LMF100 datasheet(23/28 Pages) NSC

23 / 28 page

3.0 Applications Information (Continued)

3.2 SINGLE SUPPLY OPERATION

The LMF100 can also operate with a single-ended power

supply.

Figure 20 shows the example filter with a

single-ended power supply. V

A+ and VD+ are again con-

nected to the positive power supply (4 to 15 volts), and V

A−

and V

D− are connected to ground. The AGND pin must be

tied to V

+/2 for single supply operation. This half-supply point

should be very “clean”, as any noise appearing on it will be

treated as an input to the filter. It can be derived from the

supply voltage with a pair of resistors and a bypass capacitor

(

Figure 21a), or a low-impedance half-supply voltage can be

made using a three-terminal voltage regulator or an opera-

tional amplifier (

Figure 21b and Figure 21c). The passive re-

sistor divider with a bypass capacitor is sufficient for many

applications, provided that the time constant is long enough

to reject any power supply noise. It is also important that the

half-supply reference present a low impedance to the clock

frequency, so at very low clock frequencies the regulator or

op-amp approaches may be preferable because they will re-

quire smaller capacitors to filter the clock frequency. The

main power supply voltage should be clean (preferably regu-

lated) and bypassed with 0.1 µF.

3.3 DYNAMIC CONSIDERATIONS

The maximum signal handling capability of the LMF100, like

that of any active filter, is limited by the power supply volt-

ages used. The amplifiers in the LMF100 are able to swing to

within about 1 volt of the supplies, so the input signals must

be kept small enough that none of the outputs will exceed

these limits. If the LMF100 is operating on ±5 volts, for ex-

ample, the outputs will clip at about 8V

p-p. The maximum in-

put voltage multiplied by the filter gain should therefore be

less than 8V

p-p.

Note that if the filter Q is high, the gain at the lowpass or

highpass outputs will be much greater than the nominal filter

gain (

Figure 6). As an example, a lowpass filter withaQof

10 will have a 20 dB peak in its amplitude response at f

0.If

the nominal gain of the filter (H

OLP) is equal to 1, the gain at

0 will be 10. The maximum input signal at f0 must therefore

be less than 800 mV

p-p when the circuit is operated on ±5

volt supplies.

Also note that one output can have a reasonable small volt-

age on it while another is saturated. This is most likely for a

circuit such as the notch in Mode 1 (

Figure 7). The notch out-

put will be very small at f

0, so it might appear safe to apply a

large signal to the input. However, the bandpass will have its

maximum gain at f

0 and can clip if overdriven. If one output

clips, the performance at the other outputs will be degraded,

so avoid overdriving any filter section, even ones whose out-

puts are not being directly used. Accompanying

Figures 7, 8,

9, 10, 11, 12, 13, 14, 15, 16 and Figure 17 are equations la-

beled “circuit dynamics”, which relate the Q and the gains at

the various outputs. These should be consulted to determine

peak circuit gains and maximum allowable signals for a

given application.

3.4 OFFSET VOLTAGE

The LMF100’s switched capacitor integrators have a slightly

higher input offset voltage than found in a typical continuous

time active filter integrator. Because of National’s new LMC-

MOS process and new design techniques the internal offsets

have been minimized, compared to the industry standard

MF10.

Figure 22 shows an equivalent circuit of the LMF100

from which the output dc offsets can be calculated. Typical

values for these offsets with S

A/B tied to V

+ are:

OS1 = opamp offset = ±5mV

OS2 = ±30 mV at 50:1 or 100:1

OS3 = ±15 mV at 50:1 or 100:1

When S

A/B is tied to V

−,V

OS2 will approximately halve. The

dc offset at the BP output is equal to the input offset of the

lowpass integrator (V

OS3). The offsets at the other outputs

depend on the mode of operation and the resistor ratios, as

described in the following expressions.

Mode 1 and Mode 4

Mode 1a

DS005645-32

(a) Resistive Divider with

Decoupling Capacitor

DS005645-33

(b) Voltage Regulator

DS005645-34

Divider

FIGURE 21. Three Ways of Generating V

+/2 for Single-Supply Operation

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