Datasheet LT1793 (Analog Devices) - 9

LT1793
O U U W U APPLICATI S I FOR ATIO
resulting in a change in voltage dV, which is equal to dQ/CF. for RB is determined by equating the thermal noise (4kTRS) The gain therefore is C 2 F/CS. For unity-gain, the CF should to the current noise (2qIB) times RS . Solving for RS equal the transducer capacitance plus the input capaci- results in RB = RS = 2VT/IB (VT = 26mV at 25°C). A parallel tance of the LT1793 and RF should equal RS. capacitor CB, is used to cancel the phase shift caused by the op amp input capacitance and R In the noninverting mode example, the transducer current B. is converted to a change in voltage by the transducer
Reduced Power Supply Operation
capacitance, CS. This voltage is then buffered by the LT1793 with a gain of 1 + R1/R2. A DC path is provided by To take full advantage of a wide input common mode range, R the LT1793 was designed to eliminate phase reversal. S, which is either the transducer impedance or an exter- nal resistor. Since R Referring to the photographs in Figure 5, the LT1793 is S is usually several orders of magni- tude greater than the parallel combination of R1 and R2, R shown operating in the follower mode (A B V = 1) at ±5V is added to balance the DC offset caused by the noninvert- supplies with the input swinging ±5.2V. The output of the ing input bias current and R LT1793 clips cleanly and recovers with no phase reversal. S. The input bias currents, although small at room temperature, can create significant This has the benefit of preventing lockup in servo systems errors at higher temperature, especially with transducer and minimizing distortion components. resistances of up to 1000M or more. The optimum value
Input:
±
5.2V Sine Wave LT1793 Output
LT1793 F05a LT1793 F05b
Figure 5. Voltage Follower with Input Exceeding the Common Mode Range (VS =
±
5V)
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