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參數資料
| 型號： | LMF380CIV |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 模擬濾波器 |
| 英文描述： | LMF380 Triple One-Third Octave Switched-Capacitor Active Filter |
| 中文描述： | TRIPLE SWITCHED CAPACITOR FILTER, CHEBYSHEV, BANDPASS, PQCC20 |
| 封裝： | PLASTIC, LCC-20 |
| 文件頁數： | 6/12頁 |
| 文件大?。?/td> | 232K |
| 代理商： | LMF380CIV |
Applications Information
POWER SUPPLIES
The LMF380 can operate from a total supply voltage (V
a
b
V
b
) ranging from 4.0V up to 14V, but the choice of supply
voltage can affect circuit performance. The IC depends on
MOS switches for its operation. All such switches have in-
herent ‘‘ON’’ resistances, which can cause small delays in
charging internal capacitances. Increasing the supply volt-
age reduces this ‘‘ON’’ resistance, which improves the ac-
curacy of the filter in high-frequency applications. The maxi-
mum practical center frequency improves by roughly 10% to
20% when the supply voltage increases from 5V to 10V.
Dynamic range is also affected by supply voltage. The maxi-
mum signal voltage swing capability increases as supply
voltage increases, so the dynamic range is greater with
higher power supply voltages. It is therefore recommended
that the supply voltage be kept near the maximum operating
voltage when dynamic range and/or high-frequency per-
formance are important.
As with all switched-capacitor filters, each of the LMF380’s
power supply pins should be bypassed with a minimum of
0.1
m
F located close to the chip. An additional 1
m
F to
10
m
F tantalum capacitor on each supply pin is recommend-
ed for best results.
Sampled-Data System
Considerations
CLOCK CIRCUITRY
The LMF380’s clock input circuitry accepts an external
CMOS-level clock signal at XTAL1, or can serve as a self-
contained oscillator with the addition of an external 1 MHz
crystal and two 30 pF capacitors (see Figure 3).
The Clock Output pin provides a clock signal whose fre-
quency is one-half that of the clock signal at XTAL1. This
allows multiple LMF380s to operate from a single internal or
external clock oscillator.
CLOCK FREQUENCY LIMITATIONS
The performance characteristics of a switched-capacitor fil-
ter depend on the switching (clock) frequency. At very low
clock frequencies (below 10 Hz), the time between clock
cycles is relatively long, and small parasitic leakage currents
cause the internal capacitors to discharge sufficiently to af-
fect the filter’s offset voltage and gain. This effect becomes
more pronounced at elevated operating temperatures.
At higher clock frequencies, performance deviations are
due primarily to the reduced time available for the internal
operational amplifiers to settle. For this reason, when the
filter clock is externally generated, care should be taken to
ensure that the clock waveform’s duty cycle is as close to
50% as possible, especially at high clock frequencies.
OUTPUT STEPS
Because the LMF380 uses switched-capacitor techniques,
its performance differs in several ways from non-sampled
(continuous) circuits. The analog signal at any input is sam-
pled during each filter clock cycle, and since the output volt-
age can change only once every clock cycle, the result is a
discontinuous output signal. The output signal takes the
form of a series of voltage ‘‘steps’’, as shown inFigure 2 for
clock-to-center-frequency ratios of 50:1 and 100:1.
TL/H/11123–8
FIGURE 2. Switched-Capacitor Filter Output Waveform.
Note the sampling ‘‘steps’’.
ALIASING
Another important characteristic of sampled-data systems is
their effect on signals at frequencies greater than one-half
the sampling frequency, f
S
. (The LMF380’s sampling fre-
quency is the same as the filter clock frequency). If a signal
with a frequency greater than one-half the sampling fre-
quency 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 f
S
/2
a
10 Hz will cause the system to respond as though
the input frequency was f
S
/2
b
10 Hz. If this frequency
happens to be within the passband of the filter, it will appear
at the filter’s output, even though it was not present in the
input signal. This phenomenon is known as ‘‘aliasing’’. Ali-
asing can be reduced or eliminated by limiting the input sig-
nal spectrum to less than f
S
/2. In some cases, it may be
necessary to use a bandwidth-limiting filter (often a simple
passive RC low-pass) between the signal source and the
switched-capacitor filter’s input. In the application example
shown inFigure 3, two LMF60 6th-order low-pass filters pro-
vide anti-aliasing filtering.
OFFSET VOLTAGE
Switched-capacitor filters often have higher offset voltages
than non-sampling filters with similar topologies. This is due
to charge injection from the MOS switches into the sampling
and integrating capacitors. The LMF380’s offset voltage
ranges from a minimum of
b
30 mV to a maximum of
a
120 mV.
NOISE
Switched-capacitor filters have two kinds of noise at their
outputs. There is a random, ‘‘thermal’’ noise component
whose amplitude is typically on the order of 210
m
V. The
other kind of noise is digital clock feedthrough. This will
have an amplitude in the vicinity of 10 mV peak-to-peak. In
some applications, the clock noise frequency is so high
compared to the signal frequency that it is unimportant. In
other cases, clock noise may have to be removed from the
output signal with, for example, a passive low-pass filter at
the LMF380’s output (see Figure 4).
INPUT IMPEDANCE
The LMF380’s input pins are connected directly to the inter-
nal biquad filter sections. The input impedance is purely ca-
pacitive and is approximately 6.2 pF at each input pin, in-
cluding package parasitics.
6
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