AN1988 Freescale Semiconductor / Motorola, AN1988 Datasheet - Page 2

• Manufacturer Part Number: AN1988
• Description: AN1988
• Manufacturer: Freescale Semiconductor / Motorola

10 with a 2 µF capacitor at the input. Not only can the plot be
used as a guide in the design of an input filter, it can also help
put in perspective the possible amplitudes of the signal and
about what frequencies they may occur. From looking at the
plot one can see that for a capacitance of 1uF the gains at the
two most predominate noise frequencies is unity. This means
the aliased signals will not be larger in amplitude than the input
noise that created it; giving the recommended smaller
capacitance filter in the following illustration (see
recommended filter design for reducing the PSRR. One
should note however that variations in the individual devices
and the different axes of the device produce plots that may
look significantly different. The intent here is only to give an
idea of the effects of different capacitances on the oscillator
induced frequency gains. One must also remember to absorb
any impedance the source may have into the RC filter as the
source will also add its own impedance to the network.
2
One can see the PSRR has improved by roughly a factor of
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With cost as one of the deciding factors, this is a
Figure 3. Recommended RC Input Loading
50 OHM
for PSRR at V
Figure 2. PSRR with 50 Ohm and Varying Input Capacitance
1 µF
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AN1988, Rev 1
3).
amount of noise at the output will be decreased. The series
reactance can be increased with the use of an inductor in
series with the resistor (see
advantageous then adding more resistance due to the fact
that it does not produce a large DC voltage drop.
this input voltage is allowed to drop by more than half a volt
the device may not operate correctly; hence the use of an
inductor. With the use of an application board there was found
to be greater than a factor of two drop in the output noise at
the first harmonic with the use of the 1 mH inductor. The later
harmonics incur a larger improvement due to the higher
reactance of the inductor at the higher frequencies. As a result
there was greater than a factor of eight improvement with the
third and fifth harmonics. This RC and RLC circuit produced
the following PSRR plot (see
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If the series resistance and/or reactance is increased the
The device was designed to operate with 3.3 Vdc at V
Figure 4. Recommended RCL Input Loading
50 OHM
for PSRR at V
1 mH
Figure
Figure
1 µF
Freescale Semiconductor
4). This is more
5).
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Sensor Products
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