LT1256/1256 VFS Reference

This question is mostly aimed at @creatorlars but happy to hear the opinions of others.

Why did you make the switch from 1.0V to 2.5V on the VFS pin of LT1251/1256 circuits?
Is the non-linear response near 0V of the LT1251 the reason?
Or was it just due to the ease of buffering a TL431 reference voltage?

This old circuit example below used to reference the VFS pin with 1.0V but now all of the Cadets reference it with 2.5V. It seems like adjusting the gain of any control voltages up to 2.5V is more complex than it needs to be.



from facebook:
A few reasons, none of them hard and fast rules. LT1256/LT1251 bandwidth drops the lower the CV reference voltage. So running at 2.5V maximizes performance. That and 2.5V references are more standard than 1.00V references (once you start dividing the reference voltage you lose the rated tolerance slightly.) There’s not a huge difference between 2.5V and 1.0V for the lt1256, but once you get down to 0.5V it all gets very mushy and bad.

here’s a proper +/-1.0 VREF circuit. This has been a great balanced cost vs performance design. The LM4040 and OPA2197 are perfect high precision parts that meet a reasonable price point compared to other high precision options. The OPA2197 is bounds more stable and clean than a TL072 based distribution scheme when your circuit starts getting bigger, and can drive a capacitive load all over the place without ringing.