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LT1793AIN8 Datasheet(PDF) 8 Page - Linear Technology |
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LT1793AIN8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1793 S APPLICATI I FOR ATIO voltage noise, the thermal noise of the transducer, and the op amp’s input bias current noise times the transducer impedance. Figure 3 shows total input voltage noise versus source resistance. In a low source resistance (< 5k) application the op amp voltage noise will dominate the total noise. This means the LT1793 is superior to most JFET op amps. Only the lowest noise bipolar op amps have the advantage at low source resistances. As the source resistance increases from 5k to 50k, the LT1793 will match the best bipolar op amps for noise performance, since the thermal noise of the transducer (4kTR) begins to dominate the total noise. A further increase in source resistance, above 50k, is where the op amp’s current noise component (2qIBR2) will eventually dominate the total noise. At these high source resis- tances, the LT1793 will out perform the lowest noise bipolar op amps due to the inherently low current noise of FET input op amps. Clearly, the LT1793 will extend the range of high impedance transducers that can be used for high signal-to-noise ratios. This makes the LT1793 the best choice for high impedance, capacitive transducers. Optimization Techniques for Charge Amplifiers The high input impedance JFET front end makes the LT1793 suitable in applications where very high charge sensitivity is required. Figure 4 illustrates the LT1793 in its inverting and noninverting modes of operation. A charge amplifier is shown in the inverting mode example; the gain depends on the principal of charge conservation at the input of the LT1793. The charge across the transducer capacitance CS is transferred to the feedback capacitor CF with temperature will occur when the device is nulled with a potentiometer ranging from 10k to 200k. Finer adjust- ments can be made with resistors in series with the potentiometer (Figure 2b). Amplifying Signals from High Impedance Transducers The low voltage and current noise offered by the LT1793 makes it useful in a wide range of applications, especially where high impedance, capacitive transducers are used such as hydrophones, precision accelerometers and photodiodes. The total output noise in such a system is the gain times the RMS sum of the op amp’s input referred Figure 3. Comparison of LT1793 and LT1007 Total Output 1kHz Voltage Noise vs Source Resistance Figure 4. Inverting and Noninverting Gain Configurations SOURCE RESISTANCE ( Ω) 100 1 10 1k 10k 1k 100M 1G 1793 F03 100k 100 10M 10k 1M Vn = AV √Vn 2 (OP AMP) + 4kTR + 2qIBR 2 SOURCE RESISTANCE = 2RS = R * PLUS RESISTOR † PLUS RESISTOR 1000pF CAPACITOR RESISTOR NOISE ONLY LT1793 LT1007* LT1007 † LT1793 † LT1007 LT1793* CS CS RS RS VO R2 OUTPUT RB CB R1 CS RS CB ≅ CS RB = RS RS > R1 OR R2 TRANSDUCER OUTPUT CF CB RB CB = CFCS RB = RFRS RF CS RS TRANSDUCER 1793 F04 Q = CV; = I = C dQ dt dV dt |
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