Page 345 - Analog and Digital Filter Design

342 Analog and Digital Filter Design

Microprocessor Programmable ICs MAX260/MAX261 /MAX262
Maxim produce a series of three microprocessor programmable universal
active filters. The MAX260 handles center frequencies up to 7.5kHz. The
MAX261 operates with center frequencies up to 57kHz, and the MAX262
can work up to 140 kHz. Each filter IC contains two second-order filter sections
that can be configured to provide lowpass, highpass, bandpass, bandstop, and
all-pass types. The filter response can be all-pole, such as Butterworth and
Chebyshev, or pole and zero responses, that is, Cauer (elliptic) and Inverse
Chebyshev.

The filter IC has four programmable modes controlled by the logic state in
two registers of the IC’s program memory. There is also a fifth mode known
as mode 3A; this is when the IC is in mode 3, but an external op-amp ladder
circuit is connected to the highpass and lowpass pins to provide a notch
output. The notch output is used for bandstop filters. It is also used for Cauer
or Inverse Chebyshev responses of any type (lowpass, highpass, bandpass, or
bandstop).

The ratio of clock frequency to pole frequency,f,, depends on the device and
the filter mode. A set of registers in the device’s memory stores the required
ratio. The MAX260 and MAX261 have the same ratios, varying from 100.53: 1
to 199.49: 1 inmodes 1,3, and4, andvaryingfrom71.09: 1 to 141.06: 1 inmode
2. The MAX262 can handle higher filter pole frequencies, but the ratios are
lower. These vary from 40.84: 1 to 139.80: 1 in modes 1, 3, and 4, and from
28.88 : 1 to 98.85 : 1 in mode 2. These noninteger numbers are derived from the
equations:

ratio = (64+ N).x/2, for the MAX260lMAX261 in modes
1, 3and4.N=Oto63.
ratio = (26+ N).x/2, for the MAX262 in modes 1, 3 and 4. N = 0 to 63.

In mode 2 all the ratios are divided by 4.

Mode 1 can be used to implement all-pole lowpass or bandpass filters. A limited
range of second-order notch filters can also be produced.

Mode 2 allows higher Q factors for the poles and can provide all-pole lowpass,
bandpass, or notch filters.

Mode 3 can be used to produce all-pole highpass, as well as lowpass and band-
pass filters. This is the only mode for producing highpass filters.
Mode 3A uses an external op-amp to provide a Cauer or Inverse Chebyshev
filter response. Outputs from the filter IC are those for mode 3.