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LMH6739: LMH6739 output R Value

Part Number: LMH6739

Hi Sir,

As I know the RGB out need series 75ohm to device,

Why TI Datasheet Figure 28 is 51ohm?

  • Hi Hugo,

    figure 28 has nothing to do with driving a 75R load. It only discusses how a capacitive load (CL) should be driven by the LMH6739 without causing the OPAmp to oscillate. This can only be done by inserting an "isolation" resistor, here called "Rout".

    Figure 28 of datasheet must be seen together with figure 8, which shows the recommended Rout for various capacitive loads:

    An example: If the capacitive load at output of  LMH6739 is 47pF, Rout=24R is recommended.

    Why all this hassle with Rout?

    Connecting a capacitive load directly to the output of an OPAmp can introduce a dangerous phase lag in the feedback loop, which can erode the phase margin so far that the OPAmp can become instable and oscillates. Rout "isolates" the capacitive load from the feedback loop and ensures stability.

    Kai 

  • Hi Hugo,

    Kai is correct. This figure is irrelevant. Please see the LMH6739 performance characteristics for driving a 150ohm load with Gain=+2V/V, since this is what is used for line RGB line driving with 75ohm impedance matching.

    Best regards,

    Sean
  • Hi Sir,

    Our customer have output level issue,
    the RGB input is 0.7V, But the output Change to 0.5V level.
    so we are checking this issue new.
    and Thanks for your explanation.

    Hugo.

  • Hi Kai,

    Thanks for your reply.

    In LMH6739 Fig.9 indicated the Rout vs. Capacitive load, however connecting a different VGA cable which mean has different terminated and capacitive load. In our system design, we used LMH6739 as a VGA signal buffer on Projector’s VGA output port. If we designed a Rout value between 20~33Ω on LMH6739 output, will it cause the signal reflection and any quality issue on VGA output image(ex: Image phase noise) when connected with a standard (ex: 75Ω) or a longer VGA cable respectively. Please help advise how to choose a correct Rout value to meet all kinds of these scenarios?

  • Hi Liao,

    if you have a 75R load which you connect with a 75R cable to the output of LMH6739, then you must choose a Rout of 75R. If you have a 50R load, then must take a 50R cable and a Rout of 50R.

    The cable must always be terminated with its characteristic impedance to prevent improper operation. So, a 50R cable needs 50R impedances at both ends and a 75R cable needs 75R impedances.

    Kai
  • Hi Kai,

    [The cable must always be terminated with its characteristic impedance to prevent improper operation. So, a 50R cable needs 50R impedances at both ends and a 75R cable needs 75R impedances.]

    --> This made me confused, since in your previous reply, we should refer to Fig. 28 to design a relevant Rout value with the different capacitive load. However, in the actual application, we would never know what kind of cable (50R/75R) and capacitive load will be connected by the end-user, right?

    Kent

  • In the actual application, you will need to specify what type of cable and what value of terminating resistor should be used. This is typical of impedance matching circuits. There will be no reflections and no capacitance if Rout=Rcable=Rload.
  • Hi Liao,

    there are two scenarios:

    1. The input impedance of the device to be driven is high ohmic. In this case figure 8 and figure 9 have to be taken into consideration. These figures are valid for Rload = 1k. In this case the load capacitance plays an enormous role. So, if you want to connect a 1M //10p (1:1) scope probe to the output, you should choose a output resistance of 50R, according to figure 9. To be able to connect all sorts of load capacitances up to 120p to the output, you should choose an output resistance of 75R, according to 9.

    2. If, on the other hand, the input impedance of the device to be driven is low ohmic, then this is usually a 50R input or a 75R input. In this case physics changes and you must use the technique of "characteristic impedance matching" to allow proper operation. Cable capacitance then no longer plays a role, provided you are using a 50R or 75R cable. This means that a 50R input needs a 50R cable and a 50R output resistance. Or, in the case that you have to drive a 75R input, this means that a 75R input needs a 75R cable and a 75R output resistance.

    You cannot drive a 50R input by a 75R output resistance and, vice versa, you cannot drive a 75R input by a 50R output resistance. But you can provide two outputs, one with a 50R output resistance and another with a 75R output resistance. To the 50R output you can connect a 50R cable and a 50R input and to the 75R output you can connect a 75R cable and a 75R input.

    Kai
  •  Hi Kai,

    Thanks for your reply.

    According to your detail explanation, our application is more similar to the scenario 2(driving low input impedance).

    And, here is our test condition & problem:

    ** Test condition**

    (1). Pattern generator: Quantum 804B

    (2). VGA input timing: 1920x1200-60Hz (RB)

    (3). VGA input pattern: Cross test pattern which has one pixel wide white details was used to observe the VGA transient response.

    (4). 1.8m VGA cable characteristic impedance: 75 ohm

    (5). Test jig: R/G/B signal with 75R to GND.

    ** Test result**

    (1). In Fig. 1, if we directly put R,load(75R) terminal resistor on the Main board side, the output amplitude of R/G/B signal measured on Projector’s VGA-out port was fine (Vout=1/2*Vin).

    (2). In Fig.2, if we connected Projector’s VGA-out port to the test jig through a 1.8m VGA cable, the output amplitude of R/G/B signal measured on test jig: the amplitude of High frequency*a was insufficient(Vout<1/2*Vin), but the amplitude of Low frequency*a don’t have this problem.

    Also, if we reduce the VGA input timing to 640x480-60Hz, the output amplitude of R/G/B signal measured on test jig: both High and Low frequency* don’t have the amplitude problem, too.

    Note:

    a. Below shown the measurement waveform of the Cross test pattern, that includes two parts of frequency (High frequency: 2.5MHz; Low frequency: 1KHz).

      

    This issue has been inquired and post in the below link: http://e2e.ti.com/support/amplifiers/high_speed_amplifiers/f/10/p/720761/2658805#2658805

    We have try to follow your suggestion to remove all the ferrite bead, protection diodes and filter cap from R/G/B signal trace on the LMH6739 input and output path, but there’s no help.

     

  • Hi Liao,

    then there must be something wrong with the cable, right?

    To which pins of the VGA connector have you connected the signal?

    Kai
  • Hi Kai,

    Yes, we have tried to use others 1.8M VGA cable, but get the same test result.

    We connected the R,G,B,H,V signal to the test jig through the VGA cable, there signal have the terminal resistor on the test jig. [R,G,B signal:75R to GND and H,V signal: 2.2K to GND].

     

    [8/22: We have try to follow your suggestion to remove all the ferrite bead, protection diodes and filter cap from R/G/B signal trace on the LMH6739 input and output path, but there’s no help.]

    --> Update: To remove the protection diodes and filter cap from LMH6739’s R/G/B output path, it can improve the amplitude of R/G/B signal measured on the test jag (raise 20~30mV), but still not good.