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THS7303/7353 - How to choose input/output AC coupling capacitor value

Kawai
Guru 19775 points
Other Parts Discussed in Thread: THS7353, THS7303

Hi Team,

Our customer is considering to use THS7303 or THS7353 as a replace of a video filter device. There are many typical application example in the datasheet which have several value for input and output AC coupling capacitor. How do you choose these input/output AC coupling capacitor value ? Customer is willing to use smaller value than the example(470uF, 47uF) in the datasheet. If there is an equation, could you please share us the information ?

They would not use sync tip clamp or sag correction function, only as an filter. If the device is used just as a filter, I believe there would not be a big difference between the two devices.

Best Regards,
Kawai

  • 0
    TI__Genius 15975 points

    Hi Kawai-san,

    The use of input/output AC coupling capacitor is to improve the lower frequency pass band response; with higher the value of capacitor, the lower the cut-off frequency for pass-band and thus better for the video signal which has sync signal. Sync signal is usually associated with a signal close to dc frequency. I have included a diagram below of what the ac-coupling capacitor does to the pass-band response for lower frequencies in which for a typical video system you can use the R as 75ohms.

    Both the THS7303 & THS7353 offer sync tip clamp capability which maintains the sync signal even when ac-coupled at the inputs. But, while ac coupling the outputs to pass the sync signal, you would have to use a large value capacitor like 470uF for an R of 75ohms for THS7353. This large value of capacitor is easily avoided using the THS7303, which offers SAG correction at lower frequencies. SAG correction is nothing but an increase in gain at lower frequencies by including the ac coupling capacitor in the feedback path of the internal amplifier, in-order to correct for lower frequency attenuation due to the use of 47uF ac coupling capacitor.

    I hope this provides clarification. 

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Guru 19775 points

    Hello Rohit-san,

    Thank you for the information.

    Could you let me discuss with you offline ? I appreciate it if you could send me an email to my email address.

    I will send you customer's block diagram and additional information.

    Best Regards,
    Kawai