Datasheet

AD8112
Rev. 0 | Page 20 of 28
low. This technique should be used when programming the
device for the first time after power-up when using parallel
programming.
POWER-ON RESET
When powering up the AD8112, it is usually desirable to have
the outputs in the disabled state. The
RESET
pin, when taken
low, causes all outputs to be in the disabled state. However, the
RESET
signal does not reset all registers in the AD8112. This is
important when operating in the parallel programming mode.
(Please refer to the
Parallel Programming section for information
about programming internal registers after power-up.) Serial
programming updates the entire matrix, therefore no special
considerations apply.
Because the data in the shift register is random after power-up,
it should not be used to program the matrix; otherwise the matrix
can enter an unknown state. To prevent this, do not apply logic
low signals to both
CE
and
UPDATE
immediately 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.
The
RESET
pin has a 20 kΩ pull-up resistor to DV
CC
that can be
used to create a simple power-up reset circuit. A capacitor from
RESET
to ground holds
RESET
low until the device stabilizes.
The low condition causes all the outputs to be disabled. The
capacitor then charges through the pull-up resistor to the high
state, thus allowing full programming capability of the device.
SPECIFYING AUDIO LEVELS
Several methods are used to specify audio levels. A level is actually
a power measurement, which requires not just a voltage meas-
urement, 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 1 mW. Power levels that are
measured relative to this reference level are given the designation
dBm. However, it is necessary to be sure of the reference imped-
ance used for such measurements. This can be either explicit
(for example, 0 dBm (600 Ω)) or implicit (if there is an agree-
ment on what the reference impedance is).
Because modern voltmeters have high input impedances, meas-
urements 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 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).
Because
P = V
2
/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.
The AD8112 operates as a voltage-in/voltage-out device.
Therefore, all parameters are specified in volts, but users
can convert the values to other power units or decibel-type
measurements as required by a particular application.
CREATING UNITY-GAIN CHANNELS
The channels in the AD8112 each have a gain of +2. This gain is
necessary, as opposed to a gain of unity, 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, a resistive
divider can be used at the input to divide the signals by 2. After
passing through any input/output channel combination of the
AD8112, the overall gain of unity is achieved.
1k
1k
+12
V
–12V
UNITY GAIN
AUDIO OUT
TYPICAL
OUTPUT
TYPICAL
INPUT
AUDIO
SOURCE
AD8112
G=+2
06523-032
Figure 44. Input Divide Circuit
Figure 44 shows a typical input with a divide-by-2 input divider
that creates a unity gain channel. The circuit uses 1 kΩ resistors
to form the divider. These resistors need to be high enough so
they do not overload the drive circuit. But if they are too high,
they generate an offset voltage due to the input bias current that
flows through them. Larger resistors also increase the thermal
noise of the channel.
The circuit shown in
Figure 44 can handle inputs that swing
up to ±10 V when the AD8112 operates on analog supplies of
±12 V. After passing through the divider, the maximum voltage
is ±5 V at the input. This maximum input amplitude is ±10 V at
the output after the gain of +2 of the channels.
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. Although this is not always the case, there are some
considerations when using the AD8112 to route video signals
with controlled-impedance transmission lines.
Figure 45 shows
a schematic of an input and output treatment of a typical video
channel.
75
75
75
75
TRANSMISSION
LINE
TYPICAL
OUTPUT
+5
V
OR +12V
AD8112
G=+2
–5V
OR –12V
TYPICAL
INPUT
75
VIDEO
SOURCE
06523-033
Figure 45. Video Signal Circuit