11

LTC1563-2/LTC1563-3

156323fa

Output Loading: Resistive and Capacitive

The op amps of the LTC1563-X have a rail-to-rail output

stage. To obtain maximum performance, the output load-

ing effects must be considered. Output loading issues can

be divided into resistive effects and capacitive effects.

Resistive loading affects the maximum output signal swing

and signal distortion. If the output load is excessive, the

output swing is reduced and distortion is increased. All of

the output voltage swing testing on the LTC1563-X is done

with R22 = 100k and a 10k load resistor. For best undistorted

output swing, the output load resistance should be greater

than 10k.

Capacitive loading on the output reduces the stability of

the op amp. If the capacitive loading is sufficiently high,

the stability margin is decreased to the point of oscillation

at the output. Capacitive loading should be kept below

30pF. Good, tight layout techniques should be maintained

at all times. These parts should not drive long traces and

must never drive a long coaxial cable.

When probing the

LTC1563-X, always use a 10x probe. Never use a 1x probe.

A standard 10x probe has a capacitance of 10pF to 15pF

while a 1x probe’s capacitance can be as high as 150pF.

The use of a 1x probe will probably cause oscillation.

For larger capacitive loads, a series isolation resistor can

be used between the part and the capacitive load. If the

load is too great, a buffer must be used.

Layout Precautions

The LTC1563-X is an active RC filter. The response of the

filter is determined by the on-chip capacitors and the

external resistors. Any external, stray capacitance in par-

allel with an on-chip capacitor, or to an AC ground, can

alter the transfer function.

Capacitance to an AC ground is the most likely problem.

Capacitance on the LPA or LPB pins does not affect the

transfer function but does affect the stability of the op

amps. Capacitance on the INVA and INVB pins will affect

the transfer function somewhat and will also affect the

stability of the op amps. Capacitance on the SA and SB

pins alters the transfer function of the filter. These pins are

the most sensitive to stray capacitance. Stray capacitance

on these pins results in peaking of the frequency response

near the cutoff frequency. Poor layout can give 0.5dB to

1dB of excess peaking.

To minimize the effects of parasitic layout capacitance, all

of the resistors for section A should be placed as close as

possible to the SA pin. Place the R31 resistor first so that

it is as close as possible to the SA pin on one end and as

close as possible to the INVA pin on the other end. Use the

same strategy for the layout of section B, keeping all of the

resistors as close as possible to the SB node and first

placing R32 between the SB and INVB pins. It is also best

if the signal routing and resistors are on the same layer as

the part without any vias in the signal path.

Figure 1 illustrates a good layout using the LTC1563-X

with surface mount 0805 size resistors. An even tighter

layout is possible with smaller resistors.

1653 F01

R11

LTC1563-X

R12

Figure 1. PC Board Layout

Single Pole Sections and Odd Order Filters

The LTC1563 is configured to naturally form even ordered

filters (2nd, 4th, 6th and 8th). With a little bit of work,

single pole sections and odd order filters are easily achieved.

To form a single pole section you simply use the op amp,

the on-chip C1 capacitor and two external resistors as

shown in Figure 2. This gives an inverting section with the

gain set by the R2-R1 ratio and the pole set by the R2-C1

time constant. You can use this pole with a 2nd order

section to form a noninverting gain 3rd order filter or as a

stand alone inverting gain single pole filter.

Figure 3 illustrates another way of making odd order

filters. The R1 input resistor is split into two parts with an

additional capacitor connected to ground in between the

resistors. This “TEE” network forms a single real pole. RB1