LT6600-15

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660015fb

APPLICATIONS INFORMATION

supply rails, the input/output behavior of the IC shown

in Figure 6 is relatively independent of the power supply

voltage.

The two amplifiers inside the LT6600-15 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 must be bypassed to an AC ground with a 0.01μF or

larger capacitor. Pin 7 can be driven from a low impedance

voltage of Pin 7. While the internal 11k resistors are well

matched, their absolute value can vary by ±20%. This

should be taken into consideration when connecting an

external resistor network to alter the voltage of Pin 7.

Pin 2 can be shorted to Pin 7 for simplicity. If a different

common mode output voltage is required, connect Pin 2

to a voltage source or resistor network. For 3V and 3.3V

supplies the voltage at Pin 2 must be less than or equal to

the mid supply level. For example, voltage (Pin 2) ≤ 1.65V

on a single 3.3V supply. For power supply voltages higher

than 3.3V the voltage at Pin 2 should be within the voltage

of Pin 7 – 1V to the voltage of Pin 7 + 2V. Pin 2 is a high

impedance input.

The LT6600-15 was designed to process a variety of input

signals including signals centered around the mid-sup-

ply voltage and signals that swing between ground and

a positive voltage in a single supply system (Figure 1).

The range of allowable input common mode voltage (the

the power supply level and gain setting (see Distortion vs

Input Common Mode Level in the Typical Performance

Characteristics section).

Common Mode DC Currents

In applications like Figure 1 and Figure 3 where the

LT6600-15 not only provides lowpass filtering but also level

shifts the common mode voltage of the input signal, DC

currents will be generated through the DC path between

input and output terminals. Minimize these currents to

decrease power dissipation and distortion.

Consider the application in Figure 3. Pin 7 sets the output

common mode voltage of the 1st differential amplifier

inside the LT6600-15 (see the Block Diagram section) at

2.5V. Since the input common mode voltage is near 0V, there

will be approximately a total of 2.5V drop across the series

combination of the internal 536Ω feedback resistor and the

external 133Ω input resistor. The resulting 3.7mA common

mode DC current in each input path, must be absorbed by

output voltage of the 2nd differential amplifier inside the

LT6600-15, and therefore sets the common mode output

voltage of the filter. Since, in the example of Figure 3, Pin 2

differs from Pin 7 by 0.5V, an additional 2.5mA (1.25mA

per side) of DC current will flow in the resistors coupling

the 1st differential amplifier output stage to filter output.

Thus, a total of 9.9mA is used to translate the common

mode voltages.

A simple modification to Figure 3 will reduce the DC com-

mon mode currents by 40%. If Pin 7 is shorted to Pin 2

the common mode output voltage of both op amp stages

will be 2V and the resulting DC current will be 6mA. Of

course, by AC coupling the inputs of Figure 3, the common

mode DC current can be reduced to 2.5mA.

Figure 6. Output Level vs Input Level,

Differential 1MHz Input, Gain = 1

0

20

0

–20

–40

–60

–80

–100

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660015 F06

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46

7

3RD HARMONIC

85°C

1dB COMPRESSION

POINTS

25°C

85°C

3RD HARMONIC

25°C

2ND HARMONIC, 25°C

2ND

HARMONIC

85°C