tda8757c NXP Semiconductors, tda8757c Datasheet - Page 13

• Manufacturer Part Number: tda8757c
• Description: Tda8757c Triple 8-bit Adc 205 Msps
• Manufacturer: NXP Semiconductors

Philips Semiconductors
9397 750 10111
Preliminary data
Fig 6. Clamp definition.
programming
clamp
8.2.1 Variable gain amplifiers
CLP
Three independent variable gain amplifiers are used to provide, for each channel, a
full-scale input signal to the 8-bit ADC. The gain adjustment range is designed so that
for an input range varying from 0.4 to 1.2 V (p-p), the output signal corresponds to the
ADC full-scale input of 1 V (p-p).
To ensure that the gain does not vary over the whole operating temperature range a
reference voltage V
supplied externally on pin VREF.
The calibration of the gains is done using the following principle. When an input pulse
is applied to pin HSYNC, an internal multiplexer switches from the RGB video signals
to a reference voltage (
the three corresponding outputs are compared to pre-set values loaded in three 7-bit
registers: COARSER, COARSEG and COARSEB. Depending on the result of the
comparisons, the three gains are adjusted such that the ADC outputs become equal
to the pre-set values in the registers. The three gains are controlled by changing the
values in the COARSE registers.
The signal supplied on pin HSYNC, may be selected active HIGH or active LOW. The
choice is done through the serial interface by setting bit ‘Hlevel’ in the control register
(active HIGH when bit Hlevel = 0).
This active part of the signal has to occur during the blanking period of the signal in
order not to interrupt the active video. Normally the horizontal synchronization signal,
provided by the video source, is connected to pin HSYNC.
The values loaded in the gain registers (COARSER, COARSEG, COARSEB) are
chosen from 68 values (see
= 63.5
= 128
Clamp
Clamp
Clamp
Clamp
= 64
= 0
Rev. 01 — 14 August 2002
ref
= 2.5 V (DC), with a maximum variation of 100 ppm/ C, is
video signal
1
16
V
ref
Table
). The ADCs inputs become this reference signal and
6).
© Koninklijke Philips Electronics N.V. 2002. All rights reserved.
ADC
Triple 8-bit ADC 205 Msps
255
constant level
0
constant level
TDA8757C
FCE698
13 of 38