LT6600-5

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APPLICATIONS INFORMATION

The two amplifiers inside the LT6600-5 have independent

control of their output common mode voltage (see the

Block Diagram section). The following guidelines will

optimize the performance of the filter for single supply

operation.

Pin 7 must be bypassed to an AC ground with a 0.01μF or

higher 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 can be set above mid-supply.

The voltage on Pin 2 should not be more than 1V below

the voltage on Pin 7. The voltage on Pin 2 should not be

more than 2V above the voltage on Pin 7. Pin 2 is a high

impedance input.

The LT6600-5 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 the Electrical

Characteristics section).

Common Mode DC Currents

In applications like Figure 1 and Figure 3 where the LT6600-5

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 in-

side the LT6600-5 (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 806Ω feedback resistor and the

external 200Ω input resistor. The resulting 2.5mA common

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

output voltage of the 2nd differential amplifier inside the

LT6600-5, and therefore sets the common mode output

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

differs from Pin 7 by 0.5V, an additional 1.25mA (0.625mA

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

the 1st differential amplifier output stage to filter output.

Thus, a total of 6.25mA is used to translate the common

mode voltages.

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

mon mode currents by 36%. 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 4mA. Of course,

by AC coupling the inputs of Figure 3 and shorting Pin 7

to Pin 2, the common mode DC current is eliminated.

Noise

The noise performance of the LT6600-5 can be evaluated

with the circuit of Figure 7.

Given the low noise output of the LT6600-5 and the 6dB

attenuation of the transformer coupling network, it will

be necessary to measure the noise floor of the spectrum

analyzer and subtract the instrument noise from the filter

noise measurement.

Figure 6

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1dB PASSBAND GAIN

COMPRESSION POINTS

3RD HARMONIC

3RD HARMONIC

2ND HARMONIC

2ND HARMONIC