LT6604 Linear Technology Corporation, LT6604 Datasheet - Page 11

• Manufacturer Part Number: LT6604
• Description: Dual Very Low Noise, Differential Ampli?er And 5mhz Lowpass Filter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
V
AC ground with a 0.01μF capacitor or some instability may
be 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. The voltage on V
below the voltage on V
not be more than 2V above the voltage on V
a high impedance input.
The LT6604-5 was designed to process a variety of input
signals including signals centered on the mid supply
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
the power supply level and gain setting (see “Electrical
Characteristics”).
MID
OCM
can be allowed to fl oat, but it must be bypassed to an
can be shorted to V
–100
–120
–20
–40
–60
–80
20
0
MID
Figure 6. Differential Voltage Range
IN
0
COMPRESSION POINTS
1dB PASSBAND GAIN
+
MID
. While the internal 11k resistors are well
and V
1
+
3RD HARMONIC
T
can be driven from a low impedance
A
. An internal resistor divider sets the
1MHz INPUT LEVEL (V
= 25°C
2
IN
MID
OCM
–
OCM
3
MID
in Figure 1) is determined by
. The voltage on V
3RD HARMONIC
T
2ND HARMONIC
T
A
A
= 85°C
should not be more than 1V
= 25°C, GAIN = 1
must be less than or equal
4
for simplicity. If a different
OCM
2ND HARMONIC
T
1MHz T
1MHz T
A
5
= 85°C
P-P
can be set above mid
)
A
A
= 25°C
= 85°C
6
6600 F06
7
MID
OCM
. V
–
MID
OCM
should
OCM
and at
.
OCM
) ≤
is
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the LT6604-5
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 LT6604-5 (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 806Ω feedback resistor and the
external 200Ω input resistor. The resulting 2.5mA common
mode DC current in each input path, must be absorbed by
the sources V
output voltage of the 2nd differential amplifi er inside the
LT6604-5 channel, and therefore sets the common mode
output voltage of the fi lter. Since, in the example of Figure
3, V
(625μA 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 6.25mA 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 4mA. Of course,
by AC coupling the inputs of Figure 3 and shorting V
V
Noise
The noise performance of the LT6604-5 channel can be
evaluated with the circuit of Figure 7. Given the low noise
output of the LT6604-5 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 7, measure the total integrated noise
OCM
OCM
, the common mode DC current is eliminated.
differs from V
IN
+
and V
MID
IN
–
. V
by 0.5V, an additional 1.25mA
OCM
MID
sets the common mode
is shorted to V
MID
LT6604-5
sets the output
OCM
11
MID
66045f
the
to