Effect of trim pot's tempco on a voltage reference

[mwilson]

I'm building a simple little "precison" voltage reference based on a MAX6350 voltage reference (0.02% accuracy, 1ppm/C tempco).

Reading the data sheet, I came across the following statement (p. 8, http://datasheets.maximintegrated.com/en/ds/MAX6325-MAX6350.pdf, emphasis added):

The output voltage can be trimmed a minimum of 0.6% by connecting a 10k? potentiometer between OUT and GND, and connecting its tap to the TRIM pin, as shown in Figure 1. The external trimming does not affect temperature stability.

That seemed odd to me: surely if I use a 500ppm/C trim pot, the temperature stability of the output voltage will be decreased because a change in temperature will change the trim value. So I'm interpretting it to mean that the "internal" stability of the chip doesn't change if you're using the optional TRIM pin feature, but they're glossing over the fact that the effective output now has higher sensitivity to temperature changes.

Or is it simply a matter of the effect of the trim being so small that even as the trim pot's value changes over the temperature range, the trim pot is affecting such a small fraction of overall voltage output that even with a poor tempco pot, it's not going to push the output outside of the chip's 1ppm/C spec?

Or am I missing some other "magic" that will keep the end reference voltage output within the 1ppm/C spec of the chip even when using a trim pot with lower temperature stability?

[Neilm]

It could be that they are doing something cunning like using current into to the pin rather than by measuring voltage. This could lead to the variation in overall resistance due to temperature being insignificant. While I have used that device, I have never wanted to trim it.

Neil

[Rufus]

Or am I missing some other "magic" that will keep the end reference voltage output within the 1ppm/C spec of the chip even when using a trim pot with lower temperature stability?

To me it is just saying the temperature coefficient of the reference is not altered by an applied trim. The temperature coefficient of the trim will have an effect on the applied trim.

[lewis]

The trimmer's tempco affects the resistance of the entire potentiometer track. Let's assume you've got a really shitty trimmer that has +10% tempco at some delta-T and you're using it in the potential divider configuration. The 'top' resistor in the potential divider (vith a value at some percentage of the trimmer's total resistance) will have a tempco of +10% and the 'bottom' resistor (the other part of the trimmer's track resistance) will also have a tempco of +10%. Each 'resistor' therefore changes by +10% in the same direction for a given change in temperature, and, if used in the potential divider configuration, the changes cancel out giving you the same voltage at the trimmer's wiper. In other words, the top resistor increases proportionally to the bottom one, so the net wiper voltage remains constant.

However, the input bias current of the reference pin will cause the potential divider to be loaded further with increases in resistance, and so dropping (or increasing) the voltage. So you need to use a trimmer value orders of magnitude lower than that needed to bias the chip to minimise this effect.

Also, we're assuming the temperature coefficient is linear and evenly distributed across the potentiometer's track (which if it's a carbon type it almost certainly won't be). That might be enough to be a concern for ultra-precision applications. Cermet and wirewound trimmers might be better.

[mwilson]

Thanks for the input, folks.

Lewis, thanks for the insight of thinking about it like a normal two-resistor divider... yeah, that makes total sense that the temperature isn't necessarily changing the ratio nearly as much as the absolute resistance.