AS1701, AS1706
Data Sheet - Application Information
Input Filter
Input capacitor CIN (if used), in conjunction with RIN, forms a high-pass filter that removes the DC bias from an incom-
ing signal. CIN allows the amplifier to bias the signal to an optimum DC level. Assuming zero source impedance, the -
3dB point of the high-pass filter is given by:
1
(EQ 12)
f-3dB =
2πRINCIN
Select the value for RIN as specified in Gain-Setting Resistors on page 12. Choose the value for CIN such that f-3dB is
well below the lowest frequency of interest. Setting f-3dB too high can affect the low-frequency response of the device.
Capacitors with dielectrics that have low-voltage coefficients such as tantalum or aluminum electrolytic should be used,
since capacitors with high-voltage coefficients, such as ceramics, can increase distortion at low frequencies.
Note: Other considerations when designing the input filter include the overall constraints of the system, the frequency
band of interest, and click-and-pop suppression. Although hi-fi audio specifies a flat gain response between
20Hz and 20kHz, portable voice reproduction devices such as mobile phones and two-way radios only need
address the frequency range of the human voice (~ 300Hz to 3.5kHz). Additionally, speakers used in portable
devices typically have poor response below 150Hz. In practice, the input filter may not need to be designed for
the 20Hz to 20kHz range, which could save PCB space and design costs since only small capacitors would be
required.
Bias Bypass Capacitor
The bias bypass capacitor, CBIAS, improves PSRR and THD+N by reducing power supply noise at the common-mode
bias node, and serves as the primary click- and pop-suppression component. CBIAS is fed from an internal 25kΩ
source, and controls the rate at which the common-mode bias voltage rises at power-up and falls during shutdown. For
optimal click- and pop-suppression, ensure that the input capacitor (CIN) is fully charged (ten time constants) before
CBIAS.
The value of CBIAS for best click- and pop-suppression is given by:
CINRIN
(EQ 13)
CBIAS ≤ 10
25kΩ
Note: A larger CBIAS value yields higher PSRR.
Click- and Pop-Less Operation
AC-coupling capacitors (CIN) along with CBIAS facilitate click- and pop-less power-up and shutdown. The value of
CBIAS determines the rate at which the mid-rail bias voltage rises on power-up and falls when entering shutdown.
On power-up, CIN is charged to its quiescent DC voltage through the RF from the output. The current generated cre-
ates a voltage transient at the amplifier output, which can result in an audible pop. Minimizing the value of CIN reduces
this effect, optimizing click-and-pop suppression.
For more information see Bias on page 10 and Bias Bypass Capacitor on page 13.
Supply Bypassing
Proper power supply bypassing – connect a 0.1µF ceramic capacitor in parallel with a 10µF ceramic capacitor from
VDD to GND – will ensure low-noise, low-distortion performance of the device. Additional bulk capacitance can be
added as required.
Note: Place the capacitors as close to the device as possible.
Volume Control
The addition of a digital potentiometer (AS1500 family) used as an input attenuator, can provide simple volume control
for the AS1701/AS1706.
Connect the high terminal of the AS150x to the audio input, the low terminal to ground and the AS150x wiper to CIN (as
shown in Figure 31). Setting the wiper to the top position passes the audio signal unattenuated; setting the wiper to the
lowest position fully attenuates the input.
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