OPA1677DBVT datasheet(19/52 Pages) TI

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8.2 Typical Applications

8.2.1 Phantom-Powered Preamplifier for Piezo Contact Microphones

Contact microphones are useful for amplifying the sound of musical instruments that 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

OPA1678 or OPA1679 product folder.

VS+

VOUT

VS±

R7 2 k

442

C3 390 pF

R9 2 k

R10

100

22 F

R11

100

22 F

100 k

1 M

22 F +

1.2 k

Piezo

Contact

Microphone

1 M

100 k

C4 390 pF

R12

100 k

R13

100 k

½ OPA1678

Microphone

Preamplifier

1.2 k

R14

100

TPD1E1B04

R15

100

ZD1

24 V

0.1 F

Figure 8-1. Phantom-Powered Preamplifier for Piezo Contact Microphones

8.2.1.1 Design Requirements

• –3-dB bandwidth: 20 Hz to 20 kHz

• Gain: 20 dB (10 V/V)

• Piezo element capacitance: 8 nF (9-kHz resonance)

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OPA1677, OPA1678, OPA1679

SBOS855D – JANUARY 2017 – REVISED DECEMBER 2021

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