LT6604-2.5
11
660425fa
APPLICATIONS INFORMATION
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 be more than 1V below the voltage on V
MID
. V
OCM
is
a high impedance input.
Table 1. Output Common Mode Range for Various Supplies
SUPPLY
VOLTAGE
DIFFERENTIAL OUT
VOLTAGE SWING
OUTPUT COMMON MODE RANGE
FOR LOW DISTORTION
3V 4V
P-P
2V
P-P
1V
P-P
1.4V ≤ V
OCM
≤ 1.6V
1V ≤ V
OCM
≤ 1.6V
0.75V ≤ V
OCM
≤ 1.6V
5V 8V
P-P
4V
P-P
2V
P-P
1V
P-P
2.4V ≤ V
OCM
≤ 2.6V
1.5V ≤ V
OCM
≤ 3.5V
1V ≤ V
OCM
≤ 3.75V
0.75V ≤ V
OCM
≤ 3.75V
±5V 9V
P-P
4V
P-P
2V
P-P
1V
P-P
–2V ≤ V
OCM
≤ 2V
–3.5V ≤ V
OCM
≤ 3.5V
–3.75V ≤ V
OCM
≤ 3.75V
–4.25V ≤ V
OCM
≤ 3.75V
NOTE: The voltage at V
OCM
should not be more than 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 LT6604-2.5 was designed to process a variety of
input signals including signals centered on the mid-sup-
ply voltage and signals that swing between ground and
a positive voltage in a single supply system (Figure 1).
The allowable range of the 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 LT6604-
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 LT6604-2.5 channel (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 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 LT6604-2.5 channel, 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 625μA (312μA per side) of DC current will
fl ow in the resistors coupling the 1st differential amplifi er
output stage to the 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
MID
is shorted to V
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 625μA.
Noise
The noise performance of the LT6604-2.5 channel can be
evaluated with the circuit of Figure 6. Given the low noise
output of the LT6604-2.5 and the 6dB attenuation of the
transformer coupling network, it is necessary to measure
the noise fl oor of the spectrum analyzer and subtract the
instrument noise from the fi lter noise measurement.
Example: With the IC removed and the 25Ω resistors
grounded, Figure 6, measure the total integrated noise (e
S
)
of the spectrum analyzer from 10kHz to 2.5MHz. With the
IC inserted, the signal source (V
IN
) disconnected, and the
Figure 6
–
+
0.1μF
0.1μF
2.5V
–2.5V
–
+
1/2
LT6604-2.5
R
IN
R
IN
25Ω
25Ω
660425 F06
SPECTRUM
ANALYZER
INPUT
50Ω
V
IN
COILCRAFT
TTWB-1010
1:1
25
27
4
34
6
2
29
7
