OPA1677DBVT datasheet(22/52 Pages) TI

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8.2.1.2.4 Output Network

The audio signal is ac-coupled onto the microphone signal lines through capacitors C5 and C6. The value of

capacitors C5 and C6 are determined by the low-frequency design requirements and the input impedance of

the microphone preamplifier that connect to the output of the circuit. Equation 9 shows an approximation of the

capacitor value requirements, and neglects the effects of R10, R11, R12, and R13 on the frequency response.

The microphone preamplifier input impedance (RIN_MIC) uses a typical value of 4.4 kΩ for the calculation.

OUT

IN _ MIC

3.6 F

20 Hz

4.4 k

20 Hz

˜ S ˜

: ˜

(9)

For simplicity, the same 22-μF capacitors selected for the power supply filtering are selected for C5 and C6

to satisfy Equation 9. At least 50-V rated capacitors must be used for C5 and C6. If polarized capacitors are

used, the positive terminal must be oriented towards the microphone preamplifier. Resistors R10 and R11 isolate

the op amp outputs from the capacitances of long cables that may cause instability. R12 and R13 discharge

ac-coupling capacitors C4 and C5 when phantom power is removed.

8.2.1.3 Application Curves

The frequency response of the preamplifier circuit is shown in Figure 8-3. The –3-dB frequencies are 15.87 Hz

and 181.1 kHz, which meet the design requirements. The gain within the passband of the circuit is 18.9 dB,

slightly less than the design goal of 20 dB. The reduction in gain is a result of the voltage division between the

output resistors of the piezo preamplifier circuit and the input impedance of the microphone preamplifier. The

A-weighted noise of the circuit (referred to the input) is 842.2 nVRMS or –119.27 dBu.

100

10k

100k

Frequency (Hz)

C001

Figure 8-3. Frequency Response of the Preamplifier Circuit for a 8-nF Piezo Element

OPA1677, OPA1678, OPA1679

SBOS855D – JANUARY 2017 – REVISED DECEMBER 2021

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OPA1677DBVT Datasheet(PDF) 22 Page - Texas Instruments