Environmental Engineering Reference
In-Depth Information
To overcome these difficulties, switched-capacitor techniques that syn-
thesize diffusion resistors using capacitors that are clocked periodically
emerged in the early 1970s. The essence of this technique can be demon-
strated conceptually using the circuit of Fig.1.1 where the capacitor
C
is connected to two constant voltages and The connection is con-
trolled by two externally clocked switches. It is trivial to show that the
average current flowing through the capacitor in a clock period is given
by
where
C
is the capacitance of the capacitor, is the clock period,
and is the equivalent resistance of the switched
capacitor. It is seen that the equivalent resistance can be controlled by
(i) the value of the capacitor and (ii) the clock frequency. For example,
with
C =
1 pF and we have
Since monolithic IC techniques can ensure that the ratio of floating
capacitors be controlled very accurately, usually in the range of ±0.01%,
switched capacitor networks are always designed in such a way that the
response of the networks is only a function of (i) the clock frequency
and (ii) the ratio of the capacitors, rather than the absolute value of the
capacitors, such that the behavior of filters can be controlled accurately.
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