OPA1677DBVT datasheet(20/51 Pages) TI

20 / 51 page

8.2.1.2 Detailed Design Procedure

8.2.1.2.1 Power Supply

In professional audio systems, phantom power is applied to the two signal lines that carry a differential audio

signal from the microphone. Figure 8-2 is a diagram of the system showing 48-V phantom power applied to

the differential signal lines between the piezo preamplifier output and the input of a professional microphone

preamplifier.

Piezo

Contact

Microphone

48 V

Phantom

Power

6.8 k

Microphone

Preamplifier

Differential

Signal Cable

Piezo

Preamplifier

Figure 8-2. System Diagram Showing the Application of Phantom Power to the Audio Signal Lines

A voltage divider is used to extract the common-mode phantom power from the differential audio signal in this

type of system. The voltage at center point of the voltage divider formed by R1 and R2 does not change when

audio signals are present on the signal lines (assuming R1 and R2 are matched). A Zener diode forces the

voltage at the center point of R1 and R2 to a regulated voltage. The values of R1 and R2 are determined by the

allowable voltage drop across these resistors from the current delivered to both op amp channels and the Zener

diode. There are two power supply current pathways in parallel, each sharing half the total current of the op amp

and Zener diode. Resistors R1 and R2 can be calculated using Equation 1:

OPA

6.8 k

(1)

A 24-V Zener diode is selected for this design, and 1 mA of current flows through the diode at idle conditions

to maintain the reverse-biased condition of the Zener diode. The maximum idle power supply current of both op

amp channels is 5 mA. Inserting these values into Equation 1 gives the values for R1 and R2 shown in Equation

OPA

24V

6.8 k

1.2 k

5.0 mA

1.0 mA

(2)

Using a value of 1.2 kΩ for resistors R1 and R2 establishes a 1-mA current through the Zener diode and

properly regulate the node to 24 V. Capacitor C1 forms a low-pass filter with resistors R1 and R2 to filter the

Zener diode noise and any residual differential audio signals. Mismatch in the values of R1 and R2 causes a

portion of the audio signal to appear at the voltage divider center point. The corner frequency of the low-pass

filter must be set below the audio band, as shown in Equation 3.

3dB

13 F

22 F

R || R

600

20 Hz

P o

˜ S ˜

: ˜

(3)

A 22-μF capacitor is selected because the capacitor meets the requirements for power supply filtering and is a

widely available denomination. A 0.1-µF capacitor (C2) is added in parallel with C1 as a high-frequency bypass

capacitor.

OPA1677, OPA1678, OPA1679

SBOS855E – JANUARY 2017 – REVISED DECEMBER 2022

www.ti.com

Submit Document Feedback

Product Folder Links: OPA1677 OPA1678 OPA1679

OPA1677DBVT Datasheet(PDF) 20 Page - Texas Instruments

OPA1677DBVT Datasheet(HTML) 20 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

Datasheet Download

Link URL