LMH6644: The high level holding time becomes shorter

Rock Su

Part Number: LMH6644

Dear colleague,

A customer use LMH6644 to shift Serial TX and RX signal. The schematic is below，RO&RI is RX signal，CO&CI is TX signal. Vsupply+ - Vsupply- = 5.5V.

RO&RI test result:

RO is yellow curve and LMH6644 D channel input and RI is green curve and LMH6644 D channel input.

And input RO high level holding time is 507ns, but output RI rise time delays 29ns, decrease time delays 9ns.

So high level holding time becomes shorter. Customer want keep the length of high level time consistent.

But CO&CI test result didn't has this problem as below:

CO is yellow curve and LMH6644 C channel input and CI is green curve and LMH6644 C channel input. The red label is inverted in the figure.

1. Why does this happen? The difference between the two test circuits is connection of LOW_LEVEL and HIGH_LEVEL.

2. How to solve this problem? Customer want keep the length of high level time consistent.

Best Regards,

Rock Su

over 5 years ago

0 Rock Su over 5 years ago

TI__Intellectual 2220 points

Is it caused by CMVR in the vicinity of 0V?

0 kai klaas69 over 5 years ago in reply to Rock Su

Guru 140200 points

Hi Rock,

what are the values of LOW_LEVEL and HIGH_LEVEL? How are these voltages generated?

Kai

0 Rock Su over 5 years ago in reply to kai klaas69

TI__Intellectual 2220 points

1. In the first stage LOW_LEVEL is GND, HIGH_LEVEL is 1.6V. Please ignore GND in test result figure, because this circuit has many stage, every stage reference ground is different. As below figure (only drew 2 stages)

2. Customer didn't say how LOW_LEVEL and HIGH_LEVEL are generated, do you have any advice?

0 kai klaas69 over 5 years ago in reply to Rock Su

Guru 140200 points

Hi Rock,

hmm, somehow this looks like the typical echoing caused by mismatch of characteristic impedance of a cable. Is there a cable connected to the output of LMH6644? Can the extra pulses be minimized by inserting a small resistance at the output? Or is there a capacitive load at the output, which should be isolated by the help of a small resistance?

Kai

0 Sean Cashin over 5 years ago in reply to kai klaas69

TI__Genius 14156 points

Hi Rock,

This waveform resembles Figure 57 of the datasheet for Overload Recovery. What are your supply rails?

Best regards,

Sean

0 Jacob Freet over 5 years ago

TI__Expert 7875 points

Hi Rock,

Can you also find out what the actual supply voltage values are for the LMH6644? It may be an input common mode range issue depending on the supply voltages used.

Regards,

0 Rock Su over 5 years ago in reply to Jacob Freet

TI__Intellectual 2220 points

Hi Jacob,

I have solved this problem, customer should have violated output swing low. They changed R998 and R996 to 8K, this could make output away from Vsupply-, then this phenomenon disappeared.

Best Regards,

Rock Su

China NWCMM FAE

0 Sean Cashin over 5 years ago in reply to Rock Su

TI__Genius 14156 points

Could you please click the button to indicate whether this resolved your issue?

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LMH6644: The high level holding time becomes shorter