THS7375IPWR datasheet(21/36 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)

THS7375

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discharge current. If more hum rejection is desired or

voltage range and the internal gain. As such, the

there is a loss of sync occurring, simply decrease the

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

0.1-

µF input coupling capacitor. A decrease from 0.1

point is the primary factor. For example, if the output

µF to 0.047 µF increases the hum rejection by a

dc bias point is desired to be 1.6 V on a 3.3-V supply,

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

then the input dc bias point is recommended to be

resistor to ground may be added that decreases the

(1.6 V – 320 mV)/5.6 = 0.228 V. Thus, the pull-up

overall

resistance

and

ultimately

increases

the

resistor calculates to approximately 10.8 M

Ω. If the

discharge current.

output dc-bias point is desired to be 1.6 V with a 5-V

power supply, then the value calculates to be

To ensure proper stability of the ac STC control loop,

approximately 16.7 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 THS7375 output. Because most DACs

this variance into account. For the 0.228-V input bias

or encoders use resistors to establish the voltage,

voltage example above using an ideal 10.8-M

Ω

which are typically less than 500

Ω, meeting the less

resistor, the input dc bias voltage is about 0.228 V

than 1-k

Ω requirement is easily done. However, if the

(±0.045 V) which translates to an output bias voltage

source impedance looking from the THS7375 input

of about 1.6 V (±0.25 V).

perspective is very high, simply adding a 1-k

Ω

If desired, an external resistor can be placed in

resistor to GND ensures proper operation of the

parallel

with

the

internal

800-k

Ω resistor.

This

THS7375.

external resistor may be required if the pull-up

resistor calculates to a value higher than desired.

There are no consequences of this configuration

other than decreasing the effective input impedance

Sync-tip clamps are ideal for signals that have

of the THS7375 system.

horizontal and/or vertical syncs associated with them.

However, some video signals do not have a sync

The value of the output bias voltage is very flexible

embedded within the signal. If ac-coupling of these

and is left to each individual design. It is important to

signals is desired, then a dc bias is required to

ensure that the signal does not clip or saturate the

properly set the dc operating point within the

video signal. Thus, it is recommended to ensure the

THS7375. This function is easily accomplished with

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

the THS7375 by simply adding an external pull-up

For 100% color saturated CVBS or signals with

resistor to the positive power supply, as shown in

Macrovision, the CVBS signal can reach up to 1.23

Figure 49.

VPP at the input, or 2.46 VPP at the output of the

THS7375. In contrast, other signals are typically 1

VPP or 0.7 VPP at the input which translate to an

output voltage of 2 VPP or 1.4 VPP, respectively. The

output bias voltage must account for a worst-case

situation depending on the signals involved.

One other issue that must be taken into account is

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

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

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

Figure 49. AC-Bias Input Mode Circuit

capacitor forms a high-pass filter with the parallel

Configuration

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

The dc voltage that appears at the input pin is equal

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

to Equation 1:

0.1-

µF input capacitor with a 10.8-MΩ pull-up resistor

equates to a 2.1-Hz high-pass corner frequency.

The THS7375 allowable input range is approximately

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

voltage range but is limited by the allowable output

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