TA0310

TA0310 TECHNICAL ARTICLE High Speed Operational Amplifiers for 75Ω Video Lines Christophe Prugne 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 75Ω 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 kΩ) in parallel with a Revision 2 TA0310/0504 1/14 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. Figure 1: Location of op-amps in video applications Cable Satellite Set-Top-Box output 150Ω Load input 75Ω line Terrestrial Cable Satellite TV input Capa Load 75Ω line Capa Load input 75Ω line SCART PLUGs DVD output 150Ω Load Capa Load output 150Ω Load 75Ω line CAMERA 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 opamps. 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 highspeed 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. 2/14 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. Figure 2: Video formats and standard plugs RCA JACK Pr, V, Q MPEG DECODER video line Filtering 75Ω 75Ω video line SCART Pb, U, I Y Filtering 75Ω 75Ω video line Filtering 75Ω 75Ω C MPEG DECODER video line MINI JACK DIN USCHIDEN Filtering 75Ω 75Ω video line Y 1 Filtering 75Ω 75Ω YUV, YPbPr,YIQ YC CVBS RCA JACK MPEG DECODER CVBS video line Filtering 75Ω 75Ω 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 3: Video spectrum for Standard Definition Amplitude 1Vp-p 6MHz Frequency 3/14 TA0310 Progressive Video (PV): Video signal bandwidth versus resolution The image is not interlaced. The aim is to increase its quality. The bandwidth of this signal is twice the standard definition bandwidth, 12MHz. Such a signal fits with progressive TVs, 720*480 pixels (type: 480P) and it in increasingly common in DVD players. However, because of competition with 100Hz TVs, this format is not popular in Europe yet. Figure 4: Video spectrum for Progressive video Amplitude 1Vp-p 12MHz Frequency High Definition (HDTV): The goal is to improve the definition of the image by increasing the quantity of lines and pixels per line. The bandwidth of the video signal is up to 30MHz and the signal fits with TV screens of progressive 1280*1920 pixels (type: 1280P) and interlaced 1920*1080 pixels (1080I). HDTV is now popular in the USA, and is starting to become so in Asia and Europe. Figure 5: Video spectrum for HDTV Amplitude 1Vp-p 30MHz Frequency 4/14 Signal amplitude TA0310 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. Figure 6: Video signal amplitudes including colours and luma Active Video 1V White Level 300mV ~0V Synchronization Black Level 7 ST’s high-speed op-amps Available in full production, ST offers 4 op-amp families in the high-speed op-amp portfolio that provide a broad choice to customers. These 4 families are complementary. l TSH7x : VFA, GBP=100MHz, 3V to 12V power supply, input/output rail to rail. l TSH8x : VFA, GBP=100MHz, 4.5V to 12V power supply, input/output rail to rail. l TSH9x : VFA, GBP=130MHz, 12V power supply, noise=4.2nV/√Hz, consumption=4.5mA l TSH11x : CFA, -3dB Bw=100MHz, 5V to 12V power supply, noise=3nV/√Hz, consumption=3mA (datasheets available on www.st.com) 5/14 TA0310 Impedance matching 8 Impedance matching Figure 7: Typical connexion between set-top-box and TV SET-TOP-BOX 1Vpp Gain=2 TV 1Vpp video line 2Vpp + R 75Ω 75Ω 0Volt R 0Volt 0Volt 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 75Ω 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 opamp. 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 opamp output. For a VFA, the value of R is not imposed. 1kΩ 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. 1 Reflection factor occurs when the line is loaded by the same value as its own characteristic impedance Zc. 6/14 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: Figure 8: Implementation of the TSH7x-8x in single supply 0/+5V 1Vpp 1Vpp +5V + TSH7x - 2Vpp 1Vp-p video line 5V 0Volt 1kΩ 1 kΩ 75Ω 75Ω 300mV 0Volt 75mV max. (garanted) Signal on the line 0V 0V 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). 7/14 TA0310 Implementation of TSH11x family in single supply 0/+5V: Figure 9: Implementation of the TSH11x in single supply 0/+5V Notes on video line driving 5Volt Removes the original DC component +5V R3 1k TSH11x Twice the DC component (2Vdc) Rail of TSH11x : +1.2V max (tested) 0Volt Cin (10µ) + 75Ω video line 1Vpp 0Volt 75Ω 680Ω 680Ω +5V Cout (220µ) 1Vpp 0Volt 0V 1Vp-p 5V Addition of a new DC component R2 Vdc = Vcc × R1 + R2 R1 + 10n Removes the DC component Fc