LTC6420IUDC-20-PBF datasheet(9/12 Pages) LINER

Part #	LTC6420IUDC-20-PBF

Description	Dual Matched 1.8GHz Differential Amplifi ers/ADC Drivers

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Manufacturer	LINER [Linear Technology]

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LTC6420-20

9

642020fa

matching network while the other input is connected to the

same matching network and a source resistor. Because the

return ratios of the two feedback paths are equal, the two

outputs have the same gain and thus symmetrical swing. In

general, the single-ended input impedance and termination

resistor RT are determined by the combination of RS, RG

and RF. For example, when RS is 50Ω, it is found that the

single-ended input impedance is 202Ω and RT is 66.5Ω

in order to match to a 50Ω source impedance.

The LTC6420-20 is unconditionally stable. However,

the overall differential gain is affected by both source

impedance and load impedance as follows:

A

V =

V

OUT

V

IN

=

2000

R

S + 200

•

R

L

25

+R

L

Output Impedance Match

The LTC6420-20 can drive an ADC directly without external

output impedance matching. Alternatively, the differential

output impedance of 25Ω can be matched to a higher

value impedance, e.g. 50Ω, by series resistors or an LC

network.

Output Common Mode Adjustment

The output common mode voltage is set by the VOCM pin,

which is a high impedance input. The output common

mode voltage is capable of tracking VOCM in a range from

1.1V to 1.6V. The bandwidth of VOCM control is typically

15MHz, which is dominated by a low pass ﬁlter connected

to the VOCM pin and is aimed to reduce common mode

noise generation at the outputs. The internal common

mode feedback loop has a –3dB bandwidth of 300MHz,

allowing fast rejection of any common mode output voltage

disturbance. The VOCM pin should be tied to a DC bias

voltage with a 0.1μF bypass capacitor. When interfacing

with A/D converters such as the LTC22xx families, the

VOCM pin can be connected to the VCM pin of the ADC.

Driving A/D Converters

The LTC6420-20 has been speciﬁcally designed to interface

directly with high speed A/D converters. The back page of

this data sheet shows the LTC6420-20 driving an LTC2285,

which is a dual 14-bit, 125Msps ADC.

The VOCM pins of the LTC6420-20 are connected to the

VCM pins of the LTC2285, which provide a DC voltage

level of 1.5V. Both ICs are powered from the same 3V

supply voltage.

The inputs to the LTC6420-20 can be conﬁgured in various

ways, as described in the Input Impedance and Matching

section of this data sheet. The outputs of the LTC6420-20

may be connected directly to the analog inputs of an ADC,

or a simple lowpass or bandpass ﬁlter network may be

inserted to reduce out-of-band noise.

Test Circuits

Due to the fully-differential design of the LTC6420 and

its usefulness in applications with differing characteristic

speciﬁcations, two test circuits are used to generate the

information in this data sheet. Test Circuit A is DC1299, a

two-port demonstration circuit for the LTC6420/LTC6421

family. The schematic and silkscreen are shown in Figure 4.

This circuit includes input and output transformers (baluns)

for single-ended-to-differential conversion and impedance

transformation, allowing direct hook-up to a 2-port network

analyzer. There are also series resistors at the output to

avoid loading the ampliﬁer directly with a 50Ω load. Due

to the input and output transformers, the –3dB bandwidth

is reduced from 1.8GHz to approximately 1.3GHz.

Test Circuit B uses a 4-port network analyzer to measure

S-parameters and gain/phase response. This removes the

effects of the wideband baluns and associated circuitry,

for a true picture of the >1GHz S-parameters and AC

characteristics.

APPLICATIONS INFORMATION

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