LT6600-2.5
10
660025fe
APPLICATIONS INFORMATION
V
MID
can be allowed to fl oat, but it must be bypassed to
an AC ground with a 0.01F capacitor or some instability
maybe observed. V
MID
can be driven from a low impedance
source, provided it remains at least 1.5V above V
–
and at
least 1.5V below V
+
. An internal resistor divider sets the
voltage of V
MID
. 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 V
MID
.
V
OCM
can be shorted to V
MID
for simplicity. If a different
common mode output voltage is required, connect V
OCM
to a voltage source or resistor network. For 3V and 3.3V
supplies the voltage at V
OCM
must be less than or equal
to the mid-supply level. For example, voltage (V
OCM
) ≤
1.65V on a single 3.3V supply. For power supply voltages
higher than 3.3V the voltage at V
OCM
can be set above
mid-supply, as shown in Table 1. The voltage on V
OCM
should not exceed 1V below the voltage on V
MID
. V
OCM
is a high impedance input.
Table 1. Output Common Range for Various Supplies
SUPPLY
VOLTAGE
DIFFERENTIAL OUT
VOLTAGE SWING
OUTPUT COMMON MODE
RANGE FOR LOW DISTORTION
3V 4V
P-P
1.4V ≤ V
OCM
≤ 1.6V
2V
P-P
1V ≤ V
OCM
≤ 1.6V
1V
P-P
0.75V ≤ V
OCM
≤ 1.6V
5V 8V
P-P
2.4V ≤ V
OCM
≤ 2.6V
4V
P-P
1.5V ≤ V
OCM
≤ 3.5V
2V
P-P
1V ≤ V
OCM
≤ 3.75V
1V
P-P
0.75V ≤ V
OCM
≤ 3.75V
±5V 9V
P-P
–2V ≤ V
OCM
≤ 2V
4V
P-P
–3.5V ≤ V
OCM
≤ 3.5V
2V
P-P
–3.75V ≤ V
OCM
≤ 3.75V
1V
P-P
–4.25V ≤ V
OCM
≤ 3.75V
NOTE: V
OCM
is set by the voltage at this R
IN
. The voltage at V
OCM
should
not exceed 1V below the voltage at V
MID
. To achieve some of the output
common mode ranges shown in the table, the voltage at V
MID
must be set
externally to a value below mid supply.
The LT6600-2.5 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
IN
+
and V
IN
–
in Figure 1) is determined by
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
MID
sets the output
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 1580 feedback
resistor and the external 402 input resistor. The result-
ing 1.25mA common mode DC current in each input
path,must be absorbed by the sources V
IN
+
and V
IN
–
.
V
OCM
sets the 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
OCM
differs from V
MID
by 0.5V,
an additional 625A (312A 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
common mode currents by 36%. If V
MID
is shorted toV
OCM
the 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 625A.