ST AN2472 APPLICATION NOTE
AN2472
Application note
STMAV340 analog video switch
Introduction
STMAV340 is a 4-channel SPDT high bandwidth, low Ron 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 On-resistance (Ron) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Cross-talk and off-isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Differential gain and phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.5 Current consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.6 Delay measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 STMAV340 measurement set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Termination for bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Measurement techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1 Video display (TV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2 Notebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.3 Graphic cards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.4 DVD R/W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5 PCB layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1 Supply and ground effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2 PCB demo board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
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AN2472 Video switch parameters
1 Video switch parameters
1.1 On-resistance (Ron)
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 s witch must provide a minimum
R
within this range. The higher value of the resistance will reduce the gain, add noise and
on
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
, the pass transistor has to be large which gives a higher capacitance, thus limiting the
on
bandwidth of the de vice . Thus a good tr ade-o ff bet ween the R
an important consideration in the design of an analog video switch.
If the on-resistance of the s witch is higher, the need to use an amplifier is larger as there is a
higher voltage drop across the switch.
1.2 Bandwidth
and channel capacitance is
on
The bandwidth of the video switch is an important parameter as it determines the signal
quality at the output. The higher bandwidt h of t he switch allows the signal at the input of the
switch to be reproduced at its outpu t 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 dist ortion 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 fu nction 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 t o act ive ones. The circuit that processes the video si gna l n eed s t o
have more bandwidth t han 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 attenu ation of t he signal at t he output , the cir cuit band width has
to be 3-6 times higher than the maximum fre quency 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 unterminated (left open) the value of the crosstalk measure d 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 cr osstalk measurement (similar to a real
application environment). Th is also ap plies to t he o ff- iso lat ion parameter. The higher the o ffisolation 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
occurring with a smaller differential gain. A lesser variation of on capacitance of a video
switch over various DC biases results in a lower different ial phase.
The differential gain and phase are further defined as below:
Differential gain is the perce ntage error in the magnitude/amplitude change in the analog
output voltage from t he analog input v oltage when the input is betw een 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
dB = G1 (say)
● New gain (when input bias is 0.714V, f=3.58MHz) = V
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 sim ilar 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 b y the s witch itself . Th e supply of the de vice is only conn ected
to the logic control part (switch enab le an d selecti on) . The analo g p ulsing inp ut vide o sig nal
is the other source of voltage to the video switch.
flatness over the 1.0V range, with more flatness
on
= 20 log (V
out/Vin
//Vin = 20 log (V
out
out/Vin
//Vin)
out
)
The current consumption of the s witch wh en it is act ive but not switching is only dete rmined
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
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
on
and C
determine the propagation delay of the switch.
on
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