LT6600-20

8

66002fb

APPLICATIONS INFORMATION

In Figure 3 the LT6600-20 is providing 12dB of gain. The

gain resistor has an optional 62pF in parallel to improve

the passband flatness near 20MHz. The common mode

output voltage is set to 2V.

Use Figure 4 to determine the interface between the

LT6600-20 and a current output DAC. The gain, or “trans-

transimpedance, use the following equation:

A

=

402 •R1

R1

+R2

()

Ω

()

By setting R1 + R2 = 402Ω, 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 = 49.9Ω and R2 = 348Ω. The voltage at

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

R1

R1

+R2+ 402

R1•R2

R1

+R2

50.4Ω.

Evaluating the LT6600-20

The low impedance levels and high frequency operation

ofthe LT6600-20 require some attention to the matching

networks between the LT6600-20 and other devices. The

previous examples assume an ideal (0Ω) source impedance

and a large (1kΩ) load resistance. Among practical ex-

amples where impedance must be considered is the evalu-

ation of the LT6600-20 with a network analyzer. Figure 5

is a laboratory setup that can be used to characterize

the LT6600-20 using single-ended instruments with 50

source impedance and 50Ω input impedance. For a unity

gain configuration the LT6600-20 requires a 402Ω source

resistance yet the network analyzer output is calibrated

for a 50Ω load resistance. The 1:1 transformer, 53.6Ω

and 388Ω resistors satisfy the two constraints above.

The transformer converts the single-ended source into a

differential stimulus. Similarly, the output of the LT6600-20

will have lower distortion with larger load resistance yet

the analyzer input is typically 50Ω. The 4:1 turns (16:1

impedance) transformer and the two 402Ω resistors of

Figure 5, present the output of the LT6600-20 with a 1600Ω

differential load, or the equivalent of 800Ω to ground at

each output. The impedance seen by the network analyzer

input is still 50Ω, reducing reflections in the cabling be-

tween the transformer and analyzer input.

Differential and Common Mode Voltage Ranges

The differential amplifiers inside the LT6600-20 contain

circuitry to limit the maximum peak-to-peak differential

voltage through the filter. This limiting function prevents

excessive power dissipation in the internal circuitry and

provides output short-circuit protection. The limiting

function begins to take effect at output signal levels above

illustrated in Figure 6; the LT6600-20 was configured with

unity passband gain and the input of the filter was driven

with a 1MHz signal. Because this voltage limiting takes

Figure 4

Figure 5

–

+

0.1μF

3.3V

–

+

LT6600-20

3

4

1

0.01μF

CURRENT

OUTPUT

DAC

7

2

8

5

+

–

66002 F04

6

R2

R1

–

+

R2

R1

–

+

0.1μF

0.1μF

2.5V

–2.5V

–

+

LT6600-20

3

4

1

7

2

8

5

6

66002 F05

402Ω

402Ω

NETWORK

ANALYZER

INPUT

50Ω

COILCRAFT

TTWB-16A

4:1

NETWORK

ANALYZER

SOURCE

COILCRAFT

TTWB-1010

1:1

50Ω

53.6Ω

388Ω

388Ω