LMK00105: input voltage of LMK00105

Jimmy Lai

Part Number: LMK00105

Dear Team,

According to the LMK00105SQX specification book, the actual connection method of the input terminal, the external crystal oscillator is used as the LMK00105SQX, because the input terminal has two stages of coupling capacitors, so the test waveform has a negative voltage of about - 700mv, as shown in the following picture.
---------- We can't find the specific electrical requirements of the input end of LMK00105SQX (CLKin PIN20 ) in the specification book, high level 1.4V low level -0.7V, whether this meets the requirements of the input port?

Many Thanks,

Jimmy

over 2 years ago

0 Noel Fung over 2 years ago

TI__Guru* 81750 points

Hi Jimmy,

Were you measuring the signal at the output of the XO? This signal is AC-coupled to CLKin, and CLKin has internal bias voltage at around 1.4V, so CLKin would not see a negative input voltage.

0 Jimmy Lai over 2 years ago in reply to Noel Fung

TI__Expert 6845 points

Hi Noel,

We appreciated your great support!

Green is the test point location, the picture is the actual waveform of the location.

7X40000017_A1 (1).pdf

Please help to confirm whether the current test data is ok?

Many Thanks,

Jimmy

0 Noel Fung over 2 years ago in reply to Jimmy Lai

TI__Guru* 81750 points

Hi Jimmy,

What was the scope impedance (50Ω ?) and was it setup to DC-couple?

0 Jimmy Lai over 2 years ago in reply to Noel Fung

TI__Expert 6845 points

Hi Noel,

Yes, impedance is 50Ω and we set DC-couple.

Thanks a lot!

Many Thanks,

Jimmy

0 Noel Fung over 2 years ago in reply to Jimmy Lai

TI__Guru* 81750 points

Hi Jimmy,

According to the datasheet, the pins have internal 1.4V bias, but your plot did not agree with this. Could you measure the voltage at pin 20 to confirm this? If it is 1.4V, then could you remove R212 and measure the voltage at pin 21?

0 Jimmy Lai over 2 years ago in reply to Noel Fung

TI__Expert 6845 points

Hi Noel,

Thanks for your great support always!

Please refer below,

When R212 is on, test pin20 about 1.6V
After R212 is removed, test pin20 1.4V, pin21 1.2V

Many Thanks,

Jimmy

0 Julian Hagedorn over 2 years ago in reply to Jimmy Lai

TI__Mastermind 20900 points

Hi Jimmy,

the spec that you need to take care of is the one below:

The device has internal AC-coupling. The biasing is actually after the AC-coupling cap internally. That allows for the connection that you choose. You will need to lower the peak to peak swing a bit to stay below 2 Vpp. This can be done by inserting a resistance at R212.

regards,

Julian

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LMK00105: input voltage of LMK00105