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TA0310/0504 1/14 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 75 ? 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 (75 ? 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 ? ) in parallel with a high speed operational amplifiers for 75 ? video lines revision 2 TA0310 technical article
2/14 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. 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 150 ? load 75 ? line 75 ? line 75 ? line 75 ? line 150 ? load 150 ? load capa load capa load capa load input input
3/14 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. 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 75 ? 75 ? pr, v, q mpeg decoder video line 75 ? video line 75 ? pb, u, i y filtering filtering filtering mpeg decoder video line 75 ? video line 75 ? c y filtering filtering mpeg decoder video line 75 ? c vbs filtering mini jack din uschiden rca jack 1 yuv, ypbpr,yiq yc c vbs scart 75 ? 75 ? 75 ? 75 ? 75 ? 6mhz 1vp-p amplitude frequency
4/14 TA0310 video signal bandwidth versus resolution progressive video (pv): 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. 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 4: video spectrum for progressive video figure 5: video spectrum for hdtv 12mhz 1vp-p amplitude frequency 30mhz frequency 1vp-p amplitude
5/14 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. 7 st?s high-speed op-amps available in full production, st offers 4 op-amp fam ilies in the high-s peed op-amp portfolio that provide a broad choice to customers. these 4 families are complementary. tsh7x : vfa, gbp=100mhz, 3v to 12v power supply, input/output rail to rail. tsh8x : vfa, gbp=100mhz, 4.5v to 12v power supply, input/output rail to rail. tsh9x : vfa, gbp=130mhz, 12v power supply, noise=4.2nv/ hz, consumption=4.5ma 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 1v black level white level active video
6/14 TA0310 impedance matching 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 ? 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 75 ? is physically implemented on the board to achieve the right value for matching. a second resistor of 75 ? (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 ? 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=680 ? 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 li ne 75 ? 75 ? r 0volt 0volt 2vpp 1vpp 0volt 1vpp r + - set-top-box tv gain=2
7/14 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: 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 1k ? video line 75 ? 75 ? 1k ? + - +5v 0volt tsh7x 2vpp 1vpp 0volt 1vpp 0v 5v 75mv max. (gar anted ) 0v 1vp-p signal on the line 300mv
8/14 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 ?vol 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 75 ? 75 ? 680 ? + - +5v tsh11x 0v c in (10) +5v + 1 10n addition of a new dc component c out (220) r3 1k 1vpp 0volt r1 r2 twice the dc component (2vdc) 0volt 5volt rail of tsh11x : +1.2v max (tested) 680 ? 2 r 1 r 2 r vcc vdc + = 1vpp 0volt removes the original dc component removes the dc component fc<5hz 5v 0v 1vp-p sign al o n th e lin e 300mv
9/14 notes on video line driving TA0310 implementation of tsh7x-tsh8x-tsh9x-tsh11x families in dual supply -5v/+5v: in this scenario, the implementation of the op-amp is much more simple because the video signal amplitudes are far away from the rails. but the drawback of this solution is the cost of the providing a power supply of -5v. note: rfb can emqual 1k ? for tsh7x-8x-9x, 680 ? for tsh11x (see datasheet). frequency response of tsh7x-8x-9x-11x: figure 10: implementation of the tsh7x-8x-9x-11x in dual supply -5v/+5v figure 11: small signal bandwidth. gain=+2, rload=150ohms r g video line 75 ? 75 ? r fb + - +5v 0volt tsh 2vpp 1vpp 0volt 1vpp -5v signal on the line +5v -5v 1vp-p 300mv 0v 1m 10m 100m 0 1 2 3 4 5 6 7 8 tsh9x, -5v/+5v -3db bw = 75mhz tsh11x, 0/+5v -3db bw = 80mhz tsh11x, -5/+5v -3db bw = 110mhz tsh7x-8x, -5v/+5v -3db bw = 48mhz tsh7x-8x, 0/+5v -3 db bw = 4 8mh z gain (db) - a v frequency (hz)
10/14 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 sufficien t 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
11/14 notes on video line driving TA0310 reconstruction filtering: the output stage of an mpeg decoder is a dac (digital analog converter). this dac makes an analog video signal from the digital one used for video treatments on previous stages. during this conversion the sampling frequency will be unfortunately in the video spectrum as a parasitic frequency which causes distortion and noise in the video band. a low-pass filter (called a reconstruction filter) is used to remove this parasitic frequency. the cut-off frequency of this filter fits with the useful video bandwidth (6mhz, 12mhz or 30mhz). the order is directly linked to the value of the sampling frequency compared to the video band. an increasing of the sampling frequency allows a lower order of the filter. some of the newer designs of dac target a higher frequency sampling in order to remove the filtering on the future solutions. examples: figure 13: filtering of the analog output figure 14: sti5518 (set-top-box chip). fs=27mhz for sd outputs. 6mhz lpf to limit the 27mhz rg video line 75 ? 75 ? rfb + - +vcc tsh -vcc lpf video chip dac fs digital-to-analog conversion filtering (reconstruction) buffering matching and line 6mhz 1vp-p amplitude frequency 27mhz (sti5518) parasitic frequency due to sampling of the dac 21mhz filter
12/14 TA0310 notes on video line driving notes concerning the video input stage behavior on capacitive load as explained in section 3 on page 1, a high speed op-amp can be used for matching the video signal to the input stage of a tv or set-top-box. the input of a chip-set (typically the input of an analog to digital converter) is mainly a pure resistor approximately 1k ? or 2k ? in parallel with a capacitor of some of 10pf. figure 15: stm5588 (dvd chip). fs=54mhz for pv outputs. 12mhz lpf to limit the 54mhz figure 16: fs=74.25mhz for hd outputs. 30mhz lpf to limit the 74.25mhz 12mhz 1vp-p amplitude frequency 54mhz (stm5588) filtering parasitic frequency due to sampling of the dac 42mhz 74.25mhz (sti7710) filtering parasitic frequency due to sampling of the dac 44.25mhz 30mhz frequency 1vp-p amplitude
13/14 notes on video line driving TA0310 because of this, in series we place a resistance, rs, which is used as isolation in order to limit the effect of the capacitor on the op-amp stability. the following graphs in figures 18 and 19 give the frequency response of the tsh7x-8x and tsh11x family when loaded by a chip-set input. 30pf is the worst case; a typical value is 10pf). figure 17: typical load figure 18: tsh7x-8x behavior on capa-load + _ r g 1k ? r s r fb tsh 30pf analog-to-digital converter input 75 ? video line tv or set-top-box input set-top-box dvd video camera 1m 10 m 10 0m 0 1 2 3 4 5 6 7 8 tsh7x-8x, gain = +2 ca pa load = 30pf r load = 1kohm s vc c = -5 v/+ 5v rs = 82oh ms vcc = 0/+5v rs = 91 ohm s gain (db) - a v frequency (hz)
14/14 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 www.st.com notes on video line driving TA0310 figure 19: tsh11x behavior on capaload 110100 0 1 2 3 4 5 6 7 8 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)

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