AK8855 Datasheet, PDF (18/81 Pages) Asahi Kasei Microsystems – NTSC/PAL Digital Video Decoder

[ASAHI KASEI]
AK8855
PGA ( Programmable Gain Amp )
PGA ( Programmable Gain Amp ) is integrated at the input stage of the AK8855.
PGA is adjustable from 0 dB to 12 dB, and its gain step is approximately 0.1 dB / step.
Signal input to the AK8855 is attenuated to 50 % level by a resistor-divider.
PGA setting is made by PGA Control Register ( R/W ) [ Sub Address 0x05 ].
By writing “1” to Control Register ( R/W ) [ Sub Address 0x04 ] AGC-bit, AGC function is enabled.
Since the set value by AGC is written at PGA Control Register ( R/W ) [ Sub Address 0x05 ], the AGC set
value is known by reading this register ( manual setting of PGA is invalid ).
When AGC function is disabled, PGA gain setting by manual is possible.
Bit allocation of PGA Control Register is as follows.
[ PGA Control Register ]
Sub Address 0x05
bit 7
bit 6
Reserved
PGA6
0
1
bit 5
PGA5
0
bit 4
bit 3
PGA4
PGA3
Default Value
0
0
bit 2
PGA2
1
Default Value: 0x46
bit 1
bit 0
PGA1
PGA0
1
0
AGC
The AK8855 has an adaptive AGC function. When AGC is enabled, input signal is controlled to a optimized level
by PGA.
When AGC is turned off, gain setting of PGA by manual is possible.
Enable / disable setting of AGC is done by Control Register ( R/W ) [ Sub Address 0x04 ].
Sub Address 0x04
Default Value: 0x00
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
CNTSEL
DTFIX
ODEV
FRMRT1 FRMRT0 COLKIL
ACC
AGC
Default Value
0
0
0
0
0
0
0
0
[ AGC ]-bit
[ AGC ]-bit sets the mode of AGC
[AGC]-bit
Fumction
Condition
0
Disable
1
Enable
note ) writing into PGA register is possible while AGC is enabled, but the written result is not valid to register.
The written result becomes valid to register when AGC is disabled.
Clamp
Input signal is Analog Sync-Tip clamped.
The Sync-Tip clamped input signal is then clamped to Pedestal level after AD conversion.
Anti-Aliasing Filter
Analog Band Limit Filter is integrated before ADC input in the AK8855.
The Anti-Aliasing Filter has following characteristics.
+/-2.0dB (~5.5MHz)
27MHz -30dB ( typ )
MS0319-E-03
18
2004 / 11