LT6600-2.5

7

660025i

APPLICATIO S I FOR ATIO

In Figure 3 the LT6600-2.5 is providing 12dB of gain. The

common mode output voltage is set to 2V.

Use Figure 4 to determine the interface between the

LT6600-2.5 and a current output DAC. The gain, or “trans-

transimpedance, use the following equation:

A

R

RR

=

+

()

Ω

()

1580

1

12

•

By setting R1 + R2 = 1580

Ω, the gain equation reduces to

A = R1(

Ω).

The voltage at the pins of the DAC is determined by R1,

R2, the voltage on Pin 7 and the DAC output current.

Consider Figure 4 with R1 = 49.9

Ω and R2 = 1540Ω. The

voltage at Pin 7 is 1.65V. The voltage at the DAC pins is

given by:

VV

R

RR

I

RR

RR

mV I

DAC

PIN

IN

IN

=

++

+

+

=+

Ω

7

1

1

2 1580

12

12

26

48 3

••

•

•.

49.6

Ω.

Evaluating the LT6600-2.5

The low impedance levels and high frequency operation of

the LT6600-2.5 require some attention to the matching

networks between the LT6600-2.5 and other devices. The

previous examples assume an ideal (0

Ω) source imped-

ance and a large (1k

Ω) load resistance. Among practical

examples where impedance must be considered is the

evaluation of the LT6600-2.5 with a network analyzer.

Figure 5

Figure 5 is a laboratory setup that can be used to charac-

terize the LT6600-2.5 using single-ended instruments

with 50

Ω source impedance and 50Ω input impedance.

For a 12dB gain configuration the LT6600-2.5 requires a

402

Ω source resistance yet the network analyzer output is

calibrated for a 50

Ω load resistance. The 1:1 transformer,

53.6

Ω and 388Ω resistors satisfy the two constraints

above. The transformer converts the single-ended source

into a differential stimulus. Similarly, the output of the

LT6600-2.5 will have lower distortion with larger load

resistance yet the analyzer input is typically 50

Ω. The 4:1

turns (16:1 impedance) transformer and the two 402

Ω

resistors of Figure 5, present the output of the LT6600-2.5

with a 1600

Ω differential load, or the equivalent of 800Ω

to ground at each output. The impedance seen by the

network analyzer input is still 50

Ω, reducing reflections in

the cabling between the transformer and analyzer input.

Differential and Common Mode Voltage Ranges

The rail-to-rail output stage of the LT6600-2.5 can process

large differential signal levels. On a 3V supply, the output

dissipation in the internal circuitry, the user must limit

The two amplifiers inside the LT6600-2.5 have indepen-

dent control of their output common mode voltage (see

the “Block Diagram” section). The following guidelines

will optimize the performance of the filter.

Pin 7 can be allowed to float; Pin 7 must be bypassed to an

AC ground with a 0.01

µF capacitor or some instability may

be observed. Pin 7 can be driven from a low impedance

Figure 4

–

+

0.1

µF

3.3V

–

+

LT6600-2.5

3

4

1

0.01

µF

CURRENT

OUTPUT

DAC

7

2

8

5

+

–

660025 F04

6

R2

R1

–

+

R2

R1

–

+

0.1

µF

0.1

µF

2.5V

– 2.5V

–

+

LT6600-2.5

3

4

1

7

2

8

5

6

660025 F05

402

Ω

402

Ω

NETWORK

ANALYZER

INPUT

50

Ω

COILCRAFT

TTWB-16A

4:1

NETWORK

ANALYZER

SOURCE

COILCRAFT

TTWB-1010

1:1

50

Ω

53.6

Ω

388

Ω

388

Ω