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VGA(R,B,G video input) input to differential R,G,B output with differential gain of 2 using AD8133 

Dear All,

Part No : AD8133 (Triple Differential Driver with Output Pull-Down)

My Application : VGA(R,B,G video input) input to differential R,G,B output with differential gain of 2.

I am Using AD8133in Single-Ended-to-Differential Application (Datasheet Figure 36 page No 15) but I want to below information

1.What is the purpose of 80.6 ohms if video source having 75 ohms series termination shunt termination has to be 75 ohms for maximum power transformations and to make reflection coefficient=0.

2.What is the purse to grounding the op amp inverting input by 38.3 ohms if do so(as per Datasheet Figure 36 page No 15 ) according to my calculations

            Say Vin+ = 1V,Vin- = 0V, Vind = Vin+ - Vin- = 1V

   

            Vout+=1/2 * (1500/(750+38.3)) * Vd

            Vout+=1/2 * (1500/(750+38.3)) * 1

            Vout+=0.951 V

            Vout-=-1/2 * (1500/750) * Vd

            Vout-=-1/2 * (1500/750) * 1

            Vout-=-1V

and Differential Output will be Vod = Vout+ -  Vout-

                                                 Vod = 0.951 - (-1) = 1.951 Which is not equal to 2 which is about 50 mV error.


3. Shall I take output by with out providing +49.9 ohms in (both Vout+ and Vout-) and providing one shunt termination as provided below.

270 ohms - one terminal connected to Grounded and other is connected to Vout+

150 ohms - one terminal connected to Vout+ and other is connected to Vout-

270 ohms -one terminal connected to Vout- and other is connected to Ground.

Regards,

Tendulakar S

Top Replies

  • FormerMember
    FormerMember
Nov 1, 2016 +1

Yes, given everything is 75 Ohm, than the negative input should have a 37.5 Ohm which is equivalent to a 75 Ohm right next to the pin and virtual 75 Ohm at a virtual negative source.

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