AN2472
Application note
STMAV340 analog video switch
Introduction
STMAV340 is a 4-channel SPDT high bandwidth, low R
on
switch which provides a simple,
inexpensive means to switch high quality video signals without corrupting them. It is a
versatile video switch which can be used in multiple applications such as televisions,
notebooks, graphic cards and DVD players.
March 2007
Rev1
1/11
www.st.com
Contents
AN2472
Contents
1
Video switch parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
1.2
1.3
1.4
1.5
1.6
On-resistance (R
on
) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Cross-talk and off-isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Differential gain and phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Current consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Delay measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
STMAV340 measurement set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1
2.2
2.3
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Termination for bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Measurement techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
4
Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1
4.2
4.3
4.4
Video display (TV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Notebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Graphic cards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
DVD R/W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
PCB layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1
5.2
Supply and ground effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
PCB demo board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6
7
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2/11
AN2472
Video switch parameters
1
1.1
Video switch parameters
On-resistance (R
on
)
The on-resistance of the switch determines the propagation delay as well as the losses
suffered by the incoming signal. The higher on-resistance of the switch increases the
insertion loss making the use of a buffer/gain-stage inevitable. Since the analog voltage
level for most video signals lies between 0 V and 1 V, the switch must provide a minimum
R
on
within this range. The higher value of the resistance will reduce the gain, add noise and
increase the propagation delay. Thus it is desirable to have the on-resistance of the video
switch only in the range of a few ohms. It is worth mentioning here that to achieve a lower
R
on
, the pass transistor has to be large which gives a higher capacitance, thus limiting the
bandwidth of the device. Thus a good trade-off between the R
on
and channel capacitance is
an important consideration in the design of an analog video switch.
If the on-resistance of the switch is higher, the need to use an amplifier is larger as there is a
higher voltage drop across the switch.
1.2
Bandwidth
The bandwidth of the video switch is an important parameter as it determines the signal
quality at the output. The higher bandwidth of the switch allows the signal at the input of the
switch to be reproduced at its output with minimum distortion on the edges and the
amplitude. The amplitude distortion is due to the losses through the switch, parasitic
resistances, and capacitances while the edge distortion comes mainly from the capacitance.
The high bandwidth of the switch maintains the high fidelity of the analog video signal.
The higher the bandwidth in the system, the higher is the detail in the video signal. The
highest frequency of the video signal depends on the rise/fall time of the signal. The
bandwidth of a video signal is a complex function depending on several factors like the
aspect ratio, number of vertical scan lines, frame rate or refresh rate and the ratio of total
horizontal/vertical pixels to active ones. The circuit that processes the video signal needs to
have more bandwidth than the actual bandwidth of the processed signal to minimize the
degradation of the signal and the resulting loss in picture quality. The amount of circuit
bandwidth needs to exceed the highest frequency in the signal to reproduce a high-quality
signal. Depending upon the attenuation of the signal at the output, the circuit bandwidth has
to be 3-6 times higher than the maximum frequency in the video signal. In addition to the
bandwidth, the circuit must slew fast enough to faithfully reproduce the video signal.
1.3
Cross-talk and off-isolation
It is seen during the crosstalk measurement that the termination on other ports can
significantly affect the crosstalk measured value on a port. When the unused ports are un-
terminated (left open) the value of the crosstalk measured is worse than when the unused
ports are terminated with proper 75 Ohm loads. Thus it is necessary to terminate the
unused ports with proper loads for an accurate crosstalk measurement (similar to a real
application environment). This also applies to the off-isolation parameter. The higher the off-
isolation value, the better the switch separates the active data from the non-active display
terminals.
3/11
Video switch parameters
AN2472
1.4
Differential gain and phase
Differential gain and differential phase refer to how the video switch attenuates the signal
differently for inputs biased at various DC levels.
This specification is associated with R
on
flatness over the 1.0V range, with more flatness
occurring with a smaller differential gain. A lesser variation of on capacitance of a video
switch over various DC biases results in a lower differential phase.
The differential gain and phase are further defined as below:
Differential gain is the percentage error in the magnitude/amplitude change in the analog
output voltage from the analog input voltage when the input is between 0 V and 0.714 V and
the switch is enabled. Load at the output is 150 Ohm. 0 V and 0.714 V represents the DC
offset.
Differential gain is expressed in % error and is calculated as follows:
●
●
●
●
Reference gain (when input bias is 0 V, f=3.58 MHz) = V
out
/V
in
= 20 log (V
out
/V
in
)
dB = G1 (say)
New gain (when input bias is 0.714V, f=3.58MHz) = V
out
//V
in
= 20 log (V
out
//V
in
)
dB = G2 (say)
Then Error = E = G2 - G1 (dB)
% Error in Gain = Differential Gain = 100 * Antilog (E/20)
The differential phase is measured in a similar way from the AC/transient simulation plot.
1.5
Current consumption
There are two parts to the current, one comes from the current consumed by the logic
control circuit and the other is by the switch itself. The supply of the device is only connected
to the logic control part (switch enable and selection). The analog pulsing input video signal
is the other source of voltage to the video switch.
The current consumption of the switch when it is active but not switching is only determined
by the static current through the logic part of the device. When it is switching, the current is
determined by the logic control elements of the switch.
The input voltage source to the switches' drain/source and the load attached at the switch
output determine the current through the switch itself.
During the standby state, the current consumption of the switch drops to very low and is
practically negligible.
1.6
Delay measurements
The magnitude of the R
on
and C
on
determine the propagation delay of the switch.
The delay measurements include the switch turn-on / turn-off times and the propagation
delays. The measurement is done using the load circuit as shown in the datasheet. For the
waveforms and the timing specifications, refer to the STMAV340 datasheet.
4/11
AN2472
STMAV340 measurement set up
2
2.1
STMAV340 measurement set up
Calibration
This is one of the most critical parts to good measurement. The AC parameters of the switch
are measured using a network analyzer which must be properly calibrated before use. The
calibration is done on the equipment itself and then with the board. Through, open and short
calibration are performed without the DUT placed on the board or with DUT enabled on the
board and with both the input and output ports of the network analyzer connected to the
switch connectors on the board. After the calibration is done, the channel output on the
analyzer is stable and is not affected by any movements of the cables or the board. This
also helps to ensure that the calibration is performed by taking the DUT and board into
account. Initially if desired the calibration can be run by connecting the two ports together.
2.2
Termination for bias
In a typical video application the bias is needed to provide the DC offset required for the
video signal. The decoupling capacitor filters the negative part of the video signal before it
reaches the switch. The inputs and outputs of the switch have to be terminated with 75 Ohm
resistors to eliminate any reflections due to impedance mismatches between the source and
the sink.
The parameters are measured with 150 Ohm load as the series resistor of 75 Ohm of the
cable is in series with the far-end termination resistor of 75 Ohm, adding them up to
150 Ohm.
2.3
Measurement techniques
Proper measurement methods and stable test set-up ensure that the real device parameters
are measured during the characterization of the device. Good contacts of the cable
assemblies to the SMA connectors on the board must be present. Care is taken to ensure
that the probes of the equipment are tightly screwed to the equipment and the device under
test (D.U.T.). Soldering with a good shining solder dot without any dry solder gives a robust
low resistance contact for the signals on the board.
5/11
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