LTC1562C datasheet(11/28 Pages) LINER

Part #	LTC1562C

Description	Very Low Noise, Low Distortion Active RC Quad Universal Filter

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Manufacturer	LINER [Linear Technology]

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11

LTC1562

APPLICATIONS INFORMATION

external component ZIN, usually a resistor or capacitor.

This component must of course be rated to sustain the

magnitude of voltage imposed on it.

Lowpass “T” Input Circuit

The virtual ground INV input in the Operational Filter block

provides a means for adding an “extra” lowpass pole to

any resistor-input application (such as the basic lowpass,

Figure 5, or bandpass, Figure 6a). The resistor that would

otherwise form ZIN is split into two parts and a capacitor

to ground added, forming an R-C-R “T” network (Figure

9). This adds an extra, independent real pole at a fre-

quency:

f

RC

P

PT

=

π

1

2

where CT is the new external capacitor and RP is the

parallel combination of the two input resistors RINA and

RINB. This pair of resistors must normally have a pre-

scribed series total value RIN to set the filter’s gain as

described above. The parallel value RP can however be set

arbitrarily (to RIN/4 or less) which allows choosing a

convenient standard capacitor value for CT and fine tuning

the new pole with RP.

INV

V1

2nd ORDER

1/4 LTC1562

V2

1562 F09

R2

RQ

RINB

RINA

CT

VIN

Figure 9. Lowpass “T” Input Circuit

The procedure therefore is to begin with the target extra

pole frequency fP. Determine the series value RIN from the

gain requirement. Select a capacitor value CT such that RP

= 1/(2

πfPCT) is no greater than RIN/4, and then choose

RINA and RINB that will simultaneously have the parallel

value RP and the series value RIN. Such RINA and RINB can

be found directly from the expression:

1

2

1

2

4

2

RR

R R

IN

IN P

±

()

–

A practical limitation of this technique is that the CT capaci-

tor values that tend to be required (hundreds or thousands

of pF) can destabilize the op amp in Figure 3 if RINB is too

small, leading to AC errors such as Q enhancement. For this

reason, when RINA and RINB are unequal, preferably the

larger of the two should be placed in the RINB position.

Highpass “T” Input Circuit

A method similar to the preceding technique adds an

“extra” highpass pole to any capacitor-input application

(such as the bandpass of Figure 6b or the highpass of

Figure 7). This method splits the input capacitance CIN into

two series parts CINA and CINB, with a resistor RT to ground

between them (Figure 10). This adds an extra 1st order

highpass corner with a zero at DC and a pole at the

frequency:

f

RC

P

TP

=

π

1

2

where CP = CINA + CINB is the parallel combination of the

two capacitors. At the same time, the total series capaci-

tance CIN will control the filter’s gain parameter (HH in

Basic Highpass). For a given series value CIN, the parallel

value CP can still be set arbitrarily (to 4CIN or greater).

Figure 10. Highpass “T” Input Circuit

INV

V1

2nd ORDER

1/4 LTC1562

V2

1562 F10

R2

RQ

CINB

RT

VIN

CINA

The procedure then is to begin with the target corner (pole)

frequency fP. Determine the series value CIN from the gain

requirement (for example, CIN =HH(159pF)forahighpass).

Select a resistor value RT such that CP = 1/(2πRTfP) is at

least 4CIN, and select CINAand CINBthat will simultaneously

have the parallel value CP and the series value CIN. Such

CINA and CINB can be found directly from the expression:

1

2

1

2

4

2

CC

C C

P

IN P

±

()

–

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