Datasheet LT1793 (Analog Devices) - 8

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