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LTC1562A

Very Low Noise, Low Distortion Active RC Quad Universal Filter

Linear Technology 

LTC1562

APPLICATIONS INFORMATION

at frequency fO, and for Q > 0.707, a gain peak occurs at

a frequency below fO, as shown in Figure 4.

Basic Bandpass

There are two different ways to obtain a bandpass function

in Figure 3, both of which give the following transfer

function form:

( ( ) ) HBP(s)

=

s2

–HB ωO

+ ωO /Q

/Q s

s + ωO2

ωO = 2πfO and Q are set by R2 and RQ as described previ-

ously in Setting fO and Q. When ZIN is a resistor of value

RIN, a bandpass response results at the V1 output (Figure

6a) with a gain parameter HB = RQ/RIN. Alternatively, a

capacitor of value CIN gives a bandpass response at the V2

output (Figure 6b), with the same HBP(s) expression, and

the gain parameter now HB = (RQ/10kΩ)(CIN/159pF). This

transfer function has a gain magnitude of HB (its peak value)

when the frequency equals fO and has a phase shift of 180°

at that frequency. Q measures the sharpness of the peak

(the ratio of fO to – 3dB bandwidth) in a 2nd order bandpass

function, as illustrated in Figure 4.

RIN

VIN

CIN

VIN

RQ R2

RQ R2

VOUT

VOUT

INV V1 V2

INV V1 V2

2nd ORDER

2nd ORDER

1/4 LTC1562

1/4 LTC1562

1562 F06

(a) Resistive Input

(b) Capacitive Input

Figure 6. Basic Bandpass Configurations

Basic Highpass

When ZIN of Figure 3 is a capacitor of value CIN, a highpass

response appears at the V1 output (Figure 7).

( ) V1(s)

VIN(s)

=

HHP(s)

=

s2

+

–HHs2

ωO / Q s + ωO2

Parameters ωO = 2πfO and Q are set by R2 and RQ as

above. The highpass gain parameter is HH = CIN/159pF.

For a 2nd order highpass response the gain magnitude at

frequency fO is QHH, and approaches HH at high frequen-

cies (f >> fO). For Q > 0.707, a gain peak occurs at a

frequency above fO as shown in Figure 4. The transfer

function includes a sign inversion.

CIN

VIN

RQ R2

VOUT

INV V1 V2

2nd ORDER

1/4 LTC1562

1562 F07

Figure 7. Basic Highpass Configuration

Signal Swings

The V1 and V2 outputs are capable of swinging to within

roughly 100mV of each power supply rail. As with any

analog filter, the signal swings in each 2nd order section

must be scaled so that no output overloads (saturates),

even if it is not used as a signal output. (Filter literature

often calls this the “dynamics” issue.) When an unused

output has a larger swing than the output of interest, the

section’s gain or input amplitude must be scaled down to

avoid overdriving the unused output. The LTC1562 can

still be used with high performance in such situations as

long as this constraint is followed.

For an LTC1562 section as in Figure 3, the magnitudes of

the two outputs V2 and V1, at a frequency ω = 2πf, have

the ratio,

| V2(jω)| = (100kHz)

| V1(jω)|

f

regardless of the details of ZIN. Therefore, an input fre-

quency above or below 100kHz produces larger output

amplitude at V1 or V2, respectively. This relationship can

guide the choice of filter design for maximum dynamic

range in situations (such as bandpass responses) where

there is more than one way to achieve the desired fre-

quency response with an LTC1562 section.

9

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