I am simulating the Low-Power RGB Mux/Line Driver Application Note for the OPA3684 (Figure 6 in the OPA3684 Datasheet SBOS241C – MAY 2002 – REVISED JULY 2008).
The simulation is done in TI-TINA (Version 7.0.80.96 SF-TI Build date: 2009 April 6, 11:48:24 AM).
I'm a little confuse because if I draw (build) the circuit as in the example (Figure 6) I don't get the expected voltage levels.
For the simulation I'm using 700 mv for Input 1 ( R1,G1,B1 = 700 mv) and 400 mv for Input 2 (R2,G2,B2 = 400 mv). Let's say that if I
disable Input 2 , I would expect an output voltage of around 1400mv (due to the gain of 2, really is around 2.18) but I get another
voltage level. I also added the 75ohm load of the RGB Line.
My question is that if the 78.7ohm limiting resistor goes in each output as in Figure 6. Or it just goes in each V Out , I mean
a 78.7 ohm resistor in VOut Red,
a 78.7 ohm resistor in VOut Green,
a 78.7 ohm resistor in VOut Blue,
so instead of the six (6) 78.7 ohm resistor I would just need three(3).
My second question is that the circuit uses 681 ohm resistor and 806 ohm resistor for the feedback network , but those are not necessary
commercial values. for the 681 ohm the solution is kind of simple a 680 ohm plus a 1ohm resistor, for the 806 ohm, I could use 470 ohm plus
330 ohms , for 800 ohm , ( I could use more resistors or a combination in parallel, but I think 470+330 could be a good replacement for the 806).
With 680 and 800 the gain is about 2.17 , close to the original 2.18. Well , my real question is that in the Application note says that:
" When one channel is shutdown, the feedback network is still present, slightly attenuating the signal and combining in parallel
with the 78.7 ohm to give a 75 ohm source impedance."
so what they mean is that ( 681+806 || 78.7 ) = ( 1487 || 78.7) = 74.74 ohms (eq. 1)
or the 78.7 resistor in each output is also part of the feedback network ?
(681+806+78.7 || 78.7) = (1565.7 || 78.7 ) = 74.93 ohms (eq. 2)
The reason of my question is that if I use 680 ohms and 800 ohms resistors I guess the values of the 78.7 resistor wouldn't match correctly
the 75 ohm impedance of the load, plus 78.7 it is not a commercial value. Using eq. 1 i find a value of 79 ohms for the resistor, which would
make a 74.99 ohm in combination with the feedback network.
Figure 6
R1 ----------- [ op amp ] --------- 78.7 Resistor -------|---------> Vout Red
R2 ----------- [ op amp ] --------- 78.7 Resistor---------|
Or I should be like this ?
R1 ----------- [ op amp ] -------------|------- 78.7 Resistor--------> Vout Red
R2 ----------- [ op amp ] --------------|
Any advice, clue, suggestion, link to another application notes, the name of book, questions, everything is really appreciated and welcome.
My goal is to build a circuit to switch between the signal of a PC and a Laptop to show it in a projector, also the Monitor of the PC it's going to be working.
PC ------------------------------------|---------------------------------- Monitor
LapTop -------------------------------|-----------------------------[ PC | Laptop switch] ------ Projector
Thanks.
Ernesto Arzabala
