11

64026fa

LT6402-6

The LT6402-6 has been designed to minimize the need

for external support components such as transformers or

AC-coupling capacitors. As an ADC driver, the LT6402-6

requires no external components except for power-supply

bypass capacitors. This allows DC-coupled operation for

applications that have frequency ranges including DC.

At the outputs, the common mode voltage is set via the

output AC-coupling capacitors or transformers are needed.

At the inputs, signals can be differential or single-ended

with virtually no difference in performance. Furthermore,

DC levels at the inputs can be set independently of the

output common mode voltage. These input characteristics

often eliminate the need for an input transformer and/or

AC-coupling capacitors.

Input Impedance and Matching Networks

Calculation of the input impedance of the LT6402-6 is not

straightforward from examination of the block diagram

because of the internal feedback network. In addition, the

input impedance when driven differentially is different than

when driven single-ended.

DIFFERENTIAL

SINGLE-ENDED

LT6402-6

200Ω

133Ω

For single-ended 50Ω applications, an 80.6Ω shunt

matching resistor to ground will result in the proper input

termination (Figure 1). For differential inputs there are

several termination options. If the input source is 50Ω

differential, then the input matching can be accomplished

by either a 67Ω shunt resistor across the inputs (Figure 3),

or equivalent 33Ω shunt resistors on each of the inputs

to ground (Figure 2).

Figure 1. Input Termination for Single-Ended

50Ω Input Impedance

Single-Ended to Differential Operation

The LT6402-6’s performance with single-ended inputs is

comparable to its performance with differential inputs.

This excellent single-ended performance is largely due

to the internal topology of the LT6402-6. Referring to

the block diagram, if the +INA and +INB pins are driven

with a single-ended signal (while –INA and –INB are tied

to AC ground), then the +OUT and –OUT pins are driven

differentially without any voltage swing needed from

amplifier C. Single-ended to differential conversion using

more conventional topologies suffers from performance

limitations due to the common mode amplifier.

Driving ADCs

The LT6402-6 has been specifically designed to interface

directly with high speed Analog to Digital Converters

(ADCs). In general, these ADCs have differential inputs, with

an input impedance of 1kΩ or higher. In addition, there is

generally some form of lowpass or bandpass filtering just

prior to the ADC to limit input noise at the ADC, thereby

improving system signal to noise ratio. Both the unfiltered

and filtered outputs of the LT6402-6 can easily drive the

Figure 3. Alternate Input Termination for Differential

50Ω Input Impedance

Figure 2. Input Termination for Differential 50Ω Input Impedance

APPLICATIONS INFORMATION

64026 F01

IF IN

0.1μF

LT6402-6

–INA

–INB

–OUT

+OUT

8

5

+INB

+INA

14

13

15

80.6Ω

SINGLE-ENDED

16

64026 F02

LT6402-6

–INA

–INB

–OUT

+OUT

8

5

+INB

+INA

14

13

15

33Ω

DIFFERENTIAL

16

33Ω

64026 F03

LT6402-6

–INA

–INB

–OUT

+OUT

8

5

+INB

+INA

14

13

15

DIFFERENTIAL

16

67Ω