LT6600CS8-5-PBF LINER [Linear Technology], LT6600CS8-5-PBF Datasheet - Page 9

• Manufacturer Part Number: LT6600CS8-5-PBF
• Description: Very Low Noise, Differential Amplifi er and 5MHz Lowpass Filter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The two amplifi ers 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 fi lter 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
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 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
average of V
–100
–120
–20
–40
–60
–80
20
0
IN
0
+
COMPRESSION POINTS
1dB PASSBAND GAIN
and V
1
+
. An internal resistor divider sets the
3RD HARMONIC
T
A
1MHz INPUT LEVEL (V
= 25°C
IN
2
–
Figure 6
3
in Figure 1) is determined by
3RD HARMONIC
T
2ND HARMONIC
T
A
A
= 85°C
= 25°C
4
2ND HARMONIC
T
1MHz T
1MHz T
A
5
= 85°C
P-P
)
A
A
= 25°C
= 85°C
6
66005 F06
7
–
and at
the power supply level and gain setting (see “Electrical
Characteristics”).
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the LT6600-5
not only provides lowpass fi ltering 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 amplifi er inside
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
the sources V
output voltage of the 2nd differential amplifi er inside the
LT6600-5, and therefore sets the common mode output
voltage of the fi lter. 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 fl ow in the resistors coupling
the 1st differential amplifi er output stage to fi lter output.
Thus, a total of 6.25mA is used to translate the common
mode voltages.
A simple modifi cation 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 fl oor of the spectrum
analyzer and subtract the instrument noise from the fi lter
noise measurement.
IN
+
and V
IN
–
. Pin 2 sets the common mode
LT6600-5
66005fa
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