SLLA549 July   2021 TCAN4550 , TCAN4550-Q1 , TCAN4551-Q1

 

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6 Calculate the Load Resistance

Figure 6-1 Quarts Crystal Electrical Model

Identify the crystal's shunt capacitance C0 and motional resistance Rm from the crystal’s data sheet or ask the crystal vendor if this is not specified.

Some crystal data sheets specify the Rm as the crystal’s maximum Equivalent Series Resistance or ESR. Other crystal data sheets use the term ESR to specify the total load resistance under the specified Capacitive Load which is greater than the motional resistance of the crystal itself as defined in the next step.

Calculate the combined load of the series resistance in the crystal and the reactance of the capacitance placed on the oscillator circuit. This will be to total Load Resistance RLoad which is different than the crystal’s motional resistance Rm. Both are sometimes referred to as the ESR and therefore there can be great confusion about what the term ESR is referring to. In calculations and for clarity the term ESR will not be used in this document. The crystal’s motional resistance will be designated by Rm and the total Load resistance will be designated by RLoad.

Equation 11. R L o a d = R m ×   1 + C 0 C L o a d 2 → R m =   R L o a d 1 +   C 0 C L o a d 2

If the Motional Resistance Rm of the crystal is unknown, this can be approximated by using the maximum ESR listed in the data sheet (for example, 50 ohms), but this may not be accurate and is only a rough estimation that will cause the results to be larger than the actual or typical value (for example, 20 ohms).