LMF60CIN-50 National Semiconductor, LMF60CIN-50 Datasheet - Page 13

• Manufacturer Part Number: LMF60CIN-50
• Description: IC HIPERFM 60RD SW CAO FIL 14DIP
• Manufacturer: National Semiconductor

Specifications of LMF60CIN-50

• Filter Type: Butterworth, Lowpass Switched Capacitor
• Frequency - Cutoff Or Center: 30kHz
• Number Of Filters: 1
• Max-order: 6th
• Voltage - Supply: 4 V ~ 14 V, ±2 V ~ 7.5 V
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LMF60CIN-50

5(a) DC offset is adjusted using a potentiometer in 5(b) the
1 0 LMF60 Application Hints
1 3 OFFSET ADJUST
The V
of the filter section If this pin is not used it must be tied to
the analog ground (AGND) level either mid-supply for single
ended supply operation or ground for split supply operation
This pin sets the zero reference for the output of the filter
The implementation of this pin can be seen in Figure 5 In
Op-Amp integrator circuit keeps the average DC output lev-
el at AGND The circuit in 5(b) is therefore appropriate only
for AC-coupled signals and signals biased at AGND
1 4 INPUT IMPEDANCE
The LMF60 lowpass filter input (FILTER IN pin) is not a high
impedance buffer input This input is a switched capacitor
resistor equivalent and its effective impedance is inversely
proportional to the clock frequency The equivalent circuit of
the input to the filter can be seen in Figure 6 The input
capacitor charges to the input voltage (V
of the clock period during the second half the charge is
transferred to the feedback capacitor The total transfer of
charge in one clock cycle is therefore Q
since current is defined as the flow of charge per unit time
the average input current becomes
(where T equals one clock period) or
OS
a) Equivalent Circuit for LMF60 Filter Input
ADJ pin is used in adjusting the output offset level
I
IN
e
C
IN
I
T
IN
V
IN
e
e
Q T
C
IN
V
IN
f
CLK
IN
) during one half
e
FIGURE 5 V
(Continued)
C
FIGURE 6 LMF60 Filter Input
TL H 9294– 13
TL H 9294– 15
IN
V
IN
and
OS
13
Adjust Schemes
The equivalent input resistor (R
The input capacitor is 2 pF for the LMF60-50 and 1 pF for
the LMF60-100 so for the LMF60-100
and
for the LMF60-50 As shown in the above equations for a
given cutoff frequency (f
same for the LMF60-50 and the LMF60-100 The higher the
clock to cutoff frequency ratio the greater equivalent input
resistance for a given clock frequency As the cutoff fre-
quency increases the equivalent input impedance decreas-
es This input resistance will form a voltage divider with the
source impedance (R
portional to the cutoff frequency operation at higher cutoff
frequencies will be more likely to load the input signal which
would appear as an overall decrease in gain at the output of
the filter Since the filter’s ideal gain is unity its overall gain
is given by
R
R
b) Actual Circuit for LMF60 Filter Input
IN
IN
e
e
1
5
R
c
c
f
f
A
IN
CLK
CLK
V
10
10
e
e
SOURCE
12
11
V
C
R
IN
) the input impedance remains the
e
e
IN
I
1
f
5
a
IN
C c
f
C c
) Since R
c
c
R
R
IN
e
IN
10
10
SOURCE
) then can be defined as
100
C
50
12
11
IN
1
f
e
e
CLK
IN
1
1
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c
c
is inversely pro-
f
f
C
C
10
10
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10
10
TL H 9294 – 14
TL H 9294 – 16