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

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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

1.1On-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 switch must provide a minimum Ron 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 Ron, 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 Ron 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.

1.2Bandwidth

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.

1.3Cross-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 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 offisolation value, the better the switch separates the active data from the non-active display terminals.

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Video switch parameters	AN2472

1.4Differential 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 Ron 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) = Vout/Vin = 20 log (Vout/Vin) dB = G1 (say)

●New gain (when input bias is 0.714V, f=3.58MHz) = Vout//Vin = 20 log (Vout//Vin) 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.

1.5Current 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.

1.6Delay measurements

The magnitude of the Ron and Con 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.

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