AK4687 AKM [Asahi Kasei Microsystems], AK4687 Datasheet - Page 33

• Manufacturer Part Number: AK4687
• Description: Asynchronous Stereo CODEC with Capless Stereo Selector
• Manufacturer: AKM [Asahi Kasei Microsystems]

[AK4687]
1. Grounding and Power Supply Decoupling
The AK4687 requires careful attention to power supply and grounding arrangements. AVDD1, AVDD2 and DVDD are
usually supplied from the system’s analog supply. If AVDD1, AVDD2 and DVDD are supplied separately, the power up
sequence is not critical. VSS1, VSS2, VSS3, VSS4 and VSS5 of the AK4687 must be connected to the same analog
ground plane. System analog ground and digital ground should be wired separately and connected together as close as
possible to where the supplies are brought onto the printed circuit board. Decoupling capacitors should be as near to the
AK4687 as possible, with the small value ceramic capacitor being the nearest.
2. Voltage Reference Inputs
The differential voltage between AVDD1 and VSS1 sets the analog input range, and the differential voltage between
AVDD2 and VSS2 sets the analog output range. VREF1/VREF2 are signal common of this chip. A 1µF ceramic capacitor
attached between the VREF1 and VREF2 pins eliminates the effects of high frequency noise. No load current may be
drawn from the VREF1/VREF2 pins. All signals, especially clocks, should be kept away from the VREF1/VREF2 pins in
order to avoid unwanted coupling into the AK4687.
3. Analog Inputs
The analog input is single-ended and supplied to the Pre-amp via external resistors. Select the feedback resistance to make
the pre-amp output match to the input range (typ. 1.91Vrms) of the ADC (LO and RO pins). The ADC output data format
is 2’s complement. The internal digital HPF removes the DC offset. The AK4687 samples the analog inputs at 64fs. The
digital filter rejects noise above the stop band except for multiples of 64fs. The AK4687 includes an anti-aliasing filter
(RC filter) to attenuate a noise around 64fs.
4. Analog Outputs
The analog outputs are single-ended and centered around the VSS2 (0V typ.) voltage. The output signal range is typically
2.0Vrms (typ @AVDD2=3.3V). The internal switched-capacitor filter (SCF) and continuous-time filter (CTF) attenuate
the noise generated by the delta-sigma modulator beyond the audio passband. Using a 1st-order LPF
(Figure
22) can
reduce noise beyond the audio passband.
The output voltage is a positive full scale for 7FFFFFH (@24bit) and a negative full scale for 800000H (@24bit). The
ideal output is 0V (VSS) for 000000H (@24bit). The DC offset is within ±5mV.
AK4687
470
Analog
L/R OUT
Out
2.0Vrms (typ)
2.2nF
(fc = 154kHz, gain = -0.28dB @ 40kHz, gain = -1.04dB @ 80kHz)
Figure 22. External Circuit Example1
5. Attention to the PCB Wiring
LIN1-3 and RIN1-3 pins are the summing nodes of the Pre-Amp. Attention should be given to avoid coupling with other
signals on those nodes. This can be accomplished by making the wire length of the input resistors as short as possible. The
same theory also applies to the LI/RI pins and feedback resistors; keep the wire length to a minimum. Unused input pins
among LIN1-3 and RIN1-3 pins must be left open.
MS1307-E-00
2011/05
- 33 -