OPA1678 Datasheet, PDF (20/41 Pages) Texas Instruments – Low-Distortion Audio Operational Amplifiers

OPA1678, OPA1679
SBOS855 – FEBRUARY 2017
www.ti.com
8.2 Typical Application
Contact microphones are useful for amplifying the sound of musical instruments which do not contain electrical
pickups, such as acoustic guitars and violins. Most contact microphones use a piezo element to convert
vibrations in the body of the musical instrument to a voltage which may be amplified or recorded. The low noise
and low input bias current of the OPA1678 make the device an excellent choice for high impedance preamplifiers
for piezo elements. This preamplifier circuit provides high input impedance for the piezo element but has low
output impedance for driving long cable runs. The circuit is also designed to be powered from 48-V phantom
power which is commonly available in professional microphone preamplifiers and recording consoles.
A TINA-TI ™ simulation schematic of the circuit below is available in the Tools and Software section of the
OPA167x product folder.
R1
1.2 k
Piezo
Contact
Microphone
C2
C1
0.1 F 22 F ₊
R14
100
½ OPA1678
+ ^VS+
ZD1
24 V
R2
1.2 k
R3
1M
±
VOUT
VS±
R7 2 k
R10
100
C5
22 F
TPD1E1B04
R5
100 k
R8
442
R6
100 k
C3 390 pF
C4 390 pF
R12
100 k
R13
100 k
To
Microphone
Preamplifier
R9 2 k
R4
1M
R15
±
100
+
R11
100
C6
22 F
½ OPA1678
Copyright © 2017, Texas Instruments Incorporated
Figure 35. Phantom-Powered Preamplifier for Piezo Contact Microphones
8.2.1 Design Requirements
• –3 dB Bandwidth: 20 Hz to 20 kHz
• Gain: 20 dB (10 V/V)
• Piezo Element Capacitance: 8 nF (9-kHz resonance)
20
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