LTC5598 datasheet(18/30 Pages) LINER

18 / 30 page

LTC5588-1

55881fb

APPLICATIONS INFORMATION

It is recommended to compensate the baseband input

impedance in the baseband lowpass filter design in order

to achieve best gain flatness vs baseband frequency. The

S-parameters for (each of) the baseband inputs is given

in Table 1.

Table 1. Single-Ended BB Input Impedance vs Frequency for

EN = High and VDC = 0.5V

FREQUENCY

(MHz)

BB INPUT

IMPEDANCE

REFLECTION COEFFICIENT

MAG

ANGLE

0.1

–3700

1.03

–0.13

–3900-j340

1.03

–0.13

–3700-j950

1.03

–0.37

–3200-j1500

1.03

–0.68

–2100-j1900

1.03

–1.38

–860-j1600

1.03

–2.79

–300-j990

1.03

–5.3

–87-j520

1.03

–10.6

100

–35-j308

1.04

–18.2

140

–16-j226

1.03

–24.8

200

–6-j154

1.02

–36

250

–1.4-j120

1.01

–45

300

1.4-j102

0.99

–52

350

4.4-j87

0.96

–59

400

5.4-j74

0.94

–67

450

7-j66

0.90

–73

500

8.3-j58

0.87

–80

600

9.4-j47

0.82

–92

700

10-j38

0.77

–102

800

10-j32

0.74

–113

900

10.5-j27

0.71

–122

1000

10.5-j23

0.69

–129

The circuit is optimized for a common mode voltage of

0.5V which should be externally applied. The baseband

pins should not be left floating to cause the internal PNP’s

base current to pull the common mode voltage higher

than the 0.55V limit, generating excessive current flow. If

it occurs for an extended period, damage to the IC may

result. In shutdown mode it is recommended to terminate

to ground or to a 0.5V source with a value lower than

200Ω. The PNP’s base current is about –136μA ranging

from –250μA to –50μA.

It is recommended to drive the baseband inputs differen-

tially to reduce even-order distortion products. When a DAC

is used as the signal source, a reconstruction filter should

be placed between the DAC output and the LTC5588-1

baseband inputs to avoid aliasing.

Figure 2 shows a typical baseband interface for zero-IF

repeater application. A 5th-order lowpass ladder filter is

used with –0.3dB cut-off of 60MHz. C1A, C1B, C3A and

C3B are configured in a single-ended fashion in order to

suppress common mode noise. L3A and L3B (0402 size)

are used to compensate for passband droop due to the

finite quality factor of the inductors L1A, L1B, L2A and

L2B (0603 size). R3A and R3B improves the out-of-band

noise performance. R3A = R3B = 0Ω (L3A and L3B omit-

ted) provides best out-of-band noise performance but no

passband droop compensation. In that case, L1A, L1B,

L2A and L2B may have to be increased in size (higher

quality factor) to limit passband droop.

Figure 2: Baseband Interface with 5th-Order Filter and 0.5VCM DAC (Only I Channel is Shown)

BBPI

R2A

165

L2A

250nH

GND

0.5VDC

R2B

165

BBMI

55881 F02

R2C

249

R3A

71.5

R3B

71.5

R1A

71.5

R1B

71.5

L3A

100nH

L3B

100nH

L1A

250nH

L1B

250nH

L2B

250nH

39pF

C1A

47pF

C1B

47pF

C3A

47pF

C3B

47pF

DAC

10mA ±10mA

LTC5598 Datasheet(PDF) 18 Page - Linear Technology

LTC5598 Datasheet(HTML) 18 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Datasheet Download

Link URL