AD8113
Since the data in the shift register is random after power-up, it
should not be used to program the matrix, or the matrix can enter
unknown states. To prevent this, DO NOT APPLY LOGIC
LOW SIGNALS TO BOTH CE AND UPDATE INITIALLY
AFTER POWER-UP. The shift register should first be loaded
with the desired data, and then UPDATE can be taken LOW to
program the device.
Figure 8 shows a typical input with a divide-by-two input
divider that will create a unity gain channel. The circuit uses 1 kΩ
resistors to form the divider. These resistors need to be high
enough so they will not overload the drive circuit. But if they are
too high, they will generate an offset voltage due to the input bias
current that flows through them. Larger resistors will also increase
the thermal noise of the channel.
The RESET pin has a 20 kΩ pull-up resistor to DVCC that can
be used to create a simple power-up reset circuit. A capacitor
from RESET to ground will hold RESET low for some time
while the rest of the device stabilizes. The low condition will
cause all the outputs to be disabled. The capacitor will then
charge through the pull-up resistor to the high state, thus allow-
ing full programming capability of the device.
This circuit can handle inputs that swing up to 10 V when
the AD8113 operates on analog supplies of 12 V. After the
divider, the maximum voltage will be 5 V at the input. This
maximum input amplitude will be 10 V at the output after the
gain-of-two of the channel.
VIDEO SIGNALS
Unlike audio signals, which have lower bandwidths and longer
wavelengths, video signals often use controlled-impedance
transmission lines that are terminated in their characteristic
impedance. While this is not always the case, there are some
considerations when using the AD8113 to route video signals with
controlled-impedance transmission lines. Figure 9 shows a sche-
matic of an input and output treatment of a typical video channel.
SPECIFYING AUDIO LEVELS
Several methods are used to specify audio levels. A level is
actually a power measurement, which requires not just a voltage
measurement, but also a reference impedance. Traditionally
both 150 Ω and 600 Ω have been used as references for audio
level measurements.
The typical reference power level is one milliwatt. Power levels
that are measured relative to this reference level are given the
designation dBm. However, it is always necessary to be sure of
the reference impedance used for such measurements. This can
be either explicit, e.g., 0 dBm (600 Ω), or implicit, if there is
certain agreement on what the reference impedance is.
+5V
OR +12V
75⍀
TRANSMISSION
LINE
TYPICAL
OUTPUT
TYPICAL
INPUT
AD8113
G = 2
75⍀
75⍀
75⍀
75⍀
VIDEO
SOURCE
–5V
OR –12V
Since modern voltmeters have high input impedances, measure-
ments can be made that do not terminate the signal. Therefore,
it is not proper to consider this type of measurement a dBm, or
power measurement. However, a measurement scale that is
designated dBu is now used to measure unterminated voltages.
This scale has a voltage reference for 0 dBu that is the same as
the voltage required to produce 0 dBm (600 Ω).
Since P = V2/R, the voltage required to create 1 mW into 600 Ω
is 0.775 V rms. This is the voltage reference (0 dB) used for
dBu measurements without regard to the impedance.
Figure 9. Video Signal Circuit
Video signals usually use 75 Ω transmission lines that need to be
terminated with this value of resistance at each end. When such
a source is delivered to one of the AD8113 inputs, the high
input impedance will not properly terminate these signals. There-
fore, the line should be terminated with a 75 Ω shunt resistor to
ground. Since video signals are limited in their peak-to-peak
amplitude, there is no need to attenuate video signals before
they pass through the AD8113.
The AD8113 operates as a voltage-in, voltage-out device.
Therefore, it is easiest to specify all of its parameters in volts,
and leave it to the user to convert them to other power units or
dB-type measurements as required by the particular application.
The AD8113 outputs are very low impedance and will not prop-
erly terminate the source end of a 75 Ω transmission line. In these
cases, a series 75 Ω resistor should be inserted at an output that
will drive a video signal. Then the transmission line should be
terminated with 75 Ω at its far end. This overall termination
scheme will divide the amplitude of the AD8113 output by two.
An overall unity gain channel is produced as a result of the
channel gain-of-two of the AD8113.
CREATING UNITY-GAIN CHANNELS
The channels in the AD8113 have a gain of two. This gain is
necessary as opposed to a gain of unity in order to restrict the
voltage on internal nodes to less than the breakdown voltage. If
it is desired to create channels with an overall gain of unity,
then a resistive divider at the input will divide the signals by
two. After passing through any input/output channel combina-
tion of the AD8113, the overall gain will be unity.
Power Considerations of Video Signals
If the AD8113 is used only to route conventional video signals,
runing on analog supplies of 5 V is recommended. This is all
that is necessary for video signals because they are limited in
their amplitude to generally less than 2 V p-p at the output,
after the channel gain-of-two. There will be significant power
savings when routing video signals with lower supply voltages.
+12V
AUDIO
SOURCE
1k⍀
1k⍀
UNITY GAIN
AUDIO OUT
AD8113
G = 2
If an AD8113 is used to route a mix of audio and video signals,
then other factors must be considered. In general, the analog
supplies will be at 12 V to handle the high signal levels required
for the audio.
TYPICAL
OUTPUT
TYPICAL
INPUT
–12V
Figure 8. Input Divide Circuit
REV. A
–17–
ADI [ ADI ]
