LT1568

8

1568f

PI FU CTIO S

be tied together and bypassed with a 0.1

µF capacitor to an

adequate analog ground plane using the shortest possible

wiring.

INVA, INVB (Pins 2, 15): Inverting Input. Each of the INV

pins is an inverting input of an op amp. Note that the INV

pins are high impedance, and are susceptible to coupling

of unintended signals. External parasitic capacitance on

the INV nodes will also affect the frequency response of

the filter sections. For these reasons, printed circuit con-

nections to the INV pins must be kept as short as possible.

SA, SB (Pins 3, 14): Summing Pins. These pins are a

summing junction for input signals. Stray capacitance on

the SA or SB pins may cause “small” frequency errors of

the frequency response near the cutoff frequency (or

center frequency). The three external resistors for each

section should be located as close as possible to the SA or

SB pin to minimize stray capacitance (one picofarad of

stray capacitance may add up to 0.1% frequency error).

OUTA, OUTB (Pins 4, 13): Lowpass Output. These pins

are the rail-to-rail outputs of op amps. Each output is

designed to drive a nominal net load of 400

Ω and 30pF.

OUTA, OUTB (Pins 5, 12): These pins are the inverted

versions of the OUTA and OUTB outputs respectively. Each

output is designed to drive a nominal load of 400

Ω and

30pF.

GNDA (Pin 6): GNDA serves as the common mode refer-

ence voltage for section A. It should be tied to the analog

ground plane in a dual supply system. In a single-supply

system, an internal resistor divider can be used to estab-

lish a half-supply reference point. In that case, GNDA must

µF capacitor.

NC (Pin 7): This pin is not connected internally and can be

connected to ground.

be tied together and bypassed to GND by a 0.1

µF capacitor

in a dual-supply system. In a single-supply system, tie

these pins to the ground plane.

EN (Pin 10): ENABLE. When the EN input goes high or is

open circuited, the LT1568 enters a shutdown state which

reduces the supply current to approximately 0.5mA

assume high impedance states. GNDA will continue to be

biased at half-supply. If an input signal is applied to a

complete filter circuit while the LT1568 is in shutdown,

some signal will normally flow to the output through

passive components around the inactive IC.

of approximately 40k. This defaults the LT1568 to the

shutdown state if the EN pin is left floating. Therefore, the

user must connect the EN pin to a voltage equal to or less

voltage should be 2.9V or less.)

GNDB (Pin 11): GNDB serves as the common mode

reference voltage for section B. It should be tied to the

analog ground plane in a dual supply system. In a single-

supply system, GNDB can be tied to GNDA to set the

common mode voltage at half-supply. If it is tied to

another reference voltage, GNDB should be bypassed to

µF capacitor.