LT6600-20 Linear Technology, LT6600-20 Datasheet - Page 9

• Manufacturer Part Number: LT6600-20
• Description: Very Low Noise Differential Amplifier and 20MHz Lowpass Filter
• Manufacturer: Linear Technology

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 LT6600-20 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 0.01 F or
larger capacitor. Pin 7 can be driven from a low impedance
source, provided it remains at least 1.5V above V
least 1.5V below V
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.
APPLICATIO S I FOR ATIO
–100
–120
–20
–40
–60
–80
20
Figure 6. Output Level vs Input Level,
Differential 1MHz Input, Gain = 1
0
0
COMPRESSION POINTS
3RD HARMONIC
1dB PASSBAND GAIN
1
3RD HARMONIC
+
U
. An internal resistor divider sets the
1MHz INPUT LEVEL (V
2
25 C
85 C
3
U
4
2ND HARMONIC
85 C
1MHz 85 C
2ND HARMONIC
25 C
5
P-P
1MHz 25 C
W
)
6
66002 F06
7
U
–
and at
The LT6600-20 was designed to process a variety of input
signals including signals centered around the mid-supply
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
average of V
the power supply level and gain setting (see Distortion vs
Input Common Mode Level in the Typical Performance
Characteristics).
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the
LT6600-20 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 be-
tween 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 LT6600-20 (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 402 feedback resistor
and the external 100 input resistor. The resulting 5mA
common mode DC current in each input path, must be
absorbed by the sources V
common mode output voltage of the 2nd differential
amplifier inside the LT6600-20, 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 12.5mA is
used to translate the common mode voltages.
A simple modification to Figure 3 will reduce the DC
common 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 8mA.
Of course, by AC coupling the inputs of Figure 3, the
common mode DC current can be reduced to 2.5mA.
IN
+
and V
IN
–
in Figure 1) is determined by
IN
+
and V
www.DataSheet4U.com
LT6600-20
IN
–
. Pin 2 sets the
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