TI MF5CWM

MF5
MF5 Universal Monolithic Switched Capacitor Filter
Literature Number: SNOSBC9A
MF5 Universal Monolithic Switched Capacitor Filter
General Description
Features
The MF5 consists of an extremely easy to use, general purpose CMOS active filter building block and an uncommitted
op amp. The filter building block, together with an external
clock and a few resistors, can produce various second order
functions. The filter building block has 3 output pins. One of
the output pins can be configured to perform highpass, allpass or notch functions and the remaining 2 output pins
perform bandpass and lowpass functions. The center frequency of the filter can be directly dependent on the clock
frequency or it can depend on both clock frequency and
external resistor ratios. The uncommitted op amp can be
used for cascading purposes, for obtaining additional allpass and notch functions, or for various other applications.
Higher order filter functions can be obtained by cascading
several MF5s or by using the MF5 in conjuction with the
MF10 (dual switched capacitor filter building block). The
MF5 is functionally compatible with the MF10. Any of the
classical filter configurations (such as Butterworth, Bessel,
Cauer and Chebyshev) can be formed.
Y
Y
Y
Y
Y
Y
Y
et
Y
e
Y
Low cost
14-pin DIP or 14-pin Surface Mount (SO) wide-body
package
Easy to use
Clock to center frequency ratio accuracy g 0.6%
Filter cutoff frequency stability directly dependent on
external clock quality
Low sensitivity to external component variations
Separate highpass (or notch or allpass), bandpass, lowpass outputs
fo c Q range up to 200 kHz
Operation up to 30 kHz (typical)
Additional uncommitted op-amp
bs
ol
Block and Connection Diagrams
Y
TL/H/5066 – 1
O
All Packages
Order Number MF5CN
See NS Package Number N14A
Order Number MF5CWM
See NS Package Number M14B
Top View
C1995 National Semiconductor Corporation
TL/H/5066
TL/H/5066 – 2
RRD-B30M115/Printed in U. S. A.
MF5 Universal Monolithic Switched Capacitor Filter
February 1995
Absolute Maximum Ratings
See AN-450 ‘‘Surface Mounting Methods and Their Effect
on Product Reliability’’ for other methods of soldering surface mount devices.
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Supply Voltage (V a b Vb)
Input Voltage (any pin)
14V
Power Dissipation TA e 25§ C (note 1)
Storage Temp.
Soldering Information:
N Package:
10 sec.
SO Package: Vapor phase (60 sec.)
Infrared (15 sec.)
500 mW
150§ C
Vb s Vin s V a
Operating Temp. Range
MF5CN, MF5CWM
TMIN s TA s TMAX
0§ C s TA s 70§ C
260§ C
215§ C
220§ C
Electrical Characteristics V a e 5V g 0.5%, Vb e b5V g 0.5% unless otherwise noted. Boldface limits
apply over temperature, TMIN s TA s TMAX. For all other limits TA e 25§ C.
Parameter
Tested
Limit
(Note 7)
Min
Units
8
Max
Maximum Supply Current
Design
Limit
(Note 8)
V
e
Supply Voltage
(V a b Vb)
14
Clock applied to Pin 8
No Input Signal
4.5
Filter Output
Op-amp Output
6.0
V
mA
10
mV
10
mV
et
Clock
Feedthrough
Typical
(Note 6)
Conditions
Filter Electrical Characteristics V a e 5V g 0.5%, Vb e b5V g 0.5% unless otherwise noted. Boldface
limits apply over temperature, TMIN s TA s TMAX. For all other limits TA e 25§ C.
Center Frequency
Range (fo)
Clock Frequency
Range (fCLK)
Clock to Center
Frequency Ratio
(fCLK/fo)
fCLK/fo Temp.
Coefficient
Typical
(Note 6)
Units
kHz
30
20
0.1
0.2
Hz
Max
1.5
1.0
MHz
10
Hz
Min
5.0
Ideal
Q e 10
Mode 1
Vpin9 e a 5V
FCLK e 250 kHz
50.11 g 0.2%
50.11 g 1.5%
Vpin9 e b5V
FCLK e 500 kHz
100.04 g 0.2%
100.04 g 1.5%
Vpin9 e a 5V
(50:1 CLK ratio)
g 10
ppm/§ C
Vpin9 e b5V
(100:1 CLK ratio)
g 20
ppm/§ C
Ideal
Q e 10
Mode 1
Vpin9 e a 5V
FCLK e 250 kHz
g 10
%
Vpin9 e b5V
FCLK e 500 kHz
g 10
%
Vpin9 e a 5V
(50:1 CLK ratio)
Vpin9 e b5V
(100:1 CLK ratio)
DC Lowpass Gain
Accuracy (Max)
(Note 3)
Design
Limit
(Note 8)
Min
Q Temperature
Coefficient
DC Offset
Voltage (Max)
Tested
Limit
(Note 7)
Max
O
Q Accuracy (Max)
(Note 2)
Conditions
bs
ol
Parameter
b 200
ppm/§ C
b 70
ppm/§ C
Mode 1
R1 e R2 e 10 kX
g 0.2
Vos1
dB
g 5.0
mV
Vos2
Vpin9 e a 5V
b 185
mV
Vos3
(50:1 CLK ratio)
a 115
mV
Vos2
Vpin9 e b5V
b 310
mV
Vos3
(100:1 CLK ratio)
a 240
mV
2
Filter Electrical Characteristics V a e 5V g 0.5%, Vb e b5V g 0.5% unless otherwise noted. Boldface
limits apply over temperature, TMIN s TA s TMAX. For all other limits TA e 25§ C. (Continued)
Parameter
Output
Swing (Min)
Conditions
Typical
(Note 6)
Tested
Limit
(Note 7)
BP, LP pins
RL e 5 kX
g 4.0
g 3.8
N/AP/HP pin
RL e 3.5 kX
g 4.2
g 3.8
Dynamic Range
(Note 4)
Maximum Output Short Circuit
Current (Note 5)
Design
Limit
(Note 8)
Units
V
V
Vpin9 e a 5V
(50:1 CLK ratio)
83
dB
Vpin9 e b5V
(100:1 CLK ratio)
80
dB
Source
20
mA
Sink
3.0
mA
OP-AMP Electrical Characteristics V a e a 5V g 0.5%, Vb e b5V g 0.5% unless other noted. Boldface limits apply over temperature, TMIN s TA s TMAX. For all other limits TA e 25§ C.
Conditions
Typical
(Note 6)
RL e 3.5 kX
g 4.2
Gain Bandwidth Product
2.5
Output Voltage Swing (Min)
Slew Rate
7.0
80
Input Offset Voltage (Max)
Maximum Output
Short Circuit
Current (Note 5)
Source
V
V/ms
g 20
db
mV
10
pA
20
mA
3.0
mA
bs
ol
Sink
Units
MHz
g 3.8
g 5.0
Input Bias Current
Design
Limit
(Note 8)
et
DC Open-Loop Gain
Tested
Limit
(Note 7)
e
Parameter
Logic Input Characteristics Boldface limits apply over temperature, TMIN s TA s TMAX.
All other limits TA e 25§ C.
Parameter
CMOS Clock
Input
Min Logical ‘‘1’’
Input Voltage
Max Logical ‘‘0’’
Input Voltage
Min Logical ‘‘1’’
Input Voltage
Max Logical ‘‘0’’
Input Voltage
Min Logical ‘‘1’’
Input Voltage
O
TTL Clock
Input
Max Logical ‘‘0’’
Input Voltage
Typical
(Note 6)
Conditions
Tested
Limit
(Note 7)
Design
Limit
(Note 8)
Units
3.0
V
b 3.0
V
8.0
V
2.0
V
2.0
V
0.8
V
V a e a 5V, Vb e b5V,
VL.Sh. e 0V
V a e a 10V, Vb e 0V,
VL.Sh. e a 5V
V a e a 5V, Vb e b5V,
VL.Sh. e 0V
Note 1: The typical junction-to-ambient thermal resistance (iJA) of the 14 pin N package is 160§ C/W, and 82§ C/W for the M package.
Note 2: The accuracy of the Q value is a function of the center frequency (fo). This is illustrated in the curves under the heading ‘‘Typical Performance
Characteristics’’.
Note 3: Vos1, Vos2, and Vos3 refer to the internal offsets as discussed in the Application Information section 3.4.
Note 4: For g 5V supplies the dynamic range is referenced to 2.82V rms (4V peak) where the wideband noise over a 20 kHz bandwidth is typically 200 mV rms for
the MF5 with a 50:1 CLK ratio and 280 mV rms for the MF5 with a 100:1 CLK ratio.
Note 5: The short circuit source current is measured by forcing the output that is being tested to its maximum positive voltage swing and then shorting that output to
the negative supply. The short circuit sink current is measured by forcing the output that is being tested to its maximum negative voltage swing and then shorting
that output to the positive supply. These are the worst case conditions.
Note 6: Typicals are at 25§ C and represent most likely parametric norm.
Note 7: Guaranteed and 100% tested.
Note 8: Guaranteed, but not 100% tested. These limits are not used to calculate outgoing quality levels.
3
Pin Description
SA(5):
50/100(9):
AGND(11):
CLK(8):
L. Sh(7):
These are the positive and negative
supply pins. The MF5 will operate over a
total supply range of 8V to 14V.
Decoupling the supply pins with 0.1 mF
capacitors is highly recommended.
This is the clock input for the filter. CMOS
or TTL logic level clocks can be
accomodated by setting the L. Sh pin to
the levels described in the L. Sh pin
description. For optimum filter
performance a 50% duty cycle clock is
recommended for clock frequencies
greater than 200 kHz. This gives each op
amp the maximum amount of time to
settle to a new sampled input.
This pin allows the MF5 to accommodate
either CMOS or TTL logic level clocks. For
dual supply operation (i.e., g 5V), a CMOS
or TTL logic level clock can be accepted if
the L. Sh pin is tied to mid-supply (AGND),
which should be the system ground.
For single supply operation the L. Sh pin
should be tied to mid-supply (AGND) for a
CMOS logic level clock. The mid-supply
bias should be a very low impedance
node. See Applications Information for
biasing techniques. For a TTL logic level
clock the L. Sh pin should be tied to Vb
which should be the system ground.
This is the inverting input of the
uncommitted op amp. This is a very high
impedance input, but the non-inverting
input is internally tied to AGND, making
INV2 behave like a summing junction
(low-impedance current input).
This is the output of the uncommitted op
amp. It will typically sink 1.5 mA and
source 3.0 mA. It will typically swing to
within 1V of each supply.
e
S1(4):
V a (6), Vb(10):
et
INV1(3):
The second order lowpass, bandpass,
and notch/allpass/highpass outputs. The
LP and BP outputs can typically sink 1 mA
and source 3 mA. The N/AP/HP output
can typically sink 1.5 mA and source 3
mA. Each output typically swings to within
1V of each supply.
The inverting input of the summing op
amp of the filter. This is a high impedance
input, but the non-inverting input is
internally tied to AGND, making INV1
behave like a summing junction (low
impedance current input).
S1 is a signal input pin used in the allpass
filter configurations (see modes 4 and 5).
The pin should be driven with a source
impedance of less than 1 kX. If S1 is not
driven with a signal it should be tied to
AGND (mid-supply).
This pin activates a switch that connects
one of the inputs of the filter’s second
summer to either AGND (SA tied to Vb)
or to the lowpass (LP) output (SA tied to
V a ). This offers the flexibility needed for
configuring the filter in its various modes
of operation.
This pin is used to set the internal clock to
center frequency ratio (fCLK/fo) of the
filter. By tying the pin to V a an fCLK/fo
ratio of about 50:1 (typically 50.11 g
0.2%) is obtained. Tying the 50/100 pin to
either AGND or Vb will set the fCLK/fo
ratio to about 100:1 (typically 100.04 g
0.2%).
This is the analog ground pin. This pin
should be connected to the system
ground for dual supply operation or biased
to mid-supply for single supply operation.
For a further discussion of mid-supply
biasing techniques see the Applications
Information (Section 3.2). For optimum
filter performance a ‘‘clean’’ ground must
be provided.
INV2(12):
bs
ol
LP(14), BP(1),
N/AP/HP(2):
Vo2(13):
Typical Performance Characteristics
FCLK
vs Nominal Q
Fo
O
Deviation of
Deviation of
FCLK
vs Nominal Q
Fo
OPAMP Output Voltage
Swing vs Temperature
TL/H/5066 – 3
4
observed as the frequency of a notch at the allpass output.
(Figure 10 ).
Typical Performance
Characteristics (Continued)
Q: ‘‘quality factor’’ of the 2nd order filter. Q is measured at
the bandpass output of the MF5 and is equal to fo divided by
the b3dB bandwidth of the 2nd order bandpass filter (Figure 1 ). The value of Q determines the shape of the 2nd
order filter responses as shown in Figure 6 .
Qz: the quality factor of the second order complex zero pair,
if any. Qz is related to the allpass characteristic, which is
written:
Supply Current vs Temperature
HOAP
HAP(s) e
#s
2b
s0o
a 0o2
Qz
J
s0o
a 0o2
s2 a
Q
where Qz e Q for an all-pass response.
HOBP: the gain (in V/V) of the bandpass output at f e fo.
HOLP: the gain (in V/V) of the lowpass output as f x 0 Hz
(Figure 2 ).
HOHP: the gain (in V/V) of the highpass output as
f x fclk/2 (Figure 3 ).
HON: the gain (in V/V) of the notch output as f x 0 Hz and
as f x fclk/2, when the notch filter has equal gain above
and below the center frequency (Figure 4 ). When the lowfrequency gain differs from the high-frequency gain, as in
modes 2 and 3a (Figures 11 and 8 ), the two quantities below are used in place of HON.
HON1: the gain (in V/V) of the notch output as f x 0 Hz.
HON2: the gain (in V/V) of the notch output as f x fclk/2.
TL/H/5066 – 4
1.0 Definitions of Terms
et
e
fCLK: the frequency of the external clock signal applied to
pin 8.
fo: center frequency of the second order function complex
pole pair. fo is measured at the bandpass output of the MF5,
and is the frequency of maximum bandpass gain. (Figure 1 ).
fnotch: the frequency of minimum (ideally zero) gain at the
notch output.
fz: the center frequency of the second order complex zero
pair, if any. If fz is different from fo and if Qz is high, it can be
0o
s
HOBP Q
s0o
2
a 0o2
s a
Q
bs
ol
HBP(s) e
Qe
fo
;
fH b fL
# 2Q 0 # 2Q J
1
1
f
# 2Q 0 # 2Q J
fL e fo
(a)
TL/H/5066–5
(b)
TL/H/5066 – 6
f o e 0f Lf H
b1
1
a
fH e o
2
a
a1
2
a1
0 o e 2 q fo
J
J
FIGURE 1. 2nd-Order Bandpass Response
HOLP0o2
s0o
a 0o2
Q
HLP(s) e
s2 a
fc e fo c
O
fp e fo
(a)
TL/H/5066–7
0
0 #1
1b
a
1
2Q2
b
1 2
a1
2Q2
J
1
HOP e HOLP c
(b)
J 0 #1
1
2Q2
b
1
Q
TL/H/5066 – 8
0
1
4Q2
1b
FIGURE 2. 2nd-Order Low-Pass Response
HHP(s) e
fc e fo c
Ð0 #
fp e fo c
(a)
TL/H/5066–9
HOHPs2
s0o
a 0o2
Q
s2 a
(b)
TL/H/5066 – 10
FIGURE 3. 2nd-Order High-Pass Response
5
Ð0
1b
1b
HOP e HOHP c
J 0#
(
1
a
2Q2
1
2Q2
b1
1
1
Q
0
1b
1
4Q2
1b
1 2
a1
2Q2
J
(
b1
1.0 Definition of Terms (Continued)
HN(s) e
Qe
fo
;
fH b fL
fL e fo
fH e fo
TL/H/5066–11
HON(s2 a 0o2)
s0o
a 0o2
s2 a
Q
fo e 0fLfH
# 2Q 0 # 2Q J
1
1
# 2Q 0 # 2Q J
b1
1
a
2
a1
2
a1
a
J
J
TL/H/5066 – 12
(a)
(b)
FIGURE 4. 2nd-Order Notch Response
HOAP
#s
2b
s0o
a 0o2
Q
s0o
a 0o2
Q
J
e
HAP(s) e
TL/H/5066–13
et
s2 a
TL/H/5066 – 14
(b)
(a)
bs
ol
FIGURE 5. 2nd-Order All-Pass Response
(a) Bandpass
(b) Low-Pass
(e) All-Pass
O
(d) Notch
(c) High-Pass
TL/H/5066 – 15
FIGURE 6. Responses of various 2nd-order filters
as a function of Q. Gains and center frequencies
are normalized to unity.
6
2.0 Modes of Operation
The MF5 is a switched capacitor (sampled data) filter. To
fully describe its transfer functions, a time domain approach
is appropriate. Since this is cumbersome, and since the
MF5 closely approximates continuous filters, the following
discussion is based on the well known frequency domain.
Each MF5 can produce a full 2nd order function. See Table
1 for a summary of the characteristics of the various modes.
MODE 1: Notch 1, Bandpass, Lowpass Outputs:
fnotch e fo (See Figure 7 )
fo
fo
R3
e
BW
R2
BW e the b3 dB bandwidth of the bandpass output.
Circuit dynamics:
HOBP
or HOBP e HOLP c Q e HON c Q.
HOLP e
Q
HOLP(peak) j Q c HOLP (for high Q’s)
Q
e
MODE 1a: Non-Inverting BP, LP (See Figure 8 )
f
f
e CLK or CLK
100
50
R3
e
Q
R2
HOLP e b1; HOLP(peak) j Q c HOLP (for high Q’s)
e center frequency of the complex pole pair
fo
f
f
e CLK or CLK
100
50
fnotch e center frequency of the imaginary zero pair e fo.
R2
HOLP e Lowpass gain (as f x 0) e b
R1
R3
HOBP e Bandpass gain (at f e fo) e b
R1
x fCLK/2
(
e
b R2
R1
bs
ol
et
f
x0
e
HON e Notch output gain as f
R3
R2
HOBP2 e 1 (non-inverting)
Circuit dynamics: HOBP1 e Q
Note: VIN should be driven from a low impedance ( k1 kX)
HOBP1 e b
TL/H/5066 – 16
O
FIGURE 7. MODE 1
TL/H/5066 – 17
FIGURE 8. MODE 1a
7
2.0 Modes of Operation (Continued)
MODE 2: Notch 2, Bandpass, Lowpass: fnotchkfo
(See Figure 9 )
fo
MODE 3: Highpass, Bandpass, Lowpass Outputs
(See Figure 10 )
f
R2 fCLK
R2
e CLK c
c
or
fo
100
R4
50
R4
e quality factor of the complex pole pair
Q
R2 R3
e
c
R4 R2
fCLK
R2
eb
HOHP e Highpass gain as f x
2
R1
R3
HOBP e Bandpass gain (at f e fo) e b
R1
R4
HOLP e Lowpass gain (as f x 0) e b
R1
HOHP
R2
e
e
; HOBP
Circuit dynamics:
0HOHP c HOLP c Q
R4
HOLP
HOLP(peak) j Q c HOLP (for high Q’s)
HOHP(peak) j Q c HOHP (for high Q’s)
0
e center frequency
fnotch
e
Q
e
e
HOLP e
e
0
0
f
R2
R2
a 1 or CLK
a1
100 R4
50
R4
fCLK fCLK
or
100
50
quality factor of the complex pole pair
0R2/R4 a 1
R2/R3
Lowpass output gain (as f x 0)
R2/R1
b
R2/R4 a 1
f
e CLK
0
J
J
bs
ol
et
#
#
e
HOBP e Bandpass output gain (at f e fo) e bR3/R1
HON1 e Notch output gain (as f x 0)
R2/R1
e b
R2/R4 a 1
f
HON2 e Notch output gain as f x CLK e bR2/R1
2
Filter dynamics: HOBP e Q 0HOLP HON2 e Q 0HON1 HON2
0
TL/H/5066 – 18
FIGURE 9. MODE 2
O
*In Mode 3, the feedback loop is closed around
the input summing amplifier; the finite GBW product of this op amp causes a slight Q enhancement. If this is a problem, connect a small capacitor (10 pF–100 pF) across R4 to provide some
phase lead.
TL/H/5066 – 19
FIGURE 10. MODE 3
8
2.0 Modes of Operation (Continued)
MODE 3a: HP, BP, LP and Notch with External Op amp
(See Figure 11 )
R2 fCLK
R2
f
e CLK c
c
or
fo
100
R4
50
R4
R2 R3
e
c
Q
R4 R2
R2
HOHP e b
R1
R3
HOBP e b
R1
R4
HOLP e b
R1
f
Rh fCLK
Rh
e notch frequency e CLK
or
fn
100
Rl
50
Rl
Rg
Rg
e
e
e
b
Hon
gain of notch at f fo
Q
HOLP
HOHP
Rl
Rh
Rg
c HOLP
Hn1 e gain of notch (as f x 0) e
Rl
fCLK
R
e b g c HOHP
Hn2 e gain of notch as f x
2
Rh
0
0
0
Ó #
e
fo
R3
e
;
BW
R2
Qz e quality factor of complex zero pair e
R3
R1
For AP output make R1 e R2
H*OAP e Allpass gain
# at 0
f
k f k CLK
2
0)
J
eb
R2
e b1
R1
HOLP e Lowpass gain (as f x
R2
e b
a 1 e b2
R1
e
Bandpass gain (at f e fo)
HOBP
R3
R2
R3
e b
e b2
1a
R2
R1
R2
Circuit dynamics: HOBP e (HOLP) c Q e (HOAP a 1) Q
JÓ
#
J
#
J
J
# J
*Due to the sampled data nature of the filter, a slight mismatch of fz and fo
occurs causing a 0.4 dB peaking around fo of the allpass filter amplitude
response (which theoretically should be a straight line). If this is unacceptable, Mode 5 is recommended.
bs
ol
et
#
Q
e
0
0
MODE 4: Allpass, Bandpass, Lowpass Outputs (See
Figure 12 )
e center frequency
fo
f
f
e CLK or CLK;
100
50
f*z e center frequency of the complex zero pair j fo
TL/H/5066 – 20
O
FIGURE 11. MODE 3a
TL/H/5066 – 21
FIGURE 12. MODE 4
9
2.0 Modes of Operation (Continued)
MODE 6a: Single Pole, HP, LP Filter (See Figure 14 )
e cutoff frequency of LP or HP output
R2 fCLK R2 fCLK
e
or
R3 100
R3 50
R3
HOLP e b
R1
R2
HOHP e b
R1
MODE 5: Numerator Complex Zeros, BP, LP
(See Figure 13 )
R2 fCLK
R2 fCLK
e
c
c
or 1 a
1a
fo
R4
100
R4
50
R1 fCLK
R1 fCLK
e
b
c
b
c
or 1
1
fz
R4
100
R4
50
R3
e 01 a R2/R4 c
Q
R2
R3
e 01 b R1/R4 c
Qz
R1
b R2 (R4 b R1)
H0z1 e gain at C.Z. output (as f x 0 Hz) e
R1 (R4 a R2)
b R2
fCLK
e
H0z2 e gain at C.Z. output as f x
2
R1
R2
R3
eb
e
a
c
1
HOBP
R1
R2
a
R2
R1
R4
c
HOLP eb
R2 a R4
R1
J
J
#
MODE 6b: Single Pole LP Filter (Inverting and NonInverting) (See Figure 15 )
e cutoff frequency of LP outputs
fc
R2 fCLK R2 fCLK
j
or
R3 100
R3 50
HOLP1 e 1 (non-inverting)
R3
HOLP2 e b
R2
J
bs
ol
et
#
#
0
0
e
0
0
fc
TL/H/5066 – 22
O
FIGURE 13. MODE 5
TL/H/5066 – 23
FIGURE 14. MODE 6a
TL/H/5066 – 24
FIGURE 15. MODE 6b
10
2.0 Modes of Operation (Continued)
TABLE I. Summary of Modes. Realizable filter types (e.g. low-pass) denoted by asterisks. Unless otherwise noted,
gains of various filter outputs are inverting and adjustable by resistor ratios.
LP
1
*
*
HP
HOLP e a 1
2
*
*
3
*
*
*
3a
*
*
*
4
5
*
*
*
*
6b
HOLP e a 1
b R3
HOLP2 e
R2
*
*
Number of
resistors
Adjustable
fCLK/fo
3
No
2
No
3
Yes (above
fCLK/50 or
fCLK/100)
4
Yes
Universal StateVariable Filter. Best
general-purpose mode.
7
Yes
As above, but also
includes resistortuneable notch.
3
No
Gives Allpass response with HOAP eb1
and HOLP eb2.
Notes
May need input buffer. Poor dynamics
for high Q.
4
Gives flatter allpass
response than above
if R1 e R2 e 0.02R4.
3
Single pole.
2
Single pole
bs
ol
*
(2)
*
*
*
6a
AP
*
(2)
HOBP1 e bQ
HOBP2 e a 1
1a
N
e
BP
et
Mode
3.0 Applications Information
From the specifications, the filter parameters are:
fo e 200 Hz, HOLP eb2, and, for Butterworth response,
Q e 0.707.
In section 2.0 are several modes of operation for the MF5,
each having different characteristics. Some allow adjustment of fCLK/fo, others produce different combinations of
filter types, some are inverting while others are non-inverting, etc. These characteristics are summarized in Table I. To
keep the example simple, we will use mode 1, which has
notch, bandpass, and lowpass outputs, and inverts the signal polarity. Three external resistors determine the filter’s Q
and gain. From the equations accompanying Figure 7 ,
Q e R3/R2 and the passband gain HOLP e bR2/R1. Since
the input signal is driving a summing junction through R1,
the input impedance will be equal to R1. Start by choosing a
value for R1. 10k is convenient and gives a reasonable input
impedance. For HOLP e b2, we have:
R2 e bR1HOLP e 10k c 2 e 20k.
For Q e 0.707 we have:
R3 e R2Q e 20k c 0.707 e 14.14k. Use 15k.
For operation on g 5V supplies, V a is connected to a 5V,
Vb to b5V, and AGND to ground. The power supplies
should be ‘‘clean’’ (regulated supplies are preferred) and
0.1 mF bypass capacitors are recommended.
O
The MF5 is a general-purpose second-order state variable
filter whose center frequency is proportional to the frequency of the square wave applied to the clock input (fCLK). By
connecting pin 9 to the appropriate DC voltage, the filter
center frequency fo can be made equal to either fCLK/100
or fCLK/50. fo can be very accurately set (within g 0.6%) by
using a crystal clock oscillator, or can be easily varied over
a wide frequency range by adjusting the clock frequency. If
desired, the fCLK/fo ratio can be altered by external resistors as in Figures 9, 10, 11, 13, 14 , and 15 . The filter Q and
gain are determined by external resistors.
All of the five second-order filter types can be built using the
MF5. These are illustrated in Figures 1 through 5 along with
their transfer functions and some related equations. Figure
6 shows the effect of Q on the shapes of these curves.
When filter orders greater than two are desired, two or more
MF5s can be cascaded. The MF5 also includes an uncommitted CMOS operational amplifier for additional signal processing applications.
3.1 DESIGN EXAMPLE
An example will help illustrate the MF5 design procedure.
For the example, we will design a 2nd order Butterworth
low-pass filter with a cutoff frequency of 200 Hz, and a passband gain of b2. The circuit will operate from a g 5V power
supply, and the clock amplitude will be g 5v (CMOS) levels).
11
et
e
3.0 Applications Information (Continued)
TL/H/5066 – 25
FIGURE 16. 2nd-Order Butterworth Low-Pass Filter of Design
fCLK
e 50, Connect Pin 9 to a 5V, and
Example. For
f0
O
bs
ol
Change Clock Frequency to 10 kHz.
TL/H/5066 – 26
FIGURE 17. Butterworth Low-Pass Circuit of Example, but Designed for Single-Supply Operation
12
3.0 Applications Information (Continued)
TL/H/5066–27
(a) Resistive Divider with
Decoupling Capaciter
TL/H/5066 – 28
(b) Voltage Regulator
FIGURE 18. Three Ways of Generating
TL/H/5066 – 29
(c) Operational Amplifier
with Divider
Va
for Single-supply Operation
2
these limits. If the MF5 is operating on g 5 volts, for example, the outputs will clip at about 8Vp-p. The maximum input
voltage multiplied by the filter gain should therefore be less
than 8Vp-p.
Note that if the filter has high Q, the gain at the lowpass or
highpass outputs will be much greater than the nominal filter
gain (Figure 6 ). As an example, a lowpass filter with a Q of
10 will have a 20 dB peak in its amplitude response at fo. If
the nominal gain of the filter HOLP is equal to 1, the gain at
fo will be 10. The maximum input signal at fo must therefore
be less than 800 mVp-p when the circuit is operated on g 5
volt supplies.
Also note that one output can have a reasonable small voltage on it while another is saturated. This is most likely for a
circuit such as the notch in Mode 1 (Figure 7 ). The notch
output will be very small at fo, so it might appear safe to
apply a large signal to the input. However, the bandpass will
have its maximum gain at fo and can clip if overdriven. If one
output clips, the performance at the other outputs will be
degraded, so avoid overdriving any filter section, even ones
whose outputs are not being directly used. Accompanying
Figures 7 through 15 are equations labeled ‘‘circuit dynamics’’, which relate the Q and the gains at the various outputs.
These should be consulted to determine peak circuit gains
and maximum allowable signals for a given application.
bs
ol
et
e
For a cutoff frequency of 200 Hz, the external clock can be
either 10 kHz with pin 9 connected to V a (50:1) or 20 kHz
with pin 9 tied to AGND or Vb (100:1). The voltage on the
Logic Level Shift pin (7) determines the logic threshold for
the clock input. The threshold is approximately 2V higher
than the voltage applied to pin 7. Therefore, when pin 7 is
grounded, the clock logic threshold will be 2V, making it
compatible with 0–5 volt TTL logic levels and g 5 volt
CMOS levels. Pin 7 should be connected to a clean, low-impedance (less than 1000X) voltage source.
The complete circuit of the design example is shown for a
100:1 clock ratio in Figure 16 .
3.2 SINGLE SUPPLY OPERATION
The MF5 can also operate with a single-ended power supply. Figure 17 shows the example filter with a single-ended
power supply. V a is again connected to the positive power
supply (8 to 14 volts), and Vb is connected to ground. The
AGND pin must be tied to V a /2 for single supply operation.
This half-supply point should be very ‘‘clean’’, as any noise
appearing on it will be treated as an input to the filter. It can
be derived from the supply voltage with a pair of resistors
and a bypass capacitor (Figure 18a ), or a low-impedance
half-supply voltage can be made using a three-terminal voltage regulator or an operational amplifier (Figures 18b and
18c ). The passive resistor divider with a bypass capacitor is
sufficient for many applications, provided that the time constant is long enough to reject any power supply noise. It is
also important that the half-supply reference present a low
impedance to the clock frequency, so at very low clock frequencies the regulator or op-amp approaches may be preferable because they will require smaller capacitors to filter
the clock frequency. The main power supply voltage should
be clean (preferably regulated) and bypassed with 0.1mF.
O
3.4 OFFSET VOLTAGE
The MF5’s switched capacitor integrators have a higher
equivalent input offset voltage than would be found in a
typical continuous-time active filter integrator. Figure 19
shows an equivalent circuit of the MF5 from which the output dc offsets can be calculated. Typical values for these
offsets are:
Vos1 e opamp offset e g 5mV
3.3 DYNAMIC CONSIDERATIONS
The maximum signal handling capability of the MF5, like
that of any active filter, is limited by the power supply voltages used. The amplifiers in the MF5 are able to swing to
within about 1 volt of the supplies, so the input signals must
be kept small enough that none of the outputs will exceed
b 310mV @ 100:1
Vos2 e b185mV @ 50:1
a 240mV @ 100:1
Vos3 e a 115mV @ 50:1
The dc offset at the BP output is equal to the input offset of
the lowpass integrator (Vos3). The offsets at the other outputs depend on the mode of operation and the resistor ratios, as described in the following expressions.
13
3.0 Applications Information (Continued)
Mode 1 and Mode 4
Mode 2 and Mode 5
#
Ó
1
a 1 a HOLP
Q
VOS(N)
e VOS1
VOS(BP)
VOS(LP)
e VOS3
e VOS(N) b VOS2
ÓJ
V
b OS3
Q
VOS(N)
e
# Rp 1 J V
R2
OS1 c
a VOS2
Mode 1a
1
1 a R2/R4
1
VOS3
b
:
1 a R4/R2 Q01 a R2/R4
Rp e R1//R2//R4
VOS(N.INV.BP) e
VOS(INV.BP)
VOS(LP)
a
#
1
1a
Q
J
VOS3
VOS1 b
Q
e VOS3
e VOS(N.INV.BP) b VOS2
VOS(BP)
VOS(LP)
e VOS3
e VOS(N) b VOS2
Mode 3
VOS(HP)
VOS(BP)
e VOS2
VOS(LP)
e b
e VOS3
b
# R3 V
R2
R4
1
V
R2 #
R J
R4
R2
R2
OS3 a VOS2
a
a
OS1; Rp e R1//R3//R4
bs
ol
et
e
p
J
O
FIGURE 19. Block Diagram Showing MF5
Offset Voltage Sources
TL/H/5066 – 31
FIGURE 20. Method for Trimming VOS,
See Text, Section 3.4
14
TL/H/5066 – 30
3.0 Applications Information (Continued)
ing’’, and can be reduced or eliminated by limiting the input
signal spectrum to less than fs/2. This may in some cases
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 period, 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 fCLK to fc (normally either 50:1 or 100:1) will
also affect performance. A ratio of 100:1 will reduce any
aliasing problems and is usually recommended for wideband 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 fCLK/fo ratio is dependent on the value
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 fCLK/fo will be small. If the error is too large
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 fo should
be limited to 300 kHz when fo k 5 kHz, and to 200 kHz for
fo l 5 kHz.
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 fo and Q. When operating in Mode 3, offsets can
become excessively large if R2 and R4 are used to make
fCLK/fo significantly higher than the nominal value, especially if Q is also high. An extreme example is a bandpass filter
having unity gain, a Q of 20, and fCLK/fo e 250 with pin 9
tied to Vb (100:1 nominal). R4/R2 will therefore be equal to
6.25 and the offset voltage at the lowpass output will be
about a 1.9V. Where necessary, the offset voltage can be
adjusted by using the circuit of Figure 20 . This allows adjustment of Vos1, which will have varying effects on the different
outputs as described in the above equations. Some outputs
cannot be adjusted this way in some modes, however
(Vos(BP) in modes 1a and 3, for example).
bs
ol
et
e
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 important characteristic of sampled-data systems is their effect 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 fs/2 a 100 Hz will
cause the system to respond as though the input frequency
was fs/2 - 100 Hz. This phenomenon is known as ‘‘alias-
TL/H/5066 – 32
O
FIGURE 21. The Sampled-Data Output Waveform
15
e
et
SO Package
Order Number MF5CWM
NS Package Number M14B
bs
ol
MF5 Universal Monolithic Switched Capacitor Filter
Physical Dimensions inches (millimeters)
Molded Dual-In-Line Package (N)
Order Number MF5CN
NS Package Number N14A
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