LTC5591 datasheet(15/20 Pages) LINER

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LTC5592

5592f

IF Outputs

The IF amplifiers in channels A and B are identical. The IF

amplifier for channel A, shown in Figure 7, has differen-

tial open collector outputs (IFA+ and IFA–), a DC ground

return pin (IFGNDA), and a pin for adjusting the internal

bias (IFBA). The IF outputs must be biased at the sup-

ply voltage (VCCIFA), which is applied through matching

inductors L1A and L2A. Alternatively, the IF outputs can

be biased through the center tap of a transformer (T1A).

The common node of L1A and L2A can be connected to

the center tap of the transformer. Each IF output pin draws

approximately 50.5mA of DC supply current (101mA total).

An external load resistor, R2A, can be used to improve

impedance matching if desired.

IFGNDA (Pin 23) must be grounded or the amplifier will

not draw DC current. Inductor L3A may improve LO-IF

and RF-IF leakage performance in some applications, but

is otherwise not necessary. Inductors should have small

resistance for DC. High DC resistance in L3A will reduce

the IF amplifier supply current, which will degrade RF

performance.

APPLICATIONS INFORMATION

For optimum single-ended performance, the differential

IF output must be combined through an external IF

transformer or a discrete IF balun circuit. The evaluation

board (see Figures 1 and 2) uses a 4:1 IF transformer for

impedance transformation and differential to single-ended

conversion. It is also possible to eliminate the IF transformer

and drive differential filters or amplifiers directly.

The IF output impedance can be modeled as 379Ω in

parallel with 2.2pF. The equivalent small-signal model,

including bondwire inductance, is shown in Figure 8.

Frequency-dependent differential IF output impedance is

listed in Table 3. This data is referenced to the package

pins (with no external components) and includes the ef-

fects of IC and package parasitics.

Figure 7. IF Amplifier Schematic with Bandpass Match

Figure 8. IF Output Small-Signal Model

Bandpass IF Matching

The bandpass IF matching configuration, shown in Figures

1 and 7, is best suited for IF frequencies in the 90MHz to

500MHz range. Resistor R2A may be used to reduce the IF

output resistance for greater bandwidth and inductors L1A

and L2A resonate with the internal IF output capacitance

at the desired IF frequency. The value of L1A, L2A can be

estimated as follows:

L1A = L2A =

2 fIF

()2 •2•CIF

where CIF is the internal IF capacitance (listed in Table 3).

IFA+

IF A–

0.9nH

RIF

CIF

LTC5592

5592 F08

4:1

T1A

IFA

C7A

L2A

L1A

C5A

R2A

L3A (OR SHORT)

VCCIFA

AMP

BIAS

101mA

4mA

IFBA

VCCA

LTC5592

IGNDA

IFA–

IFA+

R1A

(OPTION TO

REDUCE

DC POWER)

5592 F07

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