Page 1197 - The Mechatronics Handbook

This form of divider is useful from low frequencies up through frequencies of several megahertz. A
common application is in the scaling of large voltages.
Inductive Dividers
If the elements in the chain divider are inductors, then an autotransformer results. Inductive dividers
are useful over frequencies from a few hertz to several hundred kilohertz. Errors in the parts-per-
billion range are achievable.

Voltage Transformers
Voltage transformers constitute one of the most common means of accomplishing voltage scaling at line
frequencies. Standard double-wound configurations are useful unless voltages above about 200 kV are
to be monitored. For very high voltages, alternative configurations such as the capacitor voltage transformer
and the cascade voltage transformer are employed (Gregory, 1973).

Current Scaling
Current scaling is typically accomplished via either a current shunt or a current transformer.
A current shunt is essentially an accurately known resistance through which the current to be measured
is passed. The voltage developed across the shunt as a result of the current is the quantity measured.
Shunts are useful at DC and frequencies through the audio range. Two disadvantages are (1) the shunt
consumes power, and (2) the measurement circuitry must be operated at the same potential as the shunt.
The current transformer overcomes the mentioned disadvantages of the current shunt. Typically, the
current transformer consists of a specially constructed toroidal core upon which the secondary (sense)
winding is wrapped and through which the primary winding is passed. A single-turn primary is com-
monly used, although mutiturn primaries are available.

Other Attenuators
In addition to the aforementioned means of voltage and current scaling are attenuator pads, which
provide, in addition to voltage or power reduction, the ability to be matched in impedance to the source
and load circuits between which it is connected. The common pads include the T, L, and Π types, either
balanced or unbalanced. Resistive attenuator pads are discussed in most textbooks on circuit design (e.g.,
Cuthbert, 1983). They are useful from DC through several hundred megahertz.

Impedance Transformation
Oftentimes the impedance of the transducer must be transformed to a value more acceptable to the
remainder of the measurement system. In many cases maximum power must be transferred from the
transducer’s output signal to the remaining circuitry. In other cases it is sufficient to provide buffering
that presents a very high impedance to the transducer, a very low impedance to the rest of the system,
and a voltage gain of unity.
Matching transformers, passive matching networks such as attenuator pads, and unity-gain buffers are
standard means of accomplishing impedance transformation. Unity-gain buffers are available in IC form.

Linear Filtering
Although, in general, digital signal processing offers many advantages over analog techniques for filtering
signals, there are many relatively simple applications for which frequency-selective analog filtering is well
suited.
Filters are used within signal conditioners (1) to reduce the effects of noise that corrupts the input
signal, (2) as part of a demodulator, (3) to limit signal bandwidth, or (4) if the signal is to be sampled,
to limit its bandwidth in order to prevent aliasing. These filters can be built either entirely of passive
components or based on active devices such as op amps.

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