OPA1678 Datasheet, PDF (23/41 Pages) Texas Instruments – Low-Distortion Audio Operational Amplifiers

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OPA1678, OPA1679
SBOS855 – FEBRUARY 2017
Typical Application (continued)
8.2.2.4 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 which connect to the output of the circuit. Equation 5 gives an approximation of the
capacitor value requirements, and neglects the effects of R10, R11, R12, and R13 on the frequency response.
4.4 kΩ is used as a typical value for microphone preamplifier input impedance (R_{IN_MIC}) for the calculation.
C₅ C₆ C_OUT
C_OUT
t
2˜
2
S ˜ R_{IN _ MIC}
˜ 20
Hz
t
2 ˜ S ˜ 4.4
2
k: ˜ 20
Hz
t
3.6
PF
(5)
For simplicity, the same 22-μF capacitors selected for the power supply filtering are selected for C5 and C6 to
satisfy Equation 5. 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 which may cause instability. R12 and R13 discharge AC-
coupling capacitors C4 and C5 when phantom power is removed.
8.2.3 Application Curves
The frequency response of the preamplifier circuit is shown in Figure 37. 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 below the design goal of 20 dB. The reduction in gain is 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 nV_RMS or –119.27 dBu.
20
19
18
17
16
15
14
13
12
11
10
10
100
1k
10k
Frequency (Hz)
100k
Figure 37. Frequency Response of the Preamplifier Circuit for a 8-nF Piezo Element
1M
C001
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