AS1702, AS1703, AS1704, AS1705
austriamicrosystems
Data Sheet
Layout and Grounding Considerations
7.4.2 Input Filter
The BTL inputs can be biased at voltages other than mid-supply. However, the integrated common-mode feedback cir-
cuit adjusts for input bias, ensuring the outputs are still biased at mid-supply. Input capacitors are not required if the
common-mode input voltage (VIC) is within the range specified in Table 2 and Table 3.
Input capacitor CIN (if used), in conjunction with RIN, forms a high-pass filter that removes the DC bias from an incom-
ing signal. The AC coupling capacitor 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 8)
f-3dB =
2πRINCIN
Setting f-3dB too high affects the low-frequency response of the amplifier. Capacitors with dielectrics that have low-volt-
age coefficients such as tantalum or aluminum electrolytic should be used, since capacitors with high-voltage coeffi-
cients, such as ceramics, can increase distortion at low frequencies.
7.4.3 BIAS Capacitor
BIAS is the output of the internally generated VCC/2 bias voltage. The BIAS bypass capacitor, CBIAS, improves PSRR
and THD+N by reducing power supply noise and other noise sources at the common-mode bias node, and also gener-
ates the click- and pop-less DC bias waveform for the amplifiers. Bypass BIAS with a 0.1µF capacitor to GND. Larger
values of CBIAS (up to 1µF) improve PSRR, but increase tON/tOFF times. For example, a 1µF CBIAS capacitor increases
tON/tOFF by 10 and improves PSRR by 20dB (at 1kHz).
Note: Do not connect external loads to BIAS.
7.4.4 Supply Bypassing
Proper power supply bypassing – connect a 10µF ceramic capacitor (CBIAS) from VCC to GND – will ensure low-noise,
low-distortion performance of the device. Additional bulk capacitance can be added as required.
Note: Place CBIAS as close to the device as possible.
7.5 Layout and Grounding Considerations
Well designed PC board layout is essential for optimizing device performance. Use large traces for the power supply
inputs and amplifier outputs to minimize losses due to parasitic trace resistance and route heat away from the device.
Good grounding improves audio performance and prevents digital switching noise from coupling onto the audio signal.
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