Part Number: ONET4291VA
How does ppm of temperature coefficient of ZTC resistor impact VCSEL driver's output current granularity? Is there any quantifiable data around? The reference design suggests an external ZTC resistor.
+/- 0.2ppm/C as well as +/- 2ppm/C 30Kohm resistor from Vishay are incredibly expensive (~$6 for a single resistor in high volume). +/- 5ppm/C resistor pricing is not bad for a single resistor though still very expensive.
I'm trying to see what is the trade-off if I choose 5ppm/C resistor. My specific application using this VCSEL driver runs at, relatively, very low speed (in couple of 100's of MHz). So I'm willing to sacrifice some inaccuracy to lower cost.
The ZTC resistor is used to generate a precise zero TC current which is used as a reference current for the on-chip DACs. Changing this resistor will impact not just the current step size, but maximum bias and mod current ranges as well. For example, increasing the ZTC would result in lowering the reference current which in turn increases the resolution, but at the same time it will reduce the maximum bias and mod currents. Also, biasing the driver transistors (especially for the modulation current) at lower current may impact the high frequency performance (jitter, rise/fall times, bandwidth).
It is difficult to quantify the impact of the ppm of this ZTC resistor on the performance, it really depends on the operating temperature range of your application, and on how much variance you're able to tolerate on the above mentioned parameters. As long as the ZTC resistor value is within 1% of the nominal recommended value of 30k ohm for the temperature range, you should be good.
Hope that helps.
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