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[LMH1981] Settling time, Jitter and Lock

Benjamin Mak
Intellectual 2190 points
Other Parts Discussed in Thread: LMH1981

Hi Team!

A customer had a question regarding the LMH1981.

Some of their questions came from page 10 under Input Settling Time and Coupling Capacitor Selection.

The value of CIN is a tradeoff between start-up time and jitter performance and therefore should be evaluated based on the application requirements. Figure 11 shows a graph of typical input-referred HSync jitter vs. CIN values to use as a guideline. Refer to the Horizontal Sync Output section for more about jitter performance.

Also they had concerns with the following statement from page 14 of the datasheet under AC-Coupled Video Sources, especially the following statement:

 The LMH1981’s input clamp circuitry may have difficulty stabilizing the input signal under this type of shifting; consequently, the unstable signal at VIN may cause missing sync output pulses to result, unless a proper value for CIN is chosen.

 

Their questions are the following:

What if we don’t care about settling time?

Can I use a 4 uF input cap (with the output being 220uf) and achieve good jitter and lock (even it takes a second)?

 

Thanks,

Ben

  • 0
    TI__Genius 14317 points

    If yu are not worried about the settling time and consequently the "Sync Lock Time" associated with switching or applying the input video, then choosing a larger Cin value will decrease your jitter. If you are implementing the circuit of Fig. 18 in the Data Sheet with Cout of the driver to this circuit equal to 220 uF, Rs = 0 ohms and no Cf, then Trc-out = 150 ohms x 220 uF (if Rout series term = 75 ohms) = 33 ms. From the equation for trc-in on pg. 14, trc-in = Ri x Cin x Tline/Tclamp = 4k x 4 uF x 64 us / 250 ns = 4.096 s So no, 4 uF for Cin is too large. With Cin of 4 uF and Cout of 220 uF, changing APL of the video will cause problems for the sync tip clamp circuit and the sync "slicer" will pick off the sync at varying times which will modulate the H-sync times from line to line causing jitter. From the equation, Cin must be < .032 uF. I would encourage the customer to ask Rinzai (Avnet FAE) for an EVM board in order to test performance. See http://www.ti.com/tool/lmh1981mteval