LT6600 Linear Technology Corporation, LT6600 Datasheet - Page 11

• Manufacturer Part Number: LT6600
• Description: Very Low Noise, Differential Ampli?er And 2.5mhz Lowpass Filter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
V
an AC ground with a 0.01μF capacitor or some instability
maybe observed. V
source, provided it remains at least 1.5V above V
least 1.5V below V
voltage of V
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
to the mid supply level. For example, voltage (V
1.65V on a single 3.3V supply. For power supply voltages
higher than 3.3V the voltage at V
supply, as shown in Table 1. The voltage on V
not exceed 1V below the voltage on V
impedance input.
Table 1. Output Common Range for Various Supplies
NOTE: V
the voltage at V
voltage at V
The LT6600-2.5 was designed to process a variety of input
signals including signals centered around the mid-sup-
MID
OCM
VOLTAGE
SUPPLY
±5V
3V
5V
OCM
can be allowed to fl oat, but it must be bypassed to
can be shorted to V
MID
is set by the voltage at this R
MID
must be set externally to a value below mid supply.
. To achieve some of the output common mode ranges shown in the table, the
MID
DIFFERENTIAL OUT
VOLTAGE SWING
. While the internal 11k resistors are well
4V
2V
1V
8V
4V
2V
1V
9V
4V
2V
1V
MID
+
. An internal resistor divider sets the
P-P
P-P
P-P
P-P
P-P
P-P
P-P
P-P
P-P
P-P
P-P
can be driven from a low impedance
OCM
IN
MID
. The voltage at V
must be less than or equal
for simplicity. If a different
RANGE FOR LOW DISTORTION
OCM
OUTPUT COMMON MODE
–3.75V ≤ V
–4.25V ≤ V
0.75V ≤ V
0.75V ≤ V
–3.5V ≤ V
1.4V ≤ V
2.4V ≤ V
1.5V ≤ V
1V ≤ V
can be set above mid
1V ≤ V
OCM
–2V ≤ V
MID
should not exceed 1V below
. V
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
≤ 3.75V
≤ 1.6V
OCM
≤ 1.6V
≤ 2.6V
≤ 3.5V
≤ 3.75V
≤ 1.6V
≤ 2V
≤ 3.5V
≤ 3.75V
≤ 3.75V
is a high
–
MID
OCM
should
and at
.
OCM
) ≤
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
the power supply level and gain setting (see “Electrical
Characteristics”).
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the LT6600-2.5
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. V
common mode voltage of the 1st differential amplifi er inside
the LT6600-2.5 (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 1580Ω feedback resistor and
the external 402Ω input resistor. The resulting 1.25mA
common mode DC current in each input path,must be
absorbed by the sources V
common mode output voltage of the 2nd differential
amplifi er inside the LT6600-2.5, and therefore sets the
common mode output voltage of the fi lter. Since, in the
example of Figure 3, V
additional 625μA (312μA 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 3.125mA 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 V
common mode output voltage of both op amp stages will
be 2V and the resulting DC current will be 2mA. Of course,
by AC coupling the inputs of Figure 3, the common mode
DC current can be reduced to 625μA.
IN
+
and V
IN
OCM
–
in Figure 1) is determined by
differs from V
IN
+
and V
MID
LT6600-2.5
is shorted toV
IN
MID
–
. V
MID
sets the output
OCM
by 0.5V, an
sets the
OCM
11
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the