lmp8602mmx National Semiconductor Corporation, lmp8602mmx Datasheet - Page 16

• Manufacturer Part Number: lmp8602mmx
• Description: 60v Common Mode, Fixed Gain, Bidirectional Precision Current Sensing Amplifier
• Manufacturer: National Semiconductor Corporation

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R
first step of the design procedure with the following equation:
For the gain of 5 for the LMP8602 this results in:
For the gain of 10 for the LMP8603 this is:
For instance, the value of Q can be set to 0.5
Butterworth response, to 1/
or a 0.5 to create a critically damped response. Once the
value of R
design procedure is to calculate the required value of C to give
the desired low-pass cut-off frequency using:
Which for the gain=5 will give:
and for the gain=10:
R
100 kΩ resistor such that, for the LMP8602, the reduced gain
G
2
r
r
becomes:
can be calculated based on the desired value of Q as the
creates a resistive divider together with the internal
2
has been found, the second and last step of the
√
3 to create a Bessel response,
√
2 to create a
FIGURE 3. Reduce Gain
16
For C
Or for a gain=5:
and for a gain=10:
Note that the frequency response achieved using this proce-
dure will only be accurate if the cut-off frequency of the second
order filter is much smaller than the intrinsic 60 kHz low-pass
filter. In other words, to have the frequency response of the
LMP8602/LMP8603 circuit chosen such that the internal
poles do not affect the external second order filter.
For a desired Q = 0.707 and a cut off frequency = 3 kHz, this
will result for the LMP8602 in rounded values for R2 = 51
kΩ, C1 = 1.5 nF and C2 = 3.9 nF
And for the LMP8603 this will result in rounded values for R2
= 22 kΩ, C1 = 3.3 nF and C2 = 0.39 nF
GAIN ADJUSTMENT
The gain of the LMP8602 is 50 and the gain of the LMP8603
is 100, however, this gain can be adjusted as the signal path
in between the two internal amplifiers is available on the ex-
ternal pins.
Reduce Gain
Figure 3 shows the configuration that can be used to reduce
the gain of the LMP8602 and the LMP8603.
2
the value is calculated with:
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