LTC6948 datasheet(22/32 Pages) AD

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LTC5594

Rev A

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APPLICATIONS INFORMATION

conversion gain. To operate at lower common mode volt-

ages, a lower termination resistance can be used on each

output at the expense of conversion gain, or a negative

supply can be used at the connection of the termination

resistors. Figure 12 shows the OIP3 of the amplifier alone

with various common mode voltages.

The amplifier gain can be adjusted in eight steps of roughly

1dB from 8dB to 15dB using the AMPG register. Setting

AMPG = 0x7 sets the gain at about 15dB and setting

AMPG = 0x0 sets the gain to about 8dB.

Figure 11. Simplified Schematic of the IF Amplifier Output with Anti-Alias Filter

Figure 12. OIP3 of Amplifier Only vs

Output Common Mode Voltage (VCM)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

IF FREQUENCY (GHz)

0.5V

0.7V

0.9V

1.2V

1.5V

1.8V

IF TONESPACING = 1MHz

PIF = –1.5dB/TONE

TC = 55°C

5586 F12

A typical anti-alias filter is shown in Figure 11 for interface

with an ADC. The parallel combinations of R3||R7 and

R4||R8 set the differential impedance for the ADC. The

input and output of the filter contain a common mode

terminationforhighfrequencies.TheseareformedbyC17,

C18 and 24.9Ω at the input and C23, C24 and 24.9Ω at

theoutput.Thecommonmodeterminationattheamplifier

output ensures stability and the common mode termina-

tion at the ADC input provides a termination for the high

frequency kickback from the sampling capacitors in the

ADC. Table 5 shows some typical values vs 1dB cutoff

frequency for the anti-alias filter. To optimize the flatness

and ripple of the IF band, both the IF interstage filter and

the anti-alias filter can be designed together in a simulator

including package parasitics. The additional slope due to

RF slope and HSLO or LSLO can be compensated by us-

ing this method. The layout of the anti-alias filter should

be done so that the amplifier outputs and ADC inputs are

as close as possible. This is to prevent long line lengths

from introducing additional parasitics.

LTC5594

1.5nH

0.2pF

C17

C18

24.9

68.1

C21

C20

C23

C24

24.9

200

AC CURRENT

SOURCE

PACKAGE

PARASITICS

IFIP

IFIM

VCM

GND

5TH ORDER

ANTI-ALIAS

FILTER

5594 F11

0.9V

ZOUT = 100

–

VCC

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