OPA1692 Datasheet, PDF (13/23 Pages) Texas Instruments – Low-Power, Low-Noise and Low-Distortion Audio Operational Amplifiers

www.ti.com
OPA1692, OPA1694
SBOS566 – JUNE 2017
Typical Application (continued)
8.2.2 Detailed Design Procedure
The design requirements were selected to represent a high-performance wireless microphone application.
Wireless microphones typically employ an electret microphone element, an analog pre-amplifier circuit, and
transmit circuitry which may employ either analog or digital methods of transmission. Because these devices are
battery powered, all circuitry must be designed to consume as little power as possible while still achieving very
high audio performance. The performance specifications for the microphone used in this design are shown in
Table 1. This microphone element uses a 3-wire connection scheme with separate connections for power,
ground, and signal. The microphone datasheet specifies that the signal line be terminated with a 5.6-kΩ
resistance and a 5-V supply is recommended.
Table 1. Primo EM-173 Microphone Specifications
PARAMETER
Sensitivity
Output impedance
Signal-to-noise ratio (SNR)
Maximum input sound pressure level
Operating voltage
Operating current
VALUE
–37 dBV
600 Ω
80 dB
135 dB
5 V (3 V – 10 V)
600 µA
R1, C1, and R2 provide the correct termination impedance for the microphone; and AC-couple the microphone
signal to the amplifier input. R2 is chosen to be a large value, 100 kΩ, so that a smaller AC-coupling capacitor
can be used (C1). The high-pass corner frequency produced by C1 and R2 must be set to 20 Hz using
Equation 1:
20 Hz
1
2 ˜ S ˜ R₂ ˜ C₁
1
2 ˜ S ˜100
k: ˜ C₁
o
C₁
79.6 nF o 100 nF
(1)
R1 and R2 are in parallel for frequencies above 20 Hz. Therefore, the value of R1 is selected so that when in
parallel with R2, the combination results in a 5.6-kΩ resistance as specified in the microphone datasheet. R1 can
be calculated using Equation 2
5.6 k: ^{R1 ˜ R2}
R₁ R₂
R₁
R₁
˜100 k:
100 k:
o
R₁
5.9 k:
(2)
R3 and C2 form a low-pass filter to prevent electromagnetic interference (EMI) signals from being amplified. The
corner frequency this EMI filter is given in Equation 3
f _3dB
1
2 ˜ S ˜ R₃ ˜ C₂
1
15.9 MHz
2 ˜ S ˜100 : ˜100 pF
(3)
The input bias current of the OPA1692 through the 100-kΩ input resistor, R2 and can potentially cause a large
offset voltage to appear at the output of the amplifier. One solution to this problem is to match the DC resistance
of the circuit at each input of the amplifier. R4 and C3 accomplish this goal by providing a DC-feedback path for
the amplifier, R4, which has the same resistance as the input resistor R2. Capacitor C3 serves two functions.
First, at low-frequencies this capacitor is effectively an open circuit and therefore the gain of the amplifier is 1,
further reducing DC offsets at the output. At high frequencies where the impedance of the capacitor is low, the
feedback network of R5, R6, and C4 determine the gain of the amplifier.
The nominal gain of the preamplifier circuit is calculated by considering the output of the microphone at the
maximum input SPL. For this design, a maximum input SPL of 120 dB, or 20 pascals (Pa), is specified. The
microphone sensitivity is given as –37 dBV, measured at 1-Pa air pressure. The output signal of the microphone
at 20-Pa air pressure can be calculated by converting the –37 dBV sensitivity specification to mV per pascal of
air pressure as illustrated in Equation 4:
§ 37 dBV ·
V_OUT(MIC) 20 Pa u10^{¨© 20 ¸¹} 282.5 mV_RMS 399.5 mVp
(4)
Copyright © 2017, Texas Instruments Incorporated
Product Folder Links: OPA1692 OPA1694
Submit Documentation Feedback
13