www.austriamicrosystems.com/Audio/Amps/AS1701_06 Revision 1.53 13 - 18
AS1701, AS1706
Datasheet - Application Information
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 fre-
quency 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. C
BIAS 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 C
BIAS for best click- and pop-suppression is given by:
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, C
IN is charged to its quiescent DC voltage through the RF from the output. The current generated creates 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 C
IN (as shown in Figure 31). Set-
ting the wiper to the top position passes the audio signal unattenuated; setting the wiper to the lowest position fully attenuates the input.
CBIAS
≤
10
25kΩ
(EQ 13)
C
INRIN
