TA0310 STMICROELECTRONICS | Alldatasheet
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Technical Marketing, Standard Linear Division
1 Introduction
This paper examines the video applications in which high-speed op-amps can be found. An overview of the main consumer video applications is presented, along with a review of analog video formats and bandwidth versus resolution. Finally, we present ST’s current high-speed op-amp portfolio and give technical support for the implementation of these products in application.
2 Video applications
Currently, there are two types of video applications: broadcast video and graphics video. Broadcast video is limited to television signal transmissions with specified bandwidth (Television, Set-Top-Box, DVD player-recorder, Video Camera, etc.). On the other hand, graphics video meets the needs of computers without bandwidth limitations. This article is mainly concerned with broadcast video applications. 3 Where do we need high-speed op-amps? An amplifier stage is needed to drive analog video signals to the television via a 75 Ω video line. The applications concerned are mostly consumer applications such as set top boxes, DVD player-recorders and video cameras. In these applications, the output capabilities of the amplifier (output current and distortion versus load) are very important, as it must drive a video line characterized by low impedance (75Ω for video lines). Televisions also require high-speed op-amps. In TVs; the amplifier ensures good impedance matching between the video line and the input stage in the TV. The amplifier drives the video signal to the input stage of a chipset, which features high impedance (on the order of several k Ω) in parallel with a High Speed Operational Amplifiers for 75Ω Video Lines Revision 2 TA0310 TECHNICAL ARTICLE
TA0310 Analog video formats capacitance on the order of pF. In this situation, the driver must maintain high stability even under capacitive loads. Set-top boxes can also feature an analog video input featuring the same constraints. The choice of the video source is done via the set-top box. The source signal can be delivered from a DVD or a video camera, for example. The amplifier can be very specific, including features such as buffer+filtering (ex: STv6433) or video matrix (STv6412), or, linked to the market trend, it can be embedded in the chipset. On the other hand, the amplifier stage can also be a discrete solution using transistors or high-speed op- amps. Where the customer’s goals are speed and space-saving, high-speed op-amps provide an advantage as compared to a transistor solution. For this reason, there is currently a market for the high- speed op-amps in broadcast video applications.
4 Analog video formats
The format of the analog video signal is very important in order to evaluate the frequency and amplitude contraints required of the high-speed op-amp. There are three main division of signal formats, each giving a different quality of television image. The first type of signal format is comprised of three separate signals based on the R,G and B signals. This signal form is the "purest" video signal, providing the highest quality image. The three R, B and G signals feature the same bandwidth. This bandwidth is directly linked to the video resolution. In standard video broadcast, we use commonly YIQ, YUV or YPbPr, where appears the Luma (Y), (I,U,Pb) and (Q,V,Pr) are a component of R, B and Y. In of all these formats, three signals are driven. The second type of signal format is based on two signals, such as Luma-Chroma (Y/C) or S-Video, where C is the Chroma. Both are a coding of RGB signals linked to the NTSC, PAL and SECAM video standards developed in the USA, Europe and Asia. Figure 1: Location of op-amps in video applications DVD Set-Top-Box CAMERA SCART PLUGs output output input output TV Terrestrial Cable Satellite Cable Satellite 150Ω Load 75Ω line 75Ω line 75Ω line 75Ω line 150Ω Load 150Ω Load Capa Load Capa Load Capa Loadinput input
Video signal bandwidth versus resolution TA0310 The third type of signal format is composite video (CVBS). The aim of this signal format is to combine all the video components into only one signal. CVBS is the sum of Y and C. This signal format is the lowest quality format.
5 Video signal bandwidth versus resolution
Standard Definition (SD): Video signal used in standard interlaced video with a TV screen of 720*480 pixels (type: 480I). The bandwidth is up to 6MHz. Figure 2: Video formats and standard plugs Figure 3: Video spectrum for Standard Definition RCA JACK video line 75Ω75Ω Pr, V, Q MPEG DECODER video line 75Ω video line 75Ω Pb, U, I Y Filtering Filtering Filtering MPEG DECODER video line 75Ω video line 75Ω C Y Filtering Filtering MPEG DECODER video line 75Ω CVBS Filtering MINI JACK DIN USCHIDEN RCA JACK YUV, YPbPr,YIQ YC CVBS SCART 75Ω 75Ω 75Ω 75Ω 75Ω 6MHz 1Vp-p Amplitude Frequency
6 Signal amplitude
Figure 6 below shows the typical amplitude of a video signal including synchronization, black level (as amplitude reference 0), white level and colours.
7 ST’s high-speed op-amps
Available in full production, ST offers 4 op-amp fam ilies in the high-speed op-amp portfolio that provide a broad choice to customers. These 4 families are complementary. /c108 TSH7x: VFA, GBP=100MHz, 3V to 12V power supply, input/output rail to rail. /c108 TSH8x: VFA, GBP=100MHz, 4.5V to 12V power supply, input/output rail to rail. /c108 TSH9x: VFA, GBP=130MHz, 12V power supply, noise=4.2nV/ √Hz, consumption=4.5mA /c108 TSH11x: CFA, -3dB Bw=100MHz, 5V to 12V power supply, noise=3nV/ √Hz, consumption=3mA (datasheets available on www.st.com) Figure 6: Video signal amplitudes including colours and luma Synchronization ~0V 300mV Black Level White Level Active Video
8 Impedance matching
We can summarize as follows the constraints met when driving a signal on a line (these are constraints that can be found in any textbook on the theory of line transmission): In order to remove any reflection factors(1), the line must be loaded on both sides by its own characteristic impedance; typically 75 Ω for video lines. We call this impedance matching because the impedance is equivalent at any point in a given line. As the output impedance of the op-amp is close to zero, a resistor of 75Ω is physically implemented on the board to achieve the right value for matching. A second resistor of 75Ω (TV side) allows matching on the other side. As show in Figure 7, the network behaves like a resistor divider for the signal amplitude. Because of this, half of the output amplitude of the op-amp is lost. As the input amplitude of the op-amp must be the same as the amplitude required on the line (typically 1Vpp in video), a gain of +2 (6dB) is required on the op- amp. The value of R must be as small as possible to reduce noise and the problems of stability (assuming stray capacitances mainly on inverting input), but not too small as the 2R network is viewed as a load by the op- amp output. For a VFA, the value of R is not imposed. 1k Ω is a good choice and it satisfies the previous requirements. For a CFA, as TSH11x, the value of R is imposed and it is available in the datasheet (R=680Ω for gain=+2).
9 Power supply
A constraint belonging to every designer is the need to reduce the cost of his application. A dual power supply -5V/+5V requires an investment in a negative -5V supply circuit. One solution is to reduce the power supply to a single supply 0/+5V. As described in Figure 6, the synchronization signal descends to 0V (sometimes only 10mV). In cases such as these, the best solution is to use an input/output rail-to-rail op-amp such as TSH7x-TSH8x families. Assuming the tested value of the output rail is VOL=150mV max. Figure 7: Typical connexion between set-top-box and TV 1 Reflection factor occurs when the line is loaded by the same value as its own characteristic impedance Zc. video line 75Ω 75Ω R 0Volt 0Volt 2Vpp 1Vpp 0Volt 1Vpp R SET-TOP-BOX TV Gain=2
Notes on video line driving TA0310 (see datasheet), the minimum amplitude of the signal guaranteed on the line is 75mV. This results in a loss of the bottom signal which is only 75mV (at worst).
10 Notes on video line driving
Implementation of TSH7x-TSH8x families in single supply 0/+5V: If the op-amp is not rail-to-rail, the DC component of the video signal must be shifted to a higher value using the networks described in Figure 8. In this way, the video signal is not truncated by the output stage of the op-amp (VOL=1.2V max., see datasheet). Figure 8: Implementation of the TSH7x-8x in single supply 0/+5V 1kΩ video line 75Ω75Ω 1kΩ +5V 0Volt TSH7x 2Vpp 1Vpp 0Volt 1Vpp 75mV max. (garanted) 0V 1Vp-p Signal on the line 300mV
TA0310 Notes on video line driving Implementation of TSH11x family in single supply 0/+5V: Cin-R3 behaves like a high pass input filter (fc=16Hz) and removes the original DC component of the video signal to create a "floating" signal. The R1-R2 resistor divider provides the new DC component. The minimum level of that DC component must be ½VOL max (tested value). In order to limit the current through R1 and R2, the value of these resistances must be sufficiently high. The maximum values of R1 and R2 are calculated to in order to arrive at +Ibias max (see datasheet) equal to 1% of the current through R1 and R2. Cout removes the DC component to go back to the original video signal to fit with TV requirements. Figure 9: Implementation of the TSH11x in single supply 0/+5V video line 75Ω 75Ω 680Ω +5V TSH11x Cin(10µ) +5V +1µ 10n Addition of a new DC component Cout (220µ) 1Vpp 0Volt Twice the DC component (2Vdc) 0Volt 5Volt Rail of TSH11x : +1.2V max (tested) 680Ω 2R1R 2RVccVdc 1Vpp 0Volt Removes the original DC component Removes the DC component Fc<5Hz 1Vp-p Signal on the line 300mV
TA0310 Notes on video line driving Figure 12: Choice of the op-amp versus a video bandwidth ~6MHz ~12MHz ~30MHz Frequency Frequency Frequency HDTV Progressive Video Standard Definition (SD) 1Vp-p 1Vp-p 1Vp-p TSH7x-8x +5V TSH7x-8x? if sufficient Linearity +5V TSH11x +5V -5V or 0V TSH9x +5V -5V TSH11x +5V -5V or 0V TSH9x +5V -5V TSH11x +5V -5V or 0V TSH9x ? if sufficient Linearity +5V -5V
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publicati on are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectro nics. The ST logo is a registered trademark of STMicroelectronics All other names are the property of their respective owners © 2004 STMicroelectronics - All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Belgium - Brazil - Canada - China - Czech Repubic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States Notes on video line driving TA0310 Figure 19: TSH11x behavior on capaload 11 0 1 0 0 TSH11x, Gain = +2 Capa Load = 30pF R Load = 1kohms Vcc = 0/+5V Rs = 39ohms Vcc = -5V/+5V Rs = 30ohms Gain(dB) - A V Frequency (MHz)