THS7374IPWR datasheet(21/37 Pages) TI

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INPUT MODE OF OPERATION: AC BIAS

Level

Shift

Internal

Circuitry

+3.3V

800kW

Input

0.1 F

800kW

800k

(1)

THS7374

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discharge current. If more hum rejection is desired or

is very flexible; the output dc bias point is the primary

there is a loss of sync occurring, simply decrease the

factor. For example, if the output dc bias point is

0.1-

µF input coupling capacitor. A decrease from 0.1

desired to be mid-rail on a 3.3-V supply, then the

µF to 0.047 µF increases the hum rejection by a

input dc bias point is recommended to be (1.6 V –

factor of 2:1. Alternatively, an external pull-down

300 mV)/2 = 0.65 V. Thus, the pull-up resistor

resistor to ground may be added that decreases the

calculates to a standard 3.3-M

Ω resistor, resulting in

overall

resistance

and

ultimately

increases

the

0.644 V. If the output dc-bias point is desired to be

discharge current.

1.6 V with a 5-V power supply, then the pull-up

resistor

value

calculates

approximately

To ensure proper stability of the ac STC control loop,

5.36-M

Ω.

the source impedance must be less than 1-k

Ω with

the input capacitor in place. Otherwise, there is a

Keep in mind that the internal 800-k

Ω resistor has a

possibility for the control loop to ring; this ringing may

±20% variance. As such, the calculations should take

appear on the THS7374 output. Because most DACs

this variance into account. For the 0.644-V input bias

or encoders use resistors to establish the voltage,

voltage example above using an ideal 3.3-M

Ω

which are typically less than 300-

Ω, meeting the less

resistor, the input dc bias voltage is about 0.644 V

than 1-k

Ω requirement is easily done. However, if the

(±0.1 V).

source impedance looking from the THS7374 input

The value of the output bias is very flexible and is left

perspective is very high, simply adding a 1-k

Ω

to each individual design. It is important to ensure

resistor to GND ensures proper operation of the

that the signal does not clip or saturate the video

THS7374.

signal. Thus, it is recommended to ensure the output

bias voltage is between 0.9 V and (VS+ – 1 V). For

100%

color

saturated

CVBS

signals

with

Macrovision, the CVBS signal can reach up to 1.23

Sync tip clamps are ideal for signals that have

VPP input, or 2.46 VPP output. In contrast, other

horizontal and/or vertical syncs associated with them.

signals are typically 0.7 VPP input, or 1.4 VPP output.

However, some video signals do not have a sync

As such, the output bias voltage must account for a

embedded within the signal. If ac-coupling of these

worst-case situation depending on the potential

signals is desired, then a dc bias is required to

signals.

properly set the dc operating point within the

THS7374. This function is easily accomplished with

One other issue that must be taken into account is

the THS7374 by simply adding an external pull-up

the dc-bias point as a function of the power supply.

resistor to the positive power supply, as shown in

As such, there is an impact on the system PSRR. To

Figure 39.

help reduce this impact, the input capacitor combines

with the pull-up resistance to function as a low-pass

filter. Additionally, the time to charge the capacitor to

the final dc bias point is also a function of the pull-up

resistor and the input capacitor. Lastly, the input

capacitor forms a high-pass filter with the parallel

impedance of the pull-up resistor and the 800-k

Ω

resistor. In general, it is good to have this high-pass

filter at approximately 3-Hz to minimize any potential

droop on a P’B, P’R, or non-sync B’ or R’ signal. A

0.1-

µF input capacitor with a 3.3-MΩ pull-up resistor

equates to a 2.5-Hz high-pass corner frequency.

This mode of operation is recommended for use with

chroma (C’), P’B, P’R, U’, V’, and non-sync R'G'B’

Figure 39. AC-Bias Input Mode Circuit

signals. This method can also be utilized with signals

Configuration

with sync if desired. The benefit of using the STC

function is that it maintains a constant back porch

voltage as opposed to a back porch voltage that

The dc voltage that appears at the input pin is equal

fluctuates depending on the video content. Because

to Equation 1:

the corner frequency of the input is a very low 2.5 Hz,

then this is still very good performance, but not as

good relative to a STC configuration.

The THS7374 allowable input range is approximately

0 V to (VS+ – 1.5 V), which allows for a very wide

input voltage range. As such, the input dc bias point

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