1 R2 RB Figure 5. Model for a High Z Follower with Gain The circuit in Figure 5 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. In both circuits, resistor RB 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 ˜ Nk T R f B = 4 ∆ where k = Boltzman’s Constant = 1.381 × 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 6 shows that these circuits in Figures 4 and 5 have an identical frequency response and 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 to improve the low frequency cutoff point by the same factor. –100 –110 –120 –130 –140 –150 –160 –170 –180 –190 –200 –210 –220 0.01 0.1 110 100 1k 10k 100k FREQUENCY (Hz) TOTAL OUTPUT NOISE NOISE DUE TO RB ALONE NOISE DUE TO IB ALONE Figure 6. Noise at the Outputs of the Circuits of Figures 4 and 5. Gain = +10, CS = 3000 pF, RB = 22 M Ω However, this does not change the noise contribution of RB which, in this example, dominates at low frequencies. The graph of Figure 7 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 ( √2qIB), there is diminishing return in making R B larger. 1pA 10pA 100pA 1nA 10nA 5.2 1010 5.2 109 5.2 107 5.2 106 5.2 108 INPUT BIAS CURRENT Figure 7. 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 4 and 5. 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 4 would be equal to CS in Figure 5. 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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