lmf380 National Semiconductor Corporation, lmf380 Datasheet - Page 6

no-image

lmf380

Manufacturer Part Number
lmf380
Description
Triple One-third Octave Switched-capacitor Active Filter
Manufacturer
National Semiconductor Corporation
Datasheet
Applications Information
POWER SUPPLIES
The LMF380 can operate from a total supply voltage (V
V
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 mF located close to the chip An additional 1 mF to
10 mF 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 in Figure 2 for
clock-to-center-frequency ratios of 50 1 and 100 1
b
) ranging from 4 0V up to 14V but the choice of supply
a
b
6
ALIASING
Another important characteristic of sampled-data systems is
their effect on signals at frequencies greater than one-half
the sampling frequency f
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
the input frequency was f
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
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 in Figure 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
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 mV 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
a
FIGURE 2 Switched-Capacitor Filter Output Waveform
120 mV
S
2
a
10 Hz will cause the system to respond as though
Note the sampling ‘‘steps’’
S
S
S
2
(The LMF380’s sampling fre-
b
2 In some cases it may be
b
30 mV to a maximum of
10 Hz If this frequency
TL H 11123 – 8

Related parts for lmf380