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ANALOG SEEKrets
Transformers used for metrology applications are not required to supply significant
amounts of power. Nevertheless the core losses must be minimised in order to make the
input impedance acceptably high. For this reason, and to improve the magnetic coupling
between windings, these precision transformers often use toroidal cores wound of ultrahigh permeability material (supermalloy) in the form of a tightly wound tape,
geometrically much like a reel of sticky tape.
Since the number of turns in an inductor is an unchanging quantity, inductive voltage
dividers can be more stable than resistive dividers, providing the parasitic errors are
minimised. “8-dial” inductive voltage dividers (IVD) are commercially available that
have 0.01 ppm resolution and better than 0.1 ppm accuracy within the frequency range
of 100 Hz to 1 kHz.†
***********************************************
ANS 7.7.1:
The best risetime would be achieved if the resistor were to be removed until the capacitor had
charged to its full extent. If the input current step is I and the resistor is R then the aiming voltage for
the pulse is I⋅R. The voltage on the capacitor must slew from 0 to I⋅R, the risetime being 80% of this
time when using the standard 10%- 90% risetime rule. The full slew time is found from I = C ⋅ dV
dt
∆V
I⋅R
giving ∆t = C ⋅
=C⋅
= CR
I
I
The risetime using this switched resistor scheme is therefore 0.8×CR
The risetime of the unmodified RC network is 2.197×CR
The best risetime improvement possible is therefore 2.75×
The inductor solution does not look so wonderful now that you see what is possible. There are
networks that get much closer to this optimum, but they involve centre tapped inductors, T-coils.
ANS 7.9.1: see text
ANS 7.9.2: see text
ANS 7.9.3: see text
ANS 7.9.4:
This is stated as a ‘far field’ problem, albeit in a round about fashion. The electric and magnetic fields
are related by the wave impedance, this being the characteristic impedance of free space in the far
field.
H =
†
E
Z0
=
7.3 mV/m
= 19.4 µA/m
377Ω
Eg. Tinsley 5560K 8 dial inductive voltage divider.