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Page 56 of 97
Issue: 1.04
6.4 Solid-State Switching
One of the most common sources of failure in instruments of this complexity is
the contacts in switches, relays, connectors and potentiometers. For this reason,
the microK was designed to have no switches (apart from the on/off switch),
mechanical relays or potentiometers. The switches normally used for the operator
interface have been replaced with a combination of an industrial grade touch-
screen over a full-colour VGA LCD, making the instrument both reliable and
easy to use The internal connectors are limited to three ribbon cables (with gold
plated contacts) for signal interconnections plus a small number of connectors for
the AC power and internal DC supply.
Conventional instruments of this type use mechanical relays for some or all of
the signal routing. The microK uses only solid-state switching. The thermal
EMFs from the metal-silicon junctions in solid-state switching devices are
potentially higher than for mechanical relays under the same temperature
gradient. However, the very small size of the die within the semiconductor
devices means that there would be little opportunity for thermal gradients, giving
them a strong advantage over their mechanical counterparts. In practice the
thermal EMFs from solid-state switching are significantly less than even the best
mechanical relays. As a result, the microK achieves voltage offsets significantly
less that 250nV.
6.5 Inherent Stability
The current reversals used to eliminate thermal EMFs from resistance
measurements together with true 4-wire resistance measurement have the effect
of ensuring an intrinsically stable zero with time and temperature. The voltage at
the amplifier input when measuring a short-circuit will be the same whichever
current direction is used. The process of averaging the measurements therefore
yields zero (with uncertainty determined by the system noise).
In a similar way, the substitution technique means that the measurement system
is also inherently stable at unity ratio since the voltages measured for a reference
resistor and thermometer of the same value will be identical. There is, after all,
no difference between these two measurements apart from the fact that they are
