LMX2594: Usage of CMOS reference clock

user6136418

Part Number: LMX2594

Hello,

The reference oscillator I want to use for LMX2594 has 0-3,3V output voltage level and 6mA maximum output current. (CMOS, single-ended) Since the reference input voltage range of LMX2594 is 0,2-2V, I had to reduce the oscillator output voltage level. That's why I put 3-dB pi attenuator to OSCinP. When I was tested the circuit, I observed that the oscillator output waveform is distorted and oscillator draws maximum current.

Do you have any suggestions for this problem? Besides, what do we use the shunt 50 ohm resistor for, is it necessary to use with CMOS clock?

Best regards,

Safa

over 5 years ago

0 Noel Fung over 5 years ago

TI__Guru** 106870 points

Hi Safa,

I think you should AC-couple the CMOS XO output to the 3-dB pi, otherwise the pi will draw too much DC current from the XO.

The connection should look like.

XO --| |-- pi attenuator --| |--OSCin

0 user6136418 over 5 years ago in reply to Noel Fung

Prodigy 20 points

Thanks for quick response, but I cannot understand one issue. What exactly is the 50 ohm resistor to OSCinP pin, used for?

If I remove this 50 ohm resistor, will I have any problem?

Best regards,

Safa

0 Noel Fung over 5 years ago in reply to user6136418

TI__Guru** 106870 points

Hi Safa,

OSCin does not require external 50Ω. Say, if you have a XO having 2V output swing, you can simply AC-couple it to the OSCin of LMX2594 without the pi-attenuator.

Clock & timing

Clock & timing forum

LMX2594: Usage of CMOS reference clock