handbook of modern sensors

5.9 Noise in Sensors and Circuits 221
that moving the ground connection from sensor’s point a to the power terminal point
c prevents formation of spurious voltage across the ground conductor connected to
the sensor and a feedback resistor R 2 . A rule of thumb is to connect the ground to
the circuit board at only one point. Grounding at two or more spots may form ground
loops; which often is very difficult to diagnose.
5.9.9 Seebeck Noise
This noise is a result of the Seebeck effect (Section 3.9 of Chapter 3) which is manifested
as the generation of an electromotive force (e.m.f.) when two dissimilar metals
are joined together. The Seebeck e.m.f. is small and, for many sensors, may be simply
ignored. However, when absolute accuracy on the order of 10–100 µV is required, that
noise must be taken into account. The connection of two dissimilar metals produces
a temperature sensor. However, when temperature sensing is not a desired function,
a thermally induced e.m.f. is a spurious signal. In electronic circuits, the connection
of dissimilar metals can be found everywhere: connectors, switches, relay contacts,
sockets, wires, and so on. For instance, the copper PC board cladding connected to
Kovar TM6 input pins of an integrated circuit creates an offset voltage of 40 µV·T
where T is the temperature gradient (in ◦ C) between two dissimilar metal contacts.
The common lead–tin solder, when used with the copper cladding creates a thermoelectric
voltage between 1 and 3 µV/ ◦ C. There are special cadmium–tin solders
available to reduce these spurious signals down to 0.3 µV/ ◦ C. Figure 5.56A shows
the Seebeck e.m.f. for two types of solder. The connection of two identical wires
fabricated by different manufacturers may result in the voltage having a slope on the
order of 200 nV/ ◦ C.
In many cases, Seebeck e.m.f. may be eliminated by a proper circuit layout and
thermal balancing. It is a good practice to limit the number of junctions between the
sensor and the front stage of the interface circuit. Avoid connectors, sockets, switches,
and other potential sources of e.m.f. to the extent possible. In some cases, this will not
be possible. In these instances, attempt to balance the number and type of junctions
in the circuit’s front stage so that differential cancellations occur. Doing this may
involve deliberately creating and introducing junctions to offset necessary junctions.
Junctions which the intent to produce cancellations must be maintained at the same
temperature. Figure 5.56B shows a remote sensor connection to an amplifier where
the sensor junctions, input terminal junctions, and amplifier components junctions
are all maintained at different but properly arranged temperatures. Such thermally
balanced junctions must be maintained at close physical proximity and preferably on
common heat sinks. Air drafts and temperature gradients in the circuit boards and
sensor enclosures must be avoided.
6 Trademark of Westinghouse Electric Corp.