LTC6401-20

10

640120f

APPLICATIONS INFORMATION

Circuit Operation

The LTC6401-20 is a low noise and low distortion fully

differential op amp/ADC driver with:

• Operation from DC to 1.3GHz –3dB bandwidth

impedance

• Fixed gain of 10V/V (20dB)

• Differential input impedance 200Ω

• Differential output impedance 25Ω

• Differential impedance of output filter 100Ω

The LTC6401-20 is composed of a fully differential amplifier

with on chip feedback and output common mode voltage

control circuitry. Differential gain and input impedance are

set by 100Ω/1000Ω resistors in the feedback network.

Small output resistors of 12.5Ω improve the circuit stability

over various load conditions. They also provide a possible

external filtering option, which is often desirable when the

load is an ADC.

Filter resistors of 50Ω are available for additional filtering.

Lowpass/bandpass filters are easily implemented with just

a couple of external components. Moreover, they offer

single-ended 50Ω matching in wideband applications and

no external resistor is needed.

The LTC6401-20 is very flexible in terms of I/O coupling.

It can be AC- or DC-coupled at the inputs, the outputs or

both. Due to the internal connection between input and

output, users are advised to keep input common mode

voltage between 1V and 1.6V for proper operation. If the

inputs are AC-coupled, the input common mode voltage

circuitry is needed for bias. The LTC6401-20 provides an

driving an ADC directly without external components such

as a transformer or AC coupling capacitors. The input

signal can be either single-ended or differential with only

minor differences in distortion performance.

Input Impedance and Matching

The differential input impedance of the LTC6401-20 is

200Ω. If a 200Ω source impedance is unavailable, then

the differential inputs may need to be terminated to a lower

value impedance, e.g. 50Ω, in order to provide an imped-

ance match to the source. Several choices are available.

One approach is to use a differential shunt resistor (Figure

1). Another approach is to employ a wideband transformer

(Figure 2). Both methods provide a wideband match. The

termination resistor or the transformer must be placed

close to the input pins in order to minimize the reflection

due to input mismatch. Alternatively, one could apply a

narrowband impedance match at the inputs of the LTC6401-

20 for frequency selection and/or noise reduction.

Referring to Figure 3, LTC6401-20 can be easily configured

for single-ended input and differential output without a

balun. The signal is fed to one of the inputs through a

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

Figure 1. Input Termination for Differential 50Ω Input Impedance

Using Shunt Resistor

Figure 2. Input Termination for Differential 50Ω Input Impedance

Using a 1:4 Balun

640120 F01

IN+

OUT–

IN–

OUT+

100

Ω

66.5

Ω

12.5

Ω

1000

Ω

LTC6401-20

100

Ω

25

Ω

25

Ω

1000

Ω

12.5

Ω

50

Ω

50

Ω

13

14

15

16

7

5

6

8

+

–

+OUT

+OUTF

–OUTF

–OUT

+IN

+IN

–IN

–IN

1.7pF

640120 F02

IN+

OUT–

IN–

OUT+

100

Ω

12.5

Ω

1000

Ω

LTC6401-20

100

Ω

25

Ω

25

Ω

1000

Ω

12.5

Ω

50

Ω

50

Ω

13

14

15

16

7

5

6

8

+

–

1:4

•

•

+OUT

+OUTF

–OUTF

–OUT

+IN

+IN

–IN

–IN

1.7pF