OPA1677DBVR datasheet(21/56 Pages) TI2

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8.2.1.2.2 Input Network

Resistors R3 and R4 provide a pathway for the input bias current of the OPA1678 while maintaining the high

input impedance of the circuit. The contact microphone capacitance and the required

low-frequency response determine the values of R3 and R4. The –3-dB frequency formed by the microphone

capacitance and amplifier input impedance is shown in Equation 4:

3dB

MIC

20 Hz

R ) C

˜ S ˜

(4)

A piezo element with 8 nF of capacitance was selected for this design because the 9-kHz resonance is towards

the upper end of the audible bandwidth, and is less likely to affect the frequency response of many musical

instruments. The minimum value for resistors R3 and R4 is then calculated with Equation 5:

3dB

MIC

497.4 k

20 Hz 8 nF

˜ S ˜

(5)

1-MΩ resistors are selected for R3 and R4 to make sure the circuit meets the design requirements for –3-dB

bandwidth. The center point of resistors R3 and R4 is biased to half the supply voltage through the voltage

divider formed by R5 and R6. This sets the input common-mode voltage of the circuit to a value within the input

voltage range of the OPA1678. Piezo elements can produce very large voltages if the elements are struck with

sufficient force. To prevent damage, the input of the OPA1678 is protected by a transient voltage suppressor

(TVS) diode placed across the preamplifier inputs. The TPD1E1B04 TVS was selected due to low capacitance

and the 6.4-V clamping voltage does not clamp the desired low amplitude vibration signals. Resistors R14 and

R15 limit current flow into the amplifier inputs in the event that the internal protection diodes of the amplifier are

forward-biased.

8.2.1.2.3 Gain

R7, R8, and R9 determines the gain of the preamplifier circuit. The gain of the circuit is shown in Equation 6:

10 V/V

(6)

Resistors R7 and R9 are selected with a value of 2 kΩ to avoid loading the output of the OPA1678 and

producing distortion. The value of R8 is then calculated in Equation 7:

2 k

444.4

442

: o

(7)

Capacitors C3 and C4 limit the bandwidth of the circuit so that signals outside the audio bandwidth are not

amplified. The corner frequency produced by capacitors C3 and C4 is shown in Equation 8. This corner

frequency must be above the desired –3-dB bandwidth point to avoid attenuating high-frequency audio signals.

3dB

7/9

3.98 nF

20 kHz 2 k

˜ S ˜

(8)

C3 and C4 are 390-pF capacitors, which places the corner frequency approximately 1 decade above the desired

–3-dB bandwidth point. Capacitors C3 and C4 must be NP0 or C0G type ceramic capacitors or film capacitors.

Other ceramic dielectrics, such as X7R, are not suitable for these capacitors and produce distortion.

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OPA1677, OPA1678, OPA1679

SBOS855E – JANUARY 2017 – REVISED DECEMBER 2022

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