lt6600cs8-5-trpbf Linear Technology Corporation, lt6600cs8-5-trpbf Datasheet - Page 11

• Manufacturer Part Number: lt6600cs8-5-trpbf
• Description: Very Low Noise, Differential Amplifier And 10mhz Lowpass Filter
• Manufacturer: Linear Technology Corporation

The two amplifiers inside the LT6600-10 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 for single supply
operation.
V
higher capacitor. V
ance source, provided it remains at least 1.5V above V
and at least 1.5V below V
the voltage of V
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 V
V
common mode output voltage is required, connect V
to a voltage source or resistor network. For 3V and 3.3V
supplies the voltage at V
the mid supply level. For example, voltage (V
on a single 3.3V supply. For power supply voltages higher
than 3.3V the voltage at V
The voltage on V
voltage on V
than 2V above the voltage on V
impedance input.
The LT6600-10 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
APPLICATIO S I FOR ATIO
MID
OCM
must be bypassed to an AC ground with a 0.01μF or
can be shorted to V
–120
–100
–20
–40
–60
–80
20
0
MID
0
COMPRESSION POINTS
1dB PASSBAND GAIN
. The voltage on V
2ND HARMONIC
OCM
MID
3RD HARMONIC
1
MID
. While the internal 11k resistors are
U
1MHz INPUT LEVEL (V
should not be more than 1V below the
3RD HARMONIC
85°C
2
can be driven from a low imped-
25°C
OCM
+
OCM
Figure 6
. An internal resistor divider sets
MID
U
85°C
3
must be less than or equal to
can be set above mid supply.
for simplicity. If a different
1MHz 85°C
2ND HARMONIC
25°C
4
OCM
P-P
1MHz 25°C
MID
W
)
should not be more
5
. V
6600 F06
OCM
6
OCM
is a high
U
) ≤1.65V
MID
.
OCM
–
average of V
the power supply level and gain setting (see “Electrical
Characteristics”).
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the
LT6600-10 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. V
common mode voltage of the 1st differential amplifier
inside the LT6600-10 (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 402Ω feedback resistor
and the external 100Ω 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 LT6600-10, and therefore sets the
common mode output voltage of the filter. Since in the
example, Figure 3, V
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 V
V
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.
Noise
The noise performance of the LT6600-10 can be evaluated
with the circuit of Figure 7.
Given the low noise output of the LT6600-10 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.
OCM
the common mode output voltage of both op amp
IN
+
and V
OCM
IN
–
differs from V
in Figure 1) is determined by
IN
+
and V
LT6600-10
IN
MID
MID
–
MID
. V
sets the output
is shorted to
OCM
by 0.5V, an
sets the
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