Hi !

At the datasheet , EXT_LF's resister is 170ohms.

Is there any specification about 170Ohm?

1)

Can you tell me which type of 170Ohms should we use?

170 ± 10%(153~187Ohms) is OK?

2)

Is 10% is OK , can we use 180Ohms ± 1%(178.2~181.8Ohms)?

3)

I read the discussion.

=> https://ez.analog.com/message/156258#156258

But I couldn't understand what is "PLL stability"?

Would you explain it more concretely?

Best regards.

Kawa

1) I don't know exactly what tolerance the designers used for their simulations. But what you need to keep in mind is the capacitor tolerances effects on the filter will be greater then just the resistor tolerance. My guess he may have user 170 Ohm 5% for simulations.

2) All the 1% and 5% combinations would be OK except for the 180 Ohm. I would stay away from the 10% combinations

3) If they are using 5 or 10% 170 Ohm resistor combinations for production and are not having any problems then all is good. The capacitor tolerances are going to swamp out resistor tolerance effects. Capacitor tolerances are already 5-10% at best.

In my opinion the designer simulating the circuit simply forgot about what is available in the E96 and E24 resistor sets. He obviously would not want the customer to use two E24 resistors in series to achieve the 170 value. And over all the capacitor tolerances will swamp out the resistor tolerance effects. The fact that he used 170 for simulation just rippled through

In conclusion:if the circuit works currently don't change any thing. In the future for re-spins or new designs I would use a 169 1% resistor. In reality all resistors on the board can be 1% resistors, the cost differential between 5% and 1% are minimal. If you look at the reference schematic bom, the resistor in question is labeled 170 Ohms but they actually installed a 169 Ohm 1% resistor.Another issue to worry about is the temperature coefficients of the capacitors. Depending on the system operating temperature range and the coefficient the cap value and move out of working range. X7R caps have the worst temperature coefficient. For wide temperature ranging system I would use NPO caps (yes, they are large but are much more stable). Again if they have not seen any problems, don't worry about it. Just keep this fact for future reference.

Hope this helps