Page 344 - Analog and Digital Filter Design

Filter Integrated Circuits 3

Now? R = T7/1 or, substituting for I,

Since N =f, that is, the clock frequency, R = l/fC as stated earlier.

The equations above assume that the capacitor is fully charged (or discharged)
during the period that the switches are closed. The switching period is short so,
as you might expect, the capacitance values are small. Capacitors cannot be
large because they are formed on an integrated circuit substrate where the
surface area must be minimized to reduce costs. In practice the capacitance
values used are typically between 1 pF and ZOpF, although more likely to be at
the lower end of the value range.

The active filter circuits described earlier in this book in Chapters 4-7 can be
realized using switched capacitors to replace resistors. Thus the circuit would
comprise switched and unswitched capacitors around an op-amp, with no resis-
tors required to determine the filter cutoff frequency or response. Using this
technique. small integrated circuits have been developed with high performance;
it is possible to produce a complete eighth-order lowpass filter in an %pin IC
package.

Switched Capacitor Filter IC LT1066- 1
The Linear Technology IC LT1066-1 is a switched capacitor filter housed in an
18-pin IC package. It provides an eighth-order Cauer (elliptic) response with a
nominal 0.15dB passband ripple. The passband cutoff frequency is defined as
where the gain falls to -1 dB; the maximum cutoff frequency is supply depend-
ent and is between 36kHz and 120kHz.

The LT1066-1 has three modes of operation determined by the switching ratio
pin voltage. If the switching ratio pin is connected to the positive supply rail,
the ratio of clock frequency to cutoff frequency is 50 : 1. At this ratio, the input
signal is sampled twice in each clock period to reduce aliasing problems. If the
switchir_g ratio pin is connected to the negative supply rail the clock to cutof€
frequency ratio is 100 : 1. Finally, if the switching ratio pin is connected to analog
ground, the clock to cutoff frequency ratio is again 100: 1, but the phase
response is now linear over the lower half of the passband. In this last mode
the response is almost Inverse Chebyshev, although Linear Technology calls it
”pseudo linear phase response.”