**Category:**Datasheet/Specs

**Product Number:**AD538

I'm referring to the AD538 data sheet [1] (page 12) and the article in Analog Dialogue [2] (page 4 and 6).

In [2] (page 4) is explained that the temperature coefficient is +3330 ppm/K. It is so because the output voltage of the log section is proportional to the absolute temperature, and if we assume T=300K then the temperature coefficient is 1/(300K), which is the same as 3333 ppm/K.

In [1] (page 12) and [2] (page 6) is written that this temperature coefficient is approximated by a 90.9 Ohm resistor (which is assumed to have no temperature coefficient) in series with a 1000 Ohm PTC with +3500 ppm/K.

I think this is wrong. The formula for the temperature coefficient of two resistors in series is:

alpha = alpha1 * R1 / (R1 + R2) or R2 = R1 * (alpha1 / alpha - 1)

With R1 = 1000 Ohm, alpha1 = 3500 and alpha = 3330 the series resistor should be R2 = 51 Ohm. Not 90.9 Ohm as suggested.

Please explain why the series resistor is 90.9 Ohm, or what's wrong with my formula.

Thanks,

Michael

[1] www.analog.com/.../AD538.pdf

[2] www.analog.com/.../volume19-number1.pdf

added where 3330 ppm/K comes from

[

**edited by:**mkoch at 6:09 AM (GMT -4) on 8 Aug 2023]