3. DETAILED DESCRIPTION
3.1 SERIAL DIGITAL INPUT
SDI/SDI are high impedance differential inputs. Several
conditions must be observed when interfacing to these
inputs:
1. The differential input signal amplitude must be between
300 and 2000mVpp.
2. For DC coupling to the device, the common mode
voltage must be between 1.6+∆V
SDI(DIFF)
and VCC-
∆V
SDI(DIFF)
.
3. For input trace lengths longer than approximately 1cm,
the inputs should be terminated as shown in the Typical
Application Circuit.
Fig. 1 Static Protection Diodes
SDO
GS9068
SDO
The GS9068 inputs are self-biased, allowing for simple AC
coupling to the device. For serial digital video, a minimum
capacitor value of 4.7µF should be used to allow coupling
of pathological test signals. A tantalum capacitor is
recommended.
3.2 SERIAL DIGITAL OUTPUT
3.3 OUTPUT RETURN LOSS MEASUREMENT
The GS9068 outputs are current mode and will drive 800mV
into a 75Ω load. These outputs are protected from accidental
static damage with internal static protection diodes.
The SMPTE 259M standard requires that the output of a
cable driver have a source impedance of 75Ω and a return
loss of at least 15dB between 5MHz and 540MHz. In order
for an SDI output circuit using the GS9068 to meet this
specification, the output circuit shown in the Typical
Application Circuit is recommended.
The value of L
COMP
will vary depending on the PCB layout,
with a typical value of 5.6nH. A 4.7µF capacitor is used for
AC coupling the output of the GS9068. This value is chosen
to ensure that pathological signals can be coupled without
a significant DC component occurring.
See Section 4, Application Reference Design, for more
details.
When measuring return loss at the GS9068 output, it is
necessary to take the measurement for both a logic high
and a logic low output condition. This is because the output
protection diodes act as a varactor (voltage controlled
capacitor) as shown in Figure 1. Consequently, the output
capacitance of the GS9068 is dependent on the logic state
of the output.
To perform a practical return loss measurement, it is
necessary to force the GS9068 output to a DC high or low
condition. The actual return loss will be based on the
outputs being static at V
CC
or V
CC
-1.6V. Under normal
operating conditions the outputs of the GS9068 swing
between V
CC
-0.4V and V
CC
-1.2V, so the measured value of
return loss will not represent the actual operating return
loss.
A simple method of calculating the values of actual
operating return loss is to interpolate the two return loss
measurements. In this way, the values of return loss are
estimated at V
CC
-0.4V and V
CC
-1.2V based on the
measurements at V
CC
and V
CC
-1.6V.
The two values of return loss (high and low) will typically
differ by several decibels. If the measured return loss is R
H
for logic high and R
L
for logic low, then the two values can
be interpolated as follows:
R
IH
= R
H
- (R
H
-R
L
)/4, and
R
IL
= R
L
+(R
H
-R
L
)/4,
where R
IH
is the interpolated logic high value and R
IL
is the
interpolated logic low value.
For example, if R
H
= -18dB and R
L
= -14dB, then the
interpolated values are R
IH
= -17dB and R
IL
= -15dB.
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