From Newsgroup: sci.electronics.design
On 2026-03-01 14:15, Stuart wrote:
How did you get on? I'm doing something similar.
Wound up using a PNP high-side driver instead. It made life easier, and
it turned out that there was an easier method to achieve wide, stable bandwidth in my gizmo--the cube rooter in my recent "Cute power law
circuit" post. (See also the "Thermal Faraday Shield" thread for more detail.)
When implemented in Class H, the output of the thermal Faraday shield
gizmo wants to go like the cube of the Class H voltage: heating goes as
V**2, and the RTD bridge output goes like V, making V**3 overall.
Assuming a 1-pole rolloff, the loop bandwidth wants to go like V**2, so
for a 10:1 range of heater voltage, the bandwidth would vary 100:1.
In a MCU-supervised loop, one can use an MDAC to change the loop gain
over a wide range like that, but it's harder in analog.
The cube rooter makes the bridge output voltage linearly proportional to
the control voltage instead, and so keeps the bandwidth pretty well
constant. Since it's inside the loop, its offset voltage doesn't
matter, and the slope doesn't have to be that accurate either--I
wouldn't care about a +-20% gain variation over the full heating range.
Using three dual transistor packages, with a total cost of 30 cents or
so in modest quantity, makes a cube rooter that's easily good enough for
the job. The guaranteed datasheet specs, plus a very little thermal
design, are enough to keep the gain variation at the 10% level. The
error from the cube root curve is better than that.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
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