Datasheet logical error ?

[kellogs]

Hello,   It seems to me that all of these chips are calling for the same type of crystal; but which load capacitance is needed, 6 pF or 12 pF ?   

 Page 35 

"Crystal (XTAL) Oscillator The XTAL oscillator in the MAX41473/MAX41474 is designed to present a capacitance of approximately 12pF from the XTAL1 and XTAL2 pins to ground. In most cases, this corresponds to a 6pF load capacitance (CL) applied to the external crystal when typical PCB parasitics are included. It is very important to use a crystal with a CL equal to the capacitance of the MAX41473/MAX41474 crystal oscillator plus PCB parasitics. If a crystal designed to oscillate with a different CL is used, the crystal is pulled away from its specified oscillation frequency, introducing an error in the reference. The crystal’s natural frequency is typically below its specified frequency. However, when loaded with the CL, the crystal is pulled and oscillates at its specified frequency. This pulling is already accounted for in the specification of the CL. Accounting for typical board parasitics, a 16MHz crystal with 6pF specified CL is recommended."

   

Page 15 

"Crystal (XTAL) Oscillator The XTAL oscillator in the MAX41460 is designed to present a capacitance of approximately 12pF from the XTAL1 and XTAL2 pins to ground. In most cases, this corresponds to a 6pF load capacitance applied to the external crystal when typical PCB parasitics are included. It is very important to use a crystal with a load capacitance equal to the capacitance of the MAX41460 crystal oscillator plus PCB parasitics. If a crystal designed to oscillate with a different load capacitance is used, the crystal is pulled away from its stated operating frequency introducing an error in the reference frequency. The crystal’s natural frequency is typically below its specified frequency. However, when loaded with the specified load capacitance, the crystal is pulled and oscillates at its specified frequency. This pulling is already accounted for in the specification of the load capacitance. Accounting for typical board parasitics, a 16MHz, 12pF crystal is recommended."

After a couple of not so useful emails exchanged with support at Analog Devices, I got pointed to the eval boards documentations, which are using a 9pF crystal for both radio chips... - https://www.digikey.com/en/products/detail/epson/TSX-3225-16-0000MF18X-AC0/5260855

Eval board for MAX4147x
Eval board for MAX4146x

[u666sa]

You need to measure capacitance on your prototype board. Connect to legs of your cap (cap not installed) which goes to XTAL1 and XTAL2 pins, and measure capacitance using NanoVNA H4 or LiteVNA 64. The value you get is the value of your capacitor.

..you can temporarily solder ipex connector in place of the cap, for ease of measurements.

[PGPG]

I didn't tried to look what these ICs are.
In both descriptions they say that from each pin they have 12pF internal capacitance to GND. So if you were able to design PCB having absolutely 0pF than you should use crystal with CL=6pF.
But as you can't design such PCB you will add some extra pF so using 9pF crystal seems reasonable.
If generator in IC in enough 'strong' the better frequency stability I would expect when using 16pF crystal and adding extra capacitors out of IC. Assuming that you have better control on capacitors capacity than at PCB capacity the higher capacity comes from capacitors the resulted capacity fluctuations are smaller.

In my life I meet 6pF crystals only for 32768Hz. The task is to have in RTC as small supply current (in a range of hundreds nA) as possible and the smaller C you can drive it with lighter generator consuming less Icc.
For higher frequencies higher CL are more common, I think.

In my opinion in both descriptions there are error when they write "is typically below its specified frequency". It should say 'over', I think.

[xvr]

but which load capacitance is needed, 6 pF or 12 pF

From crystal point of view capacitors on its pins connected in series, so it seen half of it (6pF of load capacitance for 12pF capacitors)

[kellogs]

Does that mean that the first quote is calling for the incorrect crystal, purely in the context of that paragraph ?

[xvr]

Does that mean that the first quote is calling for the incorrect crystal, purely in the context of that paragraph ?

No. It says, that chip itself provides 12pF capacitance from each of crystal pin to ground (including PCB parasitics). So total load capacitance (for crystal) will be 12/2 = 6pF

It is possible to use crystal with larger load capacitance, but in this case you should provide additional capacitors at PCB.

[kellogs]

Then the second datasheet is wrong. Otherwise I am totally at loss.

[xvr]

Eval board includes 2 additional capacitors (5pF each), attached to crystal. So total load capacitance is 6+5/2=8.5pF  This is quite aligned with 9pF required to Seiko-Epson crystal.

[kellogs]

I shall try an simplify things as they currently stand in my mind:

First  datasheet:

"12 pF at XTAL pins to GND, we account for typical PCB parasitics => we choose a 6 pF crystal"

Second  datasheet:

"12 pF at XTAL pins to GND, we account for  typical PCB parasitics => we choose a 12 pF crystal"

Can these be simultaneously  true? What am I getting wrong ?

[xvr]

What datasheets are you talking about? I think it's 2 quotes in first post, am I wrong? In both of them load capacitance is 6pF (described in the same words, exactly)

[kellogs]

Hope I am not getting trolled.

first quote from first linked datasheet
"Accounting for typical board parasitics, a 16MHz crystal with 6pF specified CL is recommended."

second quote from second linked datasheet
"Accounting for typical board parasitics, a 16MHz, 12pF crystal is recommended."

[xvr]

First quote:

Crystal (XTAL) Oscillator The XTAL oscillator in the MAX41473/MAX41474 is designed to present a capacitance of approximately 12pF from the XTAL1 and XTAL2 pins to ground. In most cases, this corresponds to a 6pF load capacitance (CL) applied to the external crystal when typical PCB parasitics are included.

Second:

Crystal (XTAL) Oscillator The XTAL oscillator in the MAX41460 is designed to present a capacitance of approximately 12pF from the XTAL1 and XTAL2 pins to ground. In most cases, this corresponds to a 6pF load capacitance applied to the external crystal when typical PCB parasitics are included.

Aaa, I see. Last sentence. I think it just typo.