7

LTC1340

should require no additional filtering. Additional filtering

to reduce feedthrough noise is possible by inserting a

Hookup

The two sections of the LTC1340 are carefully shielded

from each other inside the chip, but care must also be

taken in the external hookup to minimize noise at the

amplifier output. The two halves of the chip should only

together and at the common point of AGND and PGND.

Separate PGND and AGND as much as possible. AGND is

the amplifier ground. Connect it to a ground plane and as

to PGND with a 0.1

µF capacitor. Select high quality, low

ESR and low ESL surface mount ceramic capacitors for

both the CP and the VCC bypass capacitors. Poor grade

capacitors will result in unacceptable ripple amplitude or

ringing characteristics. Connect both terminals of the

bypass capacitors as close to the chip as possible to

minimize charge pump output ripple amplitude and ground

currents in the rest of the system. Keep IN and OUT away

traces can be minimized by routing AGND PCB traces as

shield as shown in Figures 1 and 2. Connect the 1nF output

capacitor close to the varactor diode and return it to the

AGND plane. The SHDN and IN pins, should not be allowed

to go below PGND potential as the ESD diode forms an

NPN and bleeds the charge pump output.

Amplifier transconductance is typically 1800

µmho. With a

1nF external capacitor at the amplifier output, the band-

width is 125kHz. The amplifier transconductance varies

with temperature and process. The minimum recom-

The slew rate of the amplifier is:

The amplifier typically sinks or sources 20

µA, allowing it

to slew a 1nF output capacitance at 20V/ms, or 5V in

250

µs.

The on-chip amplifier feedback network is set for a DC gain

of 2.3 with an input offset of 0.35V as shown in the typical

curves. The amplifier allows a rail-to-rail input swing with

a 3V supply and provides a 5V swing at the output. The

to about 100mV above AGND. The input stage of the

common mode range. The output noise of the amplifier is

typically 15

100kHz.

There are two feedthrough signals at the amplifier OUT pin

from the charge pump, the main component at 4MHz and

the second harmonic signal at 8MHz. The 4MHz

feedthrough is typically below 50

Figure 1. Suggested Surface Mount PCB Layout for LTC1340CS8

0.1

µF

0.1

µF

LTC1340CS8

1nF

VARACTOR

DIODE

1340 F01

PIN 1

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.