tda8708a NXP Semiconductors, tda8708a Datasheet - Page 5

• Manufacturer Part Number: tda8708a
• Description: Video Analog Input Interface
• Manufacturer: NXP Semiconductors

Philips Semiconductors
FUNCTIONAL DESCRIPTION
The TDA8708A provides a simple interface for decoding
video signals.
The TDA8708A operates in configuration mode 1 (see
Fig.4) when the video signals are weak (i.e. when the gain
of the AGC amplifier has not yet reached its optimum
value). This enables a fast recovery of the synchronization
pulses in the decoder circuit. When the pulses at the
GATE A and GATE B inputs become distinct (GATE A and
GATE B pulses are synchronization pulses occurring
during the sync period and rear porch respectively) the
TDA8708A automatically switches to configuration mode 2
(see Fig.5).
When the TDA8708A is in configuration mode 1, the gain
of the AGC amplifier will be roughly adjusted (sync level to
a digital output level of 0 and the peak level to a digital
output level of 255).
In configuration mode 2 the digital output of the ADC is
compared to internal digital reference levels. The resultant
outputs control the charge or discharge current of a
capacitor connected to the AGC pin. The voltage across
this capacitor controls the gain of the video amplifier. This
is the gain control loop.
LIMITING VALUES
In accordance with the Absolute Maximum Rating System (IEC 134).
THERMAL CHARACTERISTICS
June 1994
V
V
V
V
I
T
T
T
R
O
V
stg
amb
j
CCA
CCD
CCO
I
Video analog input interface
th j-a
CC
SYMBOL
SYMBOL
analog supply voltage
digital supply voltage
output supply voltage
supply voltage difference between V
supply voltage difference between V
supply voltage difference between V
input voltage
output current
storage temperature
operating ambient temperature
junction temperature
thermal resistance from junction to ambient in free air
SOT117-1
SOT136-1
PARAMETER
PARAMETER
5
CCA
CCO
CCA
The sync level comparator is active during a positive-going
pulse at the GATE A input. This means that the sync pulse
of the composite video signal is used as an amplitude
reference. The bottom of the sync pulse is adjusted to
obtain a digital output of logic 0 at the converter output. As
the black level is at digital level 64, the sync pulse will have
a digital amplitude of 64 LSBs.
The peak-white control loop is always active. If the video
signal tends to exceed the digital code of 248, the gain will
be limited to avoid any over-range of the converter.
The use of nominal signals will prevent the output from
exceeding a digital code of 213 and the peak-white control
loop will be non-active.
The clamp level control is accomplished by using the same
techniques as used for the gain control. The black-level
digital comparator is active during a positive-going pulse at
the GATE B input. The clamp capacitor will be charged or
discharged to adjust the digital output to code 64.
and V
and V
and V
CCD
CCO
CCD
0
0
0
0.3
0.3
0.3
1.0
1.0
1.0
0.3
55
MIN.
VALUE
55
70
+7.0
+7.0
+7.0
+1.0
+1.0
+1.0
V
+10
+150
+70
+125
CCA
MAX.
Product specification
TDA8708A
UNIT
K/W
K/W
mA
V
V
V
V
V
V
V
C
C
C
UNIT