MF5CWM National Semiconductor, MF5CWM Datasheet - Page 15

• Manufacturer Part Number: MF5CWM
• Description: IC FILTER MONO SW CAP SO14
• Manufacturer: National Semiconductor

Specifications of MF5CWM

• Filter Type: Universal Switched Capacitor
• Frequency - Cutoff Or Center: 30kHz
• Number Of Filters: 1
• Max-order: 2nd
• Voltage - Supply: 8 V ~ 14 V
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *MF5CWM

3 0 Applications Information
For most applications the outputs are AC coupled and DC
offsets are not bothersome unless large signals are applied
to the filter input However larger offset voltages will cause
clipping to occur at lower ac signal levels and clipping at
any of the outputs will cause gain nonlinearities and will
change f
become excessively large if R
f
ly if Q is also high An extreme example is a bandpass filter
having unity gain a Q of 20 and f
tied to V
6 25 and the offset voltage at the lowpass output will be
about
adjusted by using the circuit of Figure 20 This allows adjust-
ment of V
outputs as described in the above equations Some outputs
cannot be adjusted this way in some modes however
(V
3 5 SAMPLED DATA SYSTEM CONSIDERATIONS
The MF5 is a sampled data filter and as such differs in
many ways from conventional continuous-time filters An im-
portant characteristic of sampled-data systems is their ef-
fect on signals at frequencies greater than one-half the
sampling frequency (The MF5’s sampling frequency is the
same as its clock frequency) If a signal with a frequency
greater than one-half the sampling frequency is applied to
the input of a sampled data system it will be ‘‘reflected’’ to
a frequency less than one-half the sampling frequency
Thus an input signal whose frequency is f
cause the system to respond as though the input frequency
was f
CLK
os(BP)
f
s
o
a
2 - 100 Hz This phenomenon is known as ‘‘alias-
significantly higher than the nominal value especial-
b
in modes 1a and 3 for example)
1 9V Where necessary the offset voltage can be
o
os1
(100 1 nominal) R
and Q When operating in Mode 3 offsets can
which will have varying effects on the different
4
2
R
and R
2
CLK
will therefore be equal to
FIGURE 21 The Sampled-Data Output Waveform
4
f
o
are used to make
s
e
2
250 with pin 9
a
(Continued)
100 Hz will
15
ing’’ and can be reduced or eliminated by limiting the input
signal spectrum to less than f
require the use of a bandwidth-limiting filter ahead of the
MF5 to limit the input spectrum However since the clock
frequency is much higher than the center frequency this will
often not be necessary
Another characteristic of sampled-data circuits is that the
output signal changes amplitude once every sampling peri-
od resulting in ‘‘steps’’ in the output voltage which occur at
the clock rate ( Figure 21 ) If necessary these can be
‘‘smoothed’’ with a simple R-C low-pass filter at the MF5
output
The ratio of f
also affect performance A ratio of 100 1 will reduce any
aliasing problems and is usually recommended for wide-
band input signals In noise sensitive applications however
a ratio of 50 1 may be better as it will result in 3 dB lower
output noise The 50 1 ratio also results in lower DC offset
voltages as discussed in 3 4
The accuracy of the f
of Q This is illustrated in the curves under the heading
‘‘Typical Performance Characteristics’’ As Q is changed
the true value of the ratio changes as well Unless the Q is
low the error in f
for a specific application use a mode that allows adjustment
of the ratio with external resistors
It should also be noted that the product of Q and f
be limited to 300 kHz when f
f
o l
5 kHz
CLK
CLK
to f
CLK
f
c
o
(normally either 50 1 or 100 1) will
will be small If the error is too large
TL H 5066– 32
f
o
ratio is dependent on the value
o k
s
2 This may in some cases
5 kHz and to 200 kHz for
o
should