Sierra Video ADA-108 - Manual (Page 19)

OG-ADA-108
13
Input Stage
The Input stage, U1, buffers the input and has a positive gain of 2 which is set by R3 and
R4. Input gain G = [1+(R4/R3)]. P1 allows us to internally terminate the input in 75 ohms
with a shorting jack between pins 1 & 2 of P1. By putting the shorting jack between pins 2
& 3, OFF position, or between pins 1 & 2, we can inform the microPIC which position the
internal termination is set. This means that the shorting jack must always be present so
that the reporting of the internal termination is correct. The R2, 22.1k, is needed for a DC
bias path to the non-inverting input of the input stage in case there is no input and 75
ohm termination at the input. R2 will be in parallel with the 75 ohm input but since it is
such a large value compared to 75 ohms it has little effect on return loss or gain at the
input.
Gain Adjust Stage
The gain stage is located at the front of the module so that the gain can be adjusted
easily. R5, 100 ohms, at the output of the input stage acts as a termination resistance for
the trace going to the gain stage. R9, 100 ohms, at the input of the gain stage acts as the
termination resistance and also creates 6dB loss from the input stage to the gain stage.
The gain stage will see a unity gain signal at its input. C58, 47pF, cap creates a low pass
filter with R5 to roll the overall response of the DA to approximately -3dB at 50MHz.
The gain of the gain stage is set by the feedback resistors around U2, R6,7, and 8. The
values were chosen to give an adjustable gain range of +/-4dB so that we would always
be able to make the +/-3dB adjustment range over the tolerance range of the feedback
resistors. The output of this stage also has a termination resistor, R10, 100 Ohms, to
drive the long trace from the front of the board to the rear of the board and the output
drivers.
Video Presence Detector
When video is present the Power LED will illuminate green, orange when video is not
present.
The output of the gain adjust stage also drives the video presence detector through a
voltage divider made of two 100 Ohm resistor, R32 and R85, then is AC coupled to U7 a
sync separator IC, LMH1981. C9 and the voltage divider creates a low pass filter for the
video going into the sync separator and will remove most of the subcarrier in a PAL or
NTSC composite signal. This helps the sync separator find sync. The vertical sync output
from the LMH1981 will only be present if the IC can find sync. Vertical sync is about 3
lines wide and negative going during the board pulses in vertical. This signal is used to
turn on PNP transistor Q1 which discharges 1uF cap C13 pulling the voltage at R36,
VID_PRESENT, high indicating that video is present. During the period of time when the
vertical sync pulse is not low the voltage at C13, R37 junction will charge toward ground
from the current coming through R37. If vertical sync does not return within two fields
then C13 charges down below the threshold of the PIC32 input and indicates no video
present.
Video Output Drivers
The adjustable gain stage has a source termination of 100 Ohms and the end of the trace
at the output drivers is also terminated in 100 Ohms, R27. The trace to the drivers is
daisy chained to each of four amplifiers with R27 being at the end of the daisy chain. A
small resistance at the non-inverting inputs of the drivers is to isolate the input
capacitance from the trace and reduce any high frequency feedback paths that could
cause oscillations in the output drivers. Each driver has a gain of two from its non-
inverting input to the output pin of the driver. Each driver drives two output BNCs at the