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INPUT MODE OF OPERATION – DC

+

-

800 k

W

+ Vs

Input

Internal

Circuitry

140mVLevel

Shifter

THS7316

SLOS521 – MARCH 2007

APPLICATION INFORMATION (continued)

The inputs to the THS7316 allows for both ac-coupled and dc-coupled inputs. Many DACs or Video Encoders

can be dc connected to the THS7316. One of the drawbacks to dc coupling is when 0-V is applied to the input.

Although the input of the THS7316 allows for a 0-V input signal with no issues, the output swing of a traditional

amplifier cannot yield a 0-V signal resulting in possible clipping. This is true for any single-supply amplifier due to

the limitations of the output transistors. Both CMOS and bipolar transistors cannot go to 0-V while sinking

current. This trait of a transistor is also the same reason why the highest output voltage is always less than the

power supply voltage when sourcing current.

This output clipping can reduce the sync amplitudes (both horizontal and vertical sync amplitudes) on the video

signal. A problem occurs if the receiver of this video signal uses an AGC loop to account for losses in the

transmission line. Some video AGC circuits derive gain from the horizontal sync amplitude. If clipping occurs on

the sync amplitude, then the AGC circuit can increase the gain too much – resulting in too much amplitude gain

correction. This may result in a picture with an overly bright display with too much color saturation.

It is good engineering design practice to ensure saturation/clipping does not take place. Transistors always take

a finite amount of time to come out of saturation. This saturation could possibly result in timing delays or other

aberrations on the signals.

To eliminate saturation/clipping problems, the THS7316 has a dc + 140-mV input shift feature. This feature takes

the input voltage and adds an internal +140-mV shift to the signal. Since the THS7316 also has a gain of 6 dB

(2 V/V), the resulting output with a 0-V applied input signal is about 280-mV. The THS7316 rail-to-rail output

stage can create this output level while connected to a typical video load. This ensures that no saturation /

clipping of the sync signals occur. This is a constant shift regardless of the input signal. For example, if a 1-V

input is applied, the output is at 2.28-V.

Because the internal gain is fixed at 6 dB, the gain dictates what the allowable linear input voltage range can be

without clipping concerns. For example, if the power supply is set to 3-V, the maximum output is about 2.9-V

while driving a significant amount of current. Thus, to avoid clipping, the allowable input is ((2.9 V / 2) – 0.14 V)

= 1.31 V. This is true for up to the maximum recommended 5-V power supply that allows about a ((4.9V / 2) –

0.14 V) = 2.31 V input range while avoiding clipping on the output.

The input impedance of the THS7316 in this mode of operation is dictated by the internal 800-k

Ω pull-down

resistor. This is shown in Figure 28. Note that the internal voltage shift does not appear at the input pin, only the

output pin.

Figure 28. Equivalent DC Input Mode Circuit

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