AK4145 Datasheet, PDF (24/27 Pages) Asahi Kasei Microsystems – Digital BTSC Stereo Encoder

[AK4145]
1. Grounding and Power Supply Decoupling
The AK4145 requires careful attention to power supply and grounding arrangements. AVDD, DVDD and TVDD are
usually supplied from analog supply in system. If AVDD1, AVDD2, DVDD1, DVDD2 and TVDD are supplied
separately, the power up sequence is not critical. VSS of the AK4145 must be connected to analog ground plane.
System analog ground and digital ground should be connected together near to where the supplies are brought onto the
printed circuit board. Decoupling capacitors should be as near to the AK4145 as possible, with the small value ceramic
capacitor being the nearest.
2. Voltage Reference Inputs
The voltage of AVDD sets the DAC analog output range. VCOM is signal ground of analog output signal. The FILT pin
is Loop filter pin for internal PLL. An electrolytic capacitor 10μF parallel with a 0.1μF ceramic capacitor attached
between the VCOM pin and VSS pin eliminate the effects of high frequency noise. An 4.7nF capacitor should be attached
between the FILTM pin and VSS pin. No load current may be drawn from these VCOM and FILT pins. All signals,
especially clocks, should be kept away from the AVDD, VCOM and FILT pins in order to avoid unwanted coupling into
the AK4145.
3. Analog Video Input
In case of composite video input mode (VCLK bit = “0”), the AK4145 receives the analog video input through the
CV27M pin using a 0.1μF ceramic capacitor in series. This video signal is used for the synchronization between the video
sync and the pilot frequency on composite audio output. The video input mode is NOT available only when the TVDD is
lower than 2.7V. In case of 27MHz clock input mode (VCLK bit = “1”), input the 27MHz clock directly without using a
0.1uF capacitor.
4. Analog Composite Audio Output
The analog output is single-ended and centered on around the VCOM voltage. The output signal range scales with the
supply voltage and nominally 2.2 x AVDD/3.3 Vpp (@100kHz deviation) at the CA pin. DC offsets on analog outputs are
eliminated by AC coupling. The modulation level of sound intermediate frequency (SIF) at RF modulator can be adjusted
by the internal composite volume control of the AK4145 (04H D7-0: ATT7-0 bits). The stereo separation can be
maximized by tuning the modulation level. If the output level of the AK4145 (typ: 2.2Vpp @100kHz deviation) is not
sufficient for the RF modulator device, the external gain stage is used for the extra gain.
AK4145
CA
10u
22k
Vop
47k
Vop
C (note)
RF Modulator
should be C value should be det ermine
47k
0.1u 10u
Note:
The C value must be determined taking account of the input impedance Z at RF modulator. The recommended fc
(cut-off frequency) of this high-pass filter is 1Hz or less for low frequency stereo separation. fc = 1/(2 x 3.14 x C x Z).
Example: C= 10uF @Z=15kohm, C= 3.3uF @Z=50kohm.
Figure 17. External Gain Stage Example.
MS0982-E-01
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2010/09