OPA1678 Datasheet, PDF (21/41 Pages) Texas Instruments – Low-Distortion Audio Operational Amplifiers

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OPA1678, OPA1679
SBOS855 – FEBRUARY 2017
Typical Application (continued)
8.2.2 Detailed Design Procedure
8.2.2.1 Power Supply
In professional audio systems, phantom power is applied to the two signal lines which carry a differential audio
signal from the microphone. Figure 36 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.
R2
R1
6.8 k
6.8 k
48 V
Phantom
Power
+
Piezo
Contact
Microphone
±
+
Differential
Signal Cable
±
Piezo
Preamplifier
Microphone
Preamplifier
Figure 36. 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 is 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 :
R₁ R₂ R_PS
V_ZD
§
¨©
I_OPA
2
I_ZD
2
·
¸¹
6.8 k:
R_PS
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. The maximum idle power supply current of both op amp
channels is 5 mA. Inserting these values into gives the values for R1 and R2 shown in .
24V
§
¨©
I_OPA
2
I_ZD
2
·
¸¹
6.8 k:
24V
§ 5.0 mA 1.0 mA ·
¨© ₂
2 ^¸¹
6.8 k:
1.2 k:
R_PS
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 .
C₁
t
1
2 ˜ S ˜ R₁ || R₂ ˜ f
3dB
t
1
2 ˜ S ˜ 600 : ˜ 20
Hz
t 13
PF o 22
PF
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.
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