tda8757a NXP Semiconductors, tda8757a Datasheet - Page 13

• Manufacturer Part Number: tda8757a
• Description: Tda8757a Triple 8-bit Adc 205 Msps
• Manufacturer: NXP Semiconductors

Philips Semiconductors
9397 750 09549
Preliminary data
8.2.1 Variable gain amplifiers
8.2.2 Important recommendations
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. On the incoming of a
pulse supplied to pin HSYNC, an internal multiplexer switches from the RGB video
signals to a reference voltage (
signal and 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 simply
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 among 68 values (see
A fine correction is also used to finely tune the gain on the three channels and to
compensate the channel-to-channel gain mismatch. The fine correction is done using
the following principle: the three binary codes, stored in the three 5-bit registers
(FINER, FINEG and FINEB) are converted into three analog voltages (with three
DACs) and are independently added to the reference voltage (
different reference voltages are used for the gain calibration of the three channels.
When the COARSE registers are set at full-scale, the resolution of the fine registers
corresponds to
The clamping and the gain calibration requisite two external signals (pulses). One
signal is connected to pin CLP and the other is connected to pin HSYNC. It is very
important that:
•
•
The active part of these two signals occur during the blanking of the video signal,
in order not to interrupt or disturb the active video.
The active part of these two signals does not overlap on each other, in order to
perform correctly the gain calibration and the clamping. Normally the clamp pulse
is sent after the end of the horizontal synchronization pulse.
1
2
Rev. 01 — 22 March 2002
LSB peak-to-peak (see
ref
= 2.5 V (DC), with a maximum variation of 100 ppm/ C, is
1
Table
16
V
ref
6).
). The ADCs inputs become this reference
Equation
3).
© Koninklijke Philips Electronics N.V. 2002. All rights reserved.
Triple 8-bit ADC 205 Msps
1
TDA8757A
16
V
ref
). Thus, three
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