2Introduction
KVD-700
1.2 Circuit Description
1.2.1 Model
AKelvin-Varley voltage divider may be thought of as
being equivalent to a digital potentiometer, except that
it has an additional, but variable resistance in series
with the wiper arm. Such a circuit model of the KVD-
700 may be seen in Figure 1.2. In the case of the
KVD-700, the resistance between the input termi-
nals 1 and LOW is 100 kΩ.
An actual digital potentiometer uses decades of re-
sistor steps each decreasing by factor of ten. The
problem with such a digital potentiometer, however,
is that its resolution becomes limited by the value of
ever smaller resistors. They become difficult to imple-
ment as the contact resistance of switches and con-
nections become significant. AKelvin-Varley circuit
overcomes this problem with its special design, de-
scribed later.
Another way to model the KVD-700 is with the
Thevenin equivalent circuit shown in Figure 1.3, where
S is the dial setting. Note that if the output is being
fed into a very high impedance, then the output im-
pedance RO, may be ignored. In general, however,
the effect of load impedance, RLmust be taken into
consideration, as will be discussed below. The ap-
proximate value of ROis shown in Figure 1.4. It may
be seen that the output impedance is maximum at
about the dial setting of 0.5 and drops to zero at both
ends, 1.0 and 0. It is the value of ROwhich will influ-
ence the effect of loading.
Note, however, that in general bridge applications,
nominally zero current flows out of the divider as the
bridge comes into balance, and therefore the divider
effectively “sees” an infinite impedance, and the ef-
fect due to ROmay be safely ignored.
1.2.2 Theory of Operation
The actual circuit diagram of the KVD-700 is shown
in Figure 1.5. This circuit is capable of dividing the
input into 107parts, i.e. 0.1 ppm. It consists of seven
decades, each of which divides its input voltage into
10 equal parts.
The implementation of this division may be seen as
follows: The input voltage across each decade is di-
vided by 10 equal resistances. Placing the resistance
of the succeeding decade in parallel with a portion of
the upstream decade reduces the effective resistance
of that portion. In particular, examine the figure and
note that first decade has 11 - not 10 - resistors (ig-
noring the 1.1 input resistor). The divider wipers from
the second decade encompass two resistors totaling
20 kΩ. This 20 kΩis shunted by 20 kΩ, the effective
total resistance of the second decade with all the
shunting in parallel with it, resulting in a total effec-
tive resistance of 10 kΩfor that step.
The 11 steps become equivalent to 10 steps of 10 kΩ
each, and in this way all the steps are kept equal.
1.1
1.0
LOW
HIGH
LOW
Figure 1.2. Digital Potentiometer Model for a Kelvin-
VarleyVoltage Divider
INPUT OUTPUT
SVIN V
OUT RL
RO
Figure 1.3.Thevenin Equivalent Circuit of a Kelvin-
VarleyVoltage Divider
R0
