THS7353_15 Datasheet, PDF (29/57 Pages) Texas Instruments – 3-Channel Low Power Video Buffer with I2C Control, Selectable Filters, External Gain Control, 2:1 Input MUX, and Selectable Input Modes

THS7353
www.ti.com
SLOS484B – NOVEMBER 2005 – REVISED AUGUST 2012
Due to the edge rates and frequencies of operation, it is recommended – but not required – to place a 0.1-μF to
0.01-μF capacitor in parallel with the large 220-μF to 1000-μF capacitors. These large value capacitors are most
commonly aluminum electrolytic. It is known that these capacitors have significantly large equivalent series
resistance (ESR), and their impedance at high frequencies is large due to the associated inductances involved
with their construction. The small 0.1-μF to 0.01-μF capacitors help pass these high frequency (>1 MHz) signals
with lower impedance than the large capacitors. This is especially true when HD and computer R'G’B’ signals are
being used. Their associated edge rates and frequency content can reach beyond 30-MHz for HD signals and
can be over 100-MHz for R'G’B’ signals – frequencies that typical aluminum electrolytic capacitors cannot pass
effectively.
Although it is common to use the same capacitor values for all the video lines, the frequency bandwidth of the
chroma signal in a S-Video system are not required to go as low or as high as the frequency of the luma
channels. Thus, the capacitor values of the chroma line(s) can be smaller – such as 0.1 μF.
OUTPUT MODES OF OPERATION – GAIN ADJUST PIN
To expand the flexibility of the THS7353, a gain adjust pin is included for each channel. This pin allows the gain
of the output buffer to be varied from 0-dB (1 V/V) to 14-dB (5 V/V) gain. This enables the user to adjust the gain
to whatever is required for the system. For example, if a source signal is attenuated, then adding a resistor
between the gain adjust pin and ground increases the channel gain according to Equation 1.
Gain = 1 +
1k
250 + R_(external)
(1)
For a gain of 6-dB, a 750-Ω resistor should be added to the system. Note that while the internal resistor matching
is very tight, less than ±1%, the absolute values of these resistors vary as much as ±10%. As such the overall
gain varies even when using tight tolerance external components. For example, if the desired gain is to be 6 dB
and a true 750-Ω external resistor is used, the overall gain is approximately 6 dB ± 0.33 dB.
One potential issue about this feature is that the dc bias point increases directly with the increase in gain. Thus, it
can be possible for the dc operating point to saturate to the power supply. For example, if ac-bias is selected, the
output dc operating point is about 1 V with a 3.3-V supply. But, if a gain of 14-dB is required, the dc operating
point saturates to the positive rail as 1 V X 5 V/V = 5 V, well above the 3.3-V power supply.
One way to counteract this is to modify the bias point as shown in the INPUT MODES OF OPERATION – AC
BIAS section of this application section. Another way to counteract this dc bias point increase is to use a
capacitor in series with the Gain Adjust pin. This capacitor blocks the dc gain and maintains it at 0-dB. Thus, the
signal gain can increase while the dc gain is only 1 – maintaining proper operation of the output amplifier. The
only stipulation with doing this is the capacitor creates a high-pass filter with the -3-dB corner frequency equal to
Equation 2. Thus, the capacitor value must be large enough to pass the desired range of frequencies.
1
2 p (R_(external) + 250) C_(external)
(2)
One point that must not be neglected is that the output buffer amplifier is a voltage feedback (VFB) amplifier.
VFB amplifiers have what is known as a gain-bandwidth (GBW) product. This means the -3-dB bandwidth of the
amplifier is indirectly proportional to the gain resulting in a theoretical constant gain X bandwidth product. For the
THS7353, the -3-dB frequency in bypass mode is about 150-MHz while in unity gain (0-dB). But, as the gain
increases, the bandwidth decreases. In a gain of 2 V/V (6-dB), the bandwidth is about 80-MHz. In the maximum
gain of 5 V/V (14 dB), the bandwidth is only about 30-MHz. Thus, there is interaction with the internal filters as it
is a composite system. The filter attenuation is added with the output amplifier attenuation, resulting in a change
in the overall system filter characteristics. Care must be taken when using high gains.
There are package parasitics and PCB parasitics in any system. Since the external gain adjustment is part of the
feedback and gain system of the output amplifier, it is possible the parasitics can cause issues with the system.
These issues can cause high frequency peaking to occur or even oscillations. Thus, it is recommended to place
a small capacitor – 2.2 pF for example, directly between the Gain Adjust pin and the Output pin when not using
the output in unity gain. In unity gain, the Gain Adjust pin should be tied directly to the output pin to ensure
stability.
Copyright © 2005–2012, Texas Instruments Incorporated
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