LT1793CS8 datasheet(9/12 Pages) LINER

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LT1793

APPLICATI

I FOR ATIO

Input:

±5.2V Sine Wave

LT1793 Output

LT1793 F05a

LT1793 F05b

Figure 5. Voltage Follower with Input Exceeding the Common Mode Range (VS = ±5V)

resulting in a change in voltage dV, which is equal to dQ/CF.

The gain therefore is CF/CS. For unity-gain, the CF should

equal the transducer capacitance plus the input capaci-

tance of the LT1793 and RF should equal RS.

In the noninverting mode example, the transducer current

is converted to a change in voltage by the transducer

capacitance, CS. This voltage is then buffered by the

LT1793 with a gain of 1 + R1/R2. A DC path is provided by

RS, which is either the transducer impedance or an exter-

nal resistor. Since RS is usually several orders of magni-

tude greater than the parallel combination of R1 and R2, RB

is added to balance the DC offset caused by the noninvert-

ing input bias current and RS. The input bias currents,

although small at room temperature, can create significant

errors at higher temperature, especially with transducer

resistances of up to 1000M or more. The optimum value

for RB isdeterminedbyequatingthethermalnoise(4kTRS)

to the current noise (2qIB) times RS2. Solving for RS

results in RB = RS = 2VT/IB (VT = 26mV at 25°C). A parallel

capacitor CB, is used to cancel the phase shift caused by

the op amp input capacitance and RB.

Reduced Power Supply Operation

To take full advantage of a wide input common mode range,

the LT1793 was designed to eliminate phase reversal.

Referring to the photographs in Figure 5, the LT1793 is

shown operating in the follower mode (AV = 1) at ±5V

supplies with the input swinging

±5.2V. The output of the

LT1793 clips cleanly and recovers with no phase reversal.

This has the benefit of preventing lockup in servo systems

and minimizing distortion components.

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