ML2111CIP datasheet(23/26 Pages) MICRO-LINEAR

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ML2111

OPERATION MODES (Continued)

Mode 1a is a good choice when Butterworth filters are

desired since they have poles in a circle with the same f0.

Figure 31 shows an example of a 4th order, 100kHz

lowpass Butterworth filter clocked at 5MHz.

A monotonic passband response with a smooth transition

band results, showing the circuit's low sensitivity, even

though 1% resistors are used which results in an

approximate value of Q.

Figure 32 gives an example of a 4th order bandpass filter

implemented by cascading 2 sections, each with a Q of

10. This figure shows the amplitude response when fCLK =

7.5MHz, resulting in a center frequency of 150kHz and a

Q of 15.5.

Figure 33 uses mode 1d of a 4th order flter where each

section has a Q of 1, independent of resistor ratios. In this

mode, the input amplifier is outside the damping (Q)

loop. Therefore, its finite bandwidth does not degrade the

response at high frequency. This allows the amplifier to be

used as an anti-aliasing and continuous smoothing fliter

by placing a capacitor across R2.

OFFSETS

Switched capacitor integrators generally exhibit higher

input offsets than discrete RC integrators.

These offsets are mainly the charge injection of the

CMOS switchers into the integrating capacitors. The

internal op amp offsets also add to the overall offset

budget.Figure 35 shows half of the ML2111 filter with its

equivalent input offsets VOS1, VOS2, & VOS3.

The DC offset at the filter bandpass output is always equal

to VOS3. The DC offsets at the remaining two outputs

(Notch and LP) depend on the mode of operation and

external resistor ratios. Table 3 illustrates this.

It is important to know the value of the DC output offsets,

especially when the filter handles input signals with large

dynamic range. As a rule of thumb, the output DC offsets

increase when:

1. The Qs decrease

2. The ratio (fCLK/fo) increases beyond 100:1. This is done

by decreasing either the (R2/R4) or the R6/(R5 + R6)

resistor ratios.

VOS1

VOS3

VOS2

(18)

(17)

(16)

(19)

(20)

Figure 35. Equivalent Input Offsets of ½ of an ML2111 Filter.

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