LTC6401-8

9

64018f

APPLICATIONS INFORMATION

Circuit Operation

The LTC6401-8 is a low noise and low distortion fully

differential op amp/ADC driver with:

• Operation from DC to 2.2GHz –3dB bandwidth

• Fixed gain of 2.5V/V (8dB)

• Differential input impedance 400Ω

• Differential output impedance 25Ω

• Differential impedance of output filter 100Ω

The LTC6401-8 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 200Ω/500Ω 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 LTC6401-8 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

and thus no external circuitry is needed for bias. The

LTC6401-8 provides an output common mode voltage

external components such as transformer or AC coupling

capacitors. The input signal can be either single-ended

or differential with only minor difference in distortion

performance.

Input Impedance and Matching

The differential input impedance of the LTC6401-8 is 400Ω.

Usually the differential inputs need to be terminated to a

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

impedance match for the source. Several choices are avail-

able. One approach is to use a differential shunt resistor

(Figure 1). Another approach is to employ a wideband

transformer and shunt resistor (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 LTC6401-8 for frequency selec-

tion and/or noise reduction.

Figure 1. Input Termination for Differential 50Ω Input Impedance

Using Shunt Resistor

Figure 2. Input Termination for Differential 50Ω Input Impedance

Using a Balun

64018 F01

+OUT

+OUTF

–OUTF

–OUT

+IN

IN+

OUT–

IN–

OUT+

+IN

–IN

–IN

200

Ω

57.6

Ω

12.5

Ω

500

Ω

LTC6401-8

200

Ω

25

Ω

25

Ω

500

Ω

12.5

Ω

50

Ω

50

Ω

2.7pF

13

14

15

16

7

5

6

8

+

–

64018 F02

+OUT

+OUTF

–OUTF

–OUT

+IN

IN+

OUT–

IN–

OUT+

+IN

–IN

MINI CIRCUITS

TCM4-19

–IN

200

Ω

12.5

Ω

500

Ω

LTC6401-8

200

Ω

25

Ω

402

Ω

25

Ω

500

Ω

12.5

Ω

50

Ω

50

Ω

2.7pF

13

14

15

16

7

5

6

8

+

–

1:4

•