STMICROELECTRONICS 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
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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
Terrestrial
Cable
Satellite
Set-Top-Box
output
150Ω Load
TV
input
75Ω line
Capa Load
75Ω line
input
Capa Load
DVD
output
150Ω Load
input
75Ω line
SCART PLUGs
Capa Load
output
75Ω line
150Ω Load
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
video line
Filtering
75Ω
MPEG
DECODER
Pb, U, I
video line
Filtering
75Ω
Y
MPEG
DECODER
Filtering
75Ω
video line
MINI JACK DIN
USCHIDEN
Filtering
75Ω
Y
75Ω
video line
75Ω
C
SCART
75Ω
75Ω
1
video line
Filtering
75Ω
YUV, YPbPr,YIQ
YC
CVBS
75Ω
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
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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.
Figure 4: Video spectrum for Progressive video
Amplitude
1Vp-p
Frequency
12MHz
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
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Frequency
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
300mV
~0V
Black Level
Synchronization
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
TV
2Vpp
1Vpp
Gain=2
1Vpp
video line
+
-
75Ω
75Ω
R
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.
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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
+5V
5V
2Vpp
1Vp-p
+
TSH7x
0Volt
video line
75Ω
75Ω
300mV
1kΩ
1kΩ
0Volt
0V
75mV max.
(garanted)
0V
Signal on the line
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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Notes on video line driving
Implementation of TSH11x family in single supply 0/+5V:
Figure 9: Implementation of the TSH11x in single supply 0/+5V
5Volt
Removes the
original DC
component
Twice the DC component (2Vdc)
Rail of TSH11x : +1.2V max (tested)
0Volt
+5V
Cin (10µ)
1Vpp
R3
1k
0Volt
+
-
75Ω
75Ω
Cout
(220µ)
680Ω 680Ω
+5V
Addition of a
new DC
component
R2
Vdc = Vcc ×
R1 + R2
video line
TSH11x
1Vpp
5V
0V
R1
1µ
+
10n
0Volt
1Vp-p
R2
Removes the DC
component
Fc<5Hz
300mV
0V
Signal on the line
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.
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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
+5V
1Vpp
+5V
2Vpp
1Vp-p
+
TSH
0Volt
Rg
video line
75Ω
75Ω
300mV
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 1kΩ for TSH7x-8x-9x, 680Ω for TSH11x (see datasheet).
Frequency response of TSH7x-8x-9x-11x:
Figure 11: Small Signal Bandwidth. Gain=+2, Rload=150ohms
8
7
TSH11x, 0/+5V
-3dB Bw = 80MHz
Gain (dB) - A V
6
5
TSH7x-8x, 0/+5V
-3dB Bw = 48MHz
TSH11x, -5/+5V
-3dB Bw = 110MHz
4
3
TSH7x-8x, -5V/+5V
-3dB Bw = 48MHz
2
1
0
1M
TSH9x, -5V/+5V
-3dB Bw = 75MHz
10M
100M
Frequency (Hz)
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Notes on video line driving
Figure 12: Choice of the op-amp versus a video bandwidth
1Vp-p
Standard
Definition
(SD)
+5V
TSH7x-8x
~6MHz
+5V
+5V
TSH11x
TSH9x
-5V
-5V
or 0V
+5V
+5V
Frequency
1Vp-p
+5V
Progressive
Video
Frequency
~12MHz
1Vp-p
HDTV
Frequency
~30MHz
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TSH7x-8x?
if sufficien t
Linearity
TSH11x
TSH9x
-5V
-5V
or 0V
+5V
+5V
TSH9x ?
if sufficient
Linearity
-5V
TSH11x
-5V
or 0V
Notes on video line driving
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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.
Figure 13: Filtering of the analog output
Video Chip
+Vcc
DAC
+
-
LPF
video line
TSH
Fs
Rg
75Ω
75Ω
Rfb
-Vcc
Digital-to-Analog
Conversion
Filtering
(Reconstruction)
Buffering
Matching and Line
Examples:
Figure 14: STi5518 (set-top-box chip). Fs=27MHz for SD outputs. 6MHz LPF to limit the 27MHz
Amplitude
Filter
Parasitic frequency due
to sampling of the DAC
1Vp-p
6MHz
27MHz
Frequency
(STi5518)
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
12MHz
Parasitic frequency due
to sampling of the DAC
54MHz
Frequency
(STm5588)
42MHz
Figure 16: Fs=74.25MHz for HD outputs. 30MHz LPF to limit the 74.25MHz
Amplitude
Filtering
1Vp-p
Parasitic frequency due
to sampling of the DAC
30MHz
74.25MHz
Frequency
(STi7710)
44.25MHz
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 1kΩ or 2kΩ 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
TV or Set-top-box input
Video Line
Set-top-box
DVD
Video Camera
+
Analog-to-Digital
Converter input
TSH
75Ω
Rs
_
1kΩ
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
Vcc = 0/+5V
Rs = 91ohms
7
Gain (dB) - A V
6
5
4
3
Vcc = -5V/+5V
Rs = 82ohms
2
1
0
1M
TSH7x-8x, Gain = +2
Capa Load = 30pF
R Load = 1kohms
10M
100M
Frequency (Hz)
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Notes on video line driving
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Figure 19: TSH11x behavior on capaload
8
Vcc = -5V/+5V
Rs = 30ohms
7
Gain(dB) - A V
6
5
4
3
Vcc = 0/+5V
Rs = 39ohms
2
TSH11x, Gain = +2
Capa Load = 30pF
R Load = 1kohms
1
0
1
10
100
Frequency (MHz)
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