TA0310 TECHNICAL ARTICLE
High Speed Operational Amplifiers for 75 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 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
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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 150 Load input
75 line
Terrestrial Cable Satellite
TV
input Capa Load
75 line
Capa Load input
75 line SCA RT PLUGs
DVD
output 150 Load
Capa Load
output 150 Load
75 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.
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Video signal bandwidth versus resolution
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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
75
75
vid eo line
SCART
Pb, U, I Y
Filtering 75 75
vid eo line
Filtering 75 75
C
MPEG DECODER
vi deo line
MINI JACK DIN USCHIDEN
Filtering 75 75
vi deo line
Y
1
Filtering 75 75
YUV, YPbPr,YIQ YC CVBS RCA JACK
MPEG DECODER
CVBS
vi deo line
Filtering 75 75
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
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T A 0310 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
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Signal amplitude
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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
300 mV ~ 0V
Synchroniz ation
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)
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Impedance matching
8 Impedance matching
Figure 7: Typical connexion between set-top-box and TV
SET-TOP-BOX
1Vpp
Gain=2
TV
1Vpp
v ide o line
2Vpp
+ R
75 75
0 Vo l t
R
0 Vo l t
0 Vo l t
W e 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 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. 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.
1
Reflection factor occurs when the line is loaded by the same value as its own characteristic impedance Zc.
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Notes on video line driving
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(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
+5 V
+ TSH 7x -
2V pp
1V p-p
v ide o line
5V
0V ol t 1k 1k
75
75
300mV 0V ol t
75mV max. (g a ra n ted) Sign a l on the lin e
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).
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T A 0310 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
5 Vo l t
Removes the original DC component
+5V
R3 1k
TSH11x
Tw ice the DC component (2Vdc) Rail of TSH11x : +1.2V max ( tested) 0 Vo l t
Cin (10 ) + 75
video line
1Vpp
0V ol t
75
680 680 +5V
Co u t ( 220 )
1Vpp
0 Vo l t
0V
1V p-p
5V
Add ition of a new DC component
R2 Vd c = Vc c × R1 + R2
R1 + 10n
Removes the DC component Fc<5Hz
1
R2
300mV
0V Sign al on the lin e
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.
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Notes on video line driving
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Implementation of TSH7x-TSH8x-TSH9x-TSH11x families in dual supply -5V/+5V:
Figure 10: Implementation of the TSH7x-8x-9x-11x in dual supply -5V/+5V
1Vpp 1Vpp
+5V
+ TS H -
2Vpp
1Vp -p
v ide o line
+5V
75
75
300 mV
0Vol t
Rg
R fb 0Volt
0V
-5V
-5V Signal on the line
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 11: Small Signal Bandwidth. Gain=+2, Rload=150ohms
8 7 6
TSH 11x, 0/+5V -3dB Bw = 80MHz TSH11x, -5/+5V - 3dB Bw = 110MHz
Gain (dB) - A V
5 4 3 2 1 0 1M
TSH7x-8x, 0/+5V -3dB Bw = 48MHz
TSH7x-8x, -5V/+5V -3dB Bw = 48MHz
T SH9x, -5V/+5V -3dB Bw = 75MHz
1 0M
10 0M
F req uen cy (Hz)
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Figure 12: Choice of the op-amp versus a video bandwidth
Notes on video line driving
1V p-p
+5V
TSH7x -8x
Standard Definition (SD)
1V p-p
+5V
TSH9x
+5V
TSH11 x
~6MHz
Frequency
-5V
-5V or 0V
Progressive Video
~12MHz
+5V
Frequency
T SH7x -8x ? i f sufficien t L in eari ty
+5V
TSH9x
+5V
TSH11 x
-5V
-5V or 0V +5V
TSH11 x
1V p-p
+5V
Frequency
TSH9x ? i f sufficient Li neari ty
HDTV
~30MHz
-5V
-5V or 0V
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Notes on video line driving Reconstruction Filtering:
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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. Figure 13: Filtering of the analog output
Video Chip
+Vcc DAC Fs LPF
Rg TS H
75
+ -
v ide o line
75
Rfb
-Vcc
Digital-to-Analog Convers ion
Exam ples: Figure 14: STi5518 (set-top-box chip). Fs=27MHz for SD outputs. 6MHz LPF to limit the 27MHz
Filtering (Reconstruction)
Buff ering
Matching and Line
Ampli tude
Filter
1 V p -p
Parasitic frequency due to sampling of the DAC
6MHz
27MHz
(S T i 5 5 1 8 )
F requency
21MHz
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Notes on video line driving
Figure 15: STm5588 (DVD chip). Fs=54MHz for PV outputs. 12MHz LPF to limit the 54MHz
Amplitude
Filtering
1Vp-p
Parasitic frequency due to sampling of the DAC
12MHz
42MHz
54MHz
Frequency
(STm5588 )
Figure 16: Fs=74.25MHz for HD outputs. 30MHz LPF to limit the 74.25MHz
A mpl itude
Filtering
1V p-p
Paras itic frequency due to sampling of the DAC
30MHz
44.25MHz
74.25MHz
(STi7 710 )
F requency
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.
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Notes on video line driving
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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. Figure 17: Typical load
T V or Set-top-box input
Set-top-box D VD Video Camera
Video Line
+
75 TSH
Analog-to-D igital Converter input
_
Rs
1k 30pF
Rfb RG
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 18: TSH7x-8x behavior on capa-load
8 7 6 V cc = 0/+5V Rs = 91ohms
G ain (dB) - A V
5 4 3 2 1 0 1M TS H7x -8 x, Gain = +2 Ca pa Load = 30pF R Load = 1kohms 10 M 10 0M Vc c = -5V/+5V Rs = 82ohms
F req uen cy (Hz)
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Notes on video line driving
Figure 19: TSH11x behavior on capaload
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8 7 6 V c c = -5V/+5V R s = 30ohms
G ain (dB) - A V
5 4 3 2 1 0 1 T SH1 1x, Gain = +2 Cap a Load = 30pF R Load = 1kohms 10 10 0 Vcc = 0/+5V Rs = 39ohms
F r eq u en cy (MHz)
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