AD743JN(2003) 查看數據表（PDF） - Analog Devices

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AD743JN
(Rev.:2003)

Ultralow Noise BiFET Op Amp

Analog Devices 

Figure 37b. An AC-Coupled, Low Noise

Hydrophone Amplifier

AD743

Where the dc gain is 1 and the gain above the low frequency

cutoff (1/(2πCC(100 Ω))) is the same as the circuit of Figure

37a. The circuit of Figure 37c uses a dc servo loop to keep the

dc output at 0 V and to maintain full dynamic range for IB’s up

to 100 nA. The time constant of R7 and C2 should be larger

than that of R1 and CT for a smooth low frequency response.

The transducer shown has a source capacitance of 7500 pF. For

smaller transducer capacitances (≤300 pF), lowest noise can be

achieved by adding a parallel RC network (R4 = R1, C1 = CT)

in series with the inverting input of the AD743.

BALANCING SOURCE IMPEDANCES

As mentioned previously, it is good practice to balance the

source impedances (both resistive and reactive) as seen by the

inputs of the AD743. Balancing the resistive components will

optimize dc performance over temperature because balancing

will mitigate the effects of any bias current errors. Balancing

input capacitance will minimize ac response errors due to the

amplifier’s input capacitance and, as shown in Figure 38, noise

performance will be optimized. Figure 39 shows the required

external components for noninverting (A) and inverting (B)

configurations.

Figure 37c. A Hydrophone Amplifier Incorporating a

DC Servo Loop

Figure 38. RTI Voltage Noise vs. Input Capacitance

REV. C

Figure 39. Optional External Components for Balancing Source Impedances

–11–

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