LT6604-2.5 Linear Technology, LT6604-2.5 Datasheet - Page 11

• Manufacturer Part Number: LT6604-2.5
• Description: Dual Very Low Noise Differential Amplifier and 2.5MHz Lowpass Filter
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
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 be more than 1V below the voltage on V
a high impedance input.
Table 1. Output Common Mode Range for Various Supplies
NOTE: The voltage at V
at V
table, the voltage at V
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
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
common mode voltage of the 1st differential amplifi er inside
the LT6604-2.5 channel (see the “Block Diagram” section)
VOLTAGE
OCM
SUPPLY
MID
±5V
3V
5V
. To achieve some of the output common mode ranges shown in the
can be shorted to V
DIFFERENTIAL OUT
VOLTAGE SWING
MID
4V
2V
1V
8V
4V
2V
1V
9V
4V
2V
1V
OCM
IN
P-P
P-P
P-P
P-P
P-P
P-P
P-P
P-P
P-P
P-P
P-P
must be set externally to a value below mid supply.
+
should not be more than 1V below the voltage
and V
OCM
MID
IN
–
OUTPUT COMMON MODE RANGE
must be less than or equal
for simplicity. If a different
in Figure 1) is determined
OCM
FOR LOW DISTORTION
–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
1V ≤ V
–2V ≤ V
can be set above mid
MID
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
OCM
sets the output
≤ 3.75V
≤ 1.6V
MID
≤ 1.6V
≤ 2.6V
≤ 3.5V
≤ 3.75V
≤ 1.6V
≤ 2V
≤ 3.5V
≤ 3.75V
≤ 3.75V
OCM
. V
OCM
should
OCM
OCM
) ≤
is
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
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
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
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
of the spectrum analyzer from 10kHz to 2.5MHz. With the
IC inserted, the signal source (V
V
IN
R
R
IN
IN
34
4
6
2
1/2
LT6604-2.5
–
+
25
–2.5V
2.5V
7
+
–
Figure 6
0.1μF
0.1μF
OCM
27
29
differs from V
IN
www.DataSheet4U.com
) disconnected, and the
25Ω
25Ω
MID
IN
LT6604-2.5
+
COILCRAFT
TTWB-1010
and V
is shorted to V
1:1
IN
–
MID
. V
SPECTRUM
ANALYZER
INPUT
OCM
by 0.5V,
11
660425 F06
50Ω
660425f
sets
OCM
S
)