LT1113 applicaTions inForMaTion R2 RF CB CF RB – – R1 OUTPUT C OUTPUT + S RS + TRANSDUCER CB ≅ CS CB = CF||CS CS RS RB = RS RB = RF||RS RS > R1 OR R2 C dV B RB Q = CV; = I = C dQ dt dt TRANSDUCER 1113 • F02 Figure 2. Noninverting and Inverting Gain Configurations (2qIB RTRANS) will eventually dominate the total noise. At parallel combination of R1 and R2, RB is added to balance these high source resistances, the LT1113 will out perform the DC offset caused by the noninverting input bias current the lowest noise bipolar op amp due to the inherently low and RS. The input bias currents, although small at room current noise of FET input op amps. Clearly, the LT1113 temperature, can create significant errors over increasing will extend the range of high impedance transducers temperature, especially with transducer resistances of up that can be used for high signal to noise ratios. This to 100M or more. The optimum value for RB is determined makes the LT1113 the best choice for high impedance, by equating the thermal noise (4kTRS) to the current noise capacitive transducers. (2qI 2 B) times RS . Solving for RS results in RB = RS = 2VT/IB The high input impedance JFET front end makes the LT1113 kT suitable in applications where very high charge sensitivity VT = = 26mV at 25°C is required. Figure 2 illustrates the LT1113 in its inverting q and noninverting modes of operation. A charge amplifier A parallel capacitor, CB, is used to cancel the phase shift is shown in the inverting mode example; here the gain caused by the op amp input capacitance and RB. depends on the principal of charge conservation at the input of the LT1113. The charge across the transducer Reduced Power Supply Operation capacitance, CS, is transferred to the feedback capacitor C The LT1113 can be operated from ±5V supplies for lower F, resulting in a change in voltage, dV, equal to dQ/CF. The gain therefore is 1 + C power dissipation resulting in lower IB and noise at the F/CS. For unity gain, CF should equal the transducer capacitance plus the input capacitance expense of reduced dynamic range. To illustrate this benefit, of the LT1113 and R let’s look at the following example: F should equal RS. In the noninvert- ing mode example, the transducer current is converted An LT1113CS8 operates at an ambient temperature of 25°C to a change in voltage by the transducer capacitance; with ±15V supplies, dissipating 318mW of power (typical this voltage is then buffered by the LT1113 with a gain of supply current = 10.6mA for the dual). The SO-8 package 1 + R1/R2. A DC path is provided by RS, which is either has a θJA of 190°C/W, which results in a die temperature the transducer impedance or an external resistor. Since increase of 60.4°C or a room temperature die operating RS is usually several orders of magnitude greater than the temperature of 85.4°C. At ±5V supplies, the die tempera- 1113fc 10 For more information www.linear.com/LT1113
