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THS7253 SAG correction circuit

Kikuo Mita
Intellectual 2515 points
Other Parts Discussed in Thread: THS7353, THS7303, THS7313

The first page of THS7353 datasheet shows the block diagram of this device with comments
“Figure 1. 3.3 V Single-Supply AC-Input/AC-Video Output System w/SAG Correction”.

So, THS7303 may support SAG correction, however, I cannot find schematic examples of
the SAG correction.
Can you let me know how to implement the SAG correction?

 

  • 0
    Prodigy 100 points

    Hi, Kikuo,

    Look at datasheet 3-Ch Low Power Video Buffer w/I2C Control Select Filter Ext Gain Control 2:1 MUX (Rev. A)  (PDF 1462 KB) :

    Figure 1. "3.3-V, Single-Supply, DC-Input/AC-Video Output System with SAG Correction"

    There are sample  (1 of 3 Channels Shown).

    same as for THS7303

    Regards!

  • 0 in reply to Georgy
    TI__Intellectual 2515 points

    Gregory-san,

    Thank you for the answer.
    I confirmed that, both THS7353 and THS7303 use the same circuit for SAG
    Correction.

    I appreciate it if I can know how to determine the compartment values such as
    47 uF and 33 uF etc.
    My concern is that if these values may be changed depending on the gain of the Amp.
    Are these value independent to the gain?

    Mita

  • 0 in reply to Kikuo Mita
    TI__Expert 3030 points

    Mita-san,

    The THS7353 illustration on Page 1 should not have the "SAG" term in the description. This is a typo error and should be removed.

    The THS7353 has a "gain adjust" pin for each channel where the gain = 1 + (1k / (250 + ext. Resistor to GND)) (See THS7353 datasheet - page 29) or = 1 if the gain adjust pin is connected to the output pin as figure 1 shows. While it is possible to do a form of SAG correction, it was not really designed explicitly for this function.

    The THS7303 (and THS7313) has a "SAG" pin for each channel. This pin can either be connected directly to the output for a simple 2V/V gain, or connected to an external resistor to GND for adjustable gain (see Figure 68 in datasheet for more info), or connect thru some capacitors to perform SAG function (see Figure 66 and associated text). Multiple Figures show how SAG feedback is performed with internal resistors where the resistor values were chosen by weighing numerous factors including capacitor values, responses, and DC gain.

    The complete transfer function of this circuit with SAG connected capacitors is very complex with alot of interaction occuring. However, it is probably easiest to state that the 47uF & 33uF capacitor values were chosen to give a respectable amount of droop (equivalent to about 270uF or traditionally AC coupled outputs) while keeping the values minimized. If more droop is acceptable then reducing the values (ex: 33uF & 22uF) can be done, or if less droop is desireable then increasing each capacitor is recommended (ex: 68uF & 47uF or 82uF & 68uF or ....).

    With the above stated, are you asking for some recommendations for the THS7303 or THS7353? If the THS7353 is the part in question, then there is No SAG function by default and the equation is very simple.

    Let me know if further are further questions.

    Cheers,

    Randy

  • 0 in reply to Randy S
    TI__Intellectual 2515 points

    Gregory-san,

    Thank you for your shuttle feedback.
    I can clear it.

    Mita