LMP8602MA NSC [National Semiconductor], LMP8602MA Datasheet - Page 15

• Manufacturer Part Number: LMP8602MA
• Description: 60V Common Mode, Fixed Gain, Bidirectional Precision Current Sensing Amplifier
• Manufacturer: NSC [National Semiconductor]

ADDITIONAL SECOND ORDER LOW PASS FILTER
The LMP8602/LMP8603 has a third order Butterworth low-
pass characteristic with a typical bandwidth of 60 kHz inte-
grated in the preamplifier stage of the part. The bandwidth of
the output buffer can be reduced by adding a capacitor on the
A1 pin to create a first order low pass filter with a time constant
determined by the 100 kΩ internal resistor and the external
filter capacitor.
When the corner frequency of the additional filter is much
lower than 60 kHz, the transfer function of the described am-
plifier van be written as:
Where K
the buffer amplifier.
The above equation can be written in the normalized frequen-
cy response for a 2
The Cutt-off frequency ω
cut-off frequency in Hz) is given by:
and the quality factor of the filter is given by:
1
equals the gain of the preamplifier and K
nd
order low pass filter:
o
in rad/sec (divide by 2
FIGURE 2. Second Order Low Pass Filter
K1 = 10, K2 = 5 for LMP8602, K2 = 10 for LMP8603
π
to get the
2
that of
15
It is also possible to create an additional second order Sallen-
Key low pass filter as shown in Figure 2 by adding external
components R
100 kΩ resistor R
pass filter characteristic.
For any filter gain K > 1x, the design procedure can be very
simple if the two capacitors are chosen to in a certain ratio.
Inserting this in the above equation for Q results in:
Which results in:
In this case, given the predetermined value of R1 = 100 kΩ
( the internal resistor), the quality factor is set solely by the
value of the resistor R
2
, C
1
, this circuit creates a second order low-
1
2
and C
.
2
. Together with the internal
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