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LT6600CS8-20#TRPBF

P1-P3P4-P6P7-P9P10-P12
LT6600-20
7
66002fb
APPLICATIONS INFORMATION
Figure 1
Interfacing to the LT6600-20
The LT6600-20 requires two equal external resistors, R
IN
,
to set the differential gain to 402/R
IN
. The inputs to the
fi lter are the voltages V
IN
+
and V
IN
–
presented to these
external components, Figure 1. The difference between
V
IN
+
and V
IN
–
is the differential input voltage. The aver-
age of V
IN
+
and V
IN
–
is the common mode input voltage.
Similarly, the voltages V
OUT
+
and V
OUT
–
appearing at
Pins 4 and 5 of the LT6600-20 are the fi lter outputs. The
difference between V
OUT
+
and V
OUT
–
is the differential
output voltage. The average of V
OUT
+
and V
OUT
–
is the
common mode output voltage.
Figure 1 illustrates the LT6600-20 operating with a single
3.3V supply and unity passband gain; the input signal is
DC coupled. The common mode input voltage is 0.5V, and
the differential input voltage is 2V
P-P
. The common mode
output voltage is 1.65V, and the differential output voltage
is 2V
P-P
for frequencies below 20MHz. The common mode
output voltage is determined by the voltage at Pin 2. Since
Pin 2 is shorted to Pin 7, the output common mode is the
mid-supply voltage. In addition, the common mode input
voltage can be equal to the mid-supply voltage of Pin 7
(see the Distortion vs Input Common Mode Level graphs
in the Typical Performance Characteristics section).
Figure 2 shows how to AC couple signals into the
LT6600-20. In this instance, the input is a single-ended
signal. AC coupling allows the processing of single-ended
or differential signals with arbitrary common mode levels.
The 0.1µF coupling capacitor and the 402Ω gain setting
resistor form a high pass fi lter, attenuating signals below
4kHz. Larger values of coupling capacitors will proportion-
ally reduce this highpass 3dB frequency.
Figure 2
Figure 3
–
+
402
402
0.01µF
0.1µF
3.3V
–
+
V
IN
–
V
IN
+
3
4
1
7
2
8
5
6
66002 F01
V
OUT
+
V
OUT
–
V
t
3
2
1
V
IN
+
V
IN
–
V
t
3
2
1
V
OUT
+
LT6600-20
V
OUT
–
0 0
–
+
402Ω
402Ω
0.01µF
0.1µF
0.1µF
0.1µF
3.3V
–
+V
IN
+
3
4
1
7
2
8
5
6
66002 F02
V
OUT
+
V
OUT
–
V
3
2
2
1
V
t
1
0
0
–1
V
IN
+
LT6600-20
V
OUT
+
V
OUT
–
–
+
100
100
0.1µF
0.01µF
5V
–
+
V
IN
–
V
IN
+
3
4
1
7
2
8
5
6
66002 F03
V
OUT
+
V
OUT
–
62pF
62pF
+
–
2V
V
t
3
2
1
0
V
IN
+
V
IN
–
V
t
3
2
1
0
V
OUT
+
V
OUT
–
LT6600-20
500mV
P-P
(DIFF)
LT6600-20
8
66002fb
APPLICATIONS INFORMATION
In Figure 3 the LT6600-20 is providing 12dB of gain. The
gain resistor has an optional 62pF in parallel to improve
the passband fl atness near 20MHz. The common mode
output voltage is set to 2V.
Use Figure 4 to determine the interface between the
LT6600-20 and a current output DAC. The gain, or “trans-
impedance,” is defi ned as A = V
OUT
/I
IN
. To compute the
transimpedance, use the following equation:
A =
402 •R1
R1+ R2
()
Ω
()
By setting R1 + R2 = 402, the gain equation reduces
to A = R1().
The voltage at the pins of the DAC is determined by R1, R2,
the voltage on Pin 7 and the DAC output current. Consider
Figure 4 with R1 = 49.9 and R2 = 348. The voltage at
Pin 7 is 1.65V. The voltage at the DAC pins is given by:
V
DAC
= V
PIN7
•
R1
R1+ R2 + 402
+I
IN
•
R1• R2
R1+ R2
= 26mV + I
IN
• 48.3Ω
I
IN
is I
IN
+
or I
IN
–
. The transimpedance in this example is
50.4.
Evaluating the LT6600-20
The low impedance levels and high frequency operation
ofthe LT6600-20 require some attention to the matching
networks between the LT6600-20 and other devices. The
previous examples assume an ideal (0) source impedance
and a large (1k) load resistance. Among practical ex-
amples where impedance must be considered is the evalu-
ation of the LT6600-20 with a network analyzer. Figure 5
is a laboratory setup that can be used to characterize
the LT6600-20 using single-ended instruments with 50
source impedance and 50 input impedance. For a unity
gain confi guration the LT6600-20 requires a 402 source
resistance yet the network analyzer output is calibrated
for a 50 load resistance. The 1:1 transformer, 53.6
and 388 resistors satisfy the two constraints above.
The transformer converts the single-ended source into a
differential stimulus. Similarly, the output of the LT6600-20
will have lower distortion with larger load resistance yet
the analyzer input is typically 50. The 4:1 turns (16:1
impedance) transformer and the two 402 resistors of
Figure 5, present the output of the LT6600-20 with a 1600
differential load, or the equivalent of 800 to ground at
each output. The impedance seen by the network analyzer
input is still 50, reducing refl ections in the cabling be-
tween the transformer and analyzer input.
Differential and Common Mode Voltage Ranges
The differential amplifi ers inside the LT6600-20 contain
circuitry to limit the maximum peak-to-peak differential
voltage through the fi lter. This limiting function prevents
excessive power dissipation in the internal circuitry and
provides output short-circuit protection. The limiting
function begins to take effect at output signal levels above
2V
P-P
and it becomes noticeable above 3.5V
P-P
. This is
illustrated in Figure 6; the LT6600-20 was confi gured with
unity passband gain and the input of the fi lter was driven
with a 1MHz signal. Because this voltage limiting takes
Figure 4 Figure 5
–
+
0.1µF
3.3V
–
+
LT6600-20
3
4
1
0.01µF
CURRENT
OUTPUT
DAC
7
2
8
5
V
OUT
+
V
OUT
–
66002 F04
6
R2
R1
I
IN
–
I
IN
+
R2
R1
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
LT6600-20
3
4
1
7
2
8
5
6
66002 F05
402
402
NETWORK
ANALYZER
INPUT
50
COILCRAFT
TTWB-16A
4:1
NETWORK
ANALYZER
SOURCE
COILCRAFT
TTWB-1010
1:1
50
53.6
388
388
LT6600-20
9
66002fb
APPLICATIONS INFORMATION
place well before the output stage of the fi lter reaches the
supply rails, the input/output behavior of the IC shown
in Figure 6 is relatively independent of the power supply
voltage.
The two amplifi ers inside the LT6600-20 have indepen-
dent control of their output common mode voltage (see
the Block Diagram section). The following guidelines will
optimize the performance of the fi lter.
Pin 7 must be bypassed to an AC ground with a 0.01µF or
larger capacitor. Pin 7 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 Pin 7. 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 Pin 7.
Pin 2 can be shorted to Pin 7 for simplicity. If a different
common mode output voltage is required, connect Pin 2
to a voltage source or resistor network. For 3V and 3.3V
supplies the voltage at Pin 2 must be less than or equal to
the mid supply level. For example, voltage (Pin 2)≤1.65V
on a single 3.3V supply. For power supply voltages higher
than 3.3V the voltage at Pin 2 should be within the voltage
of Pin 7 – 1V to the voltage of Pin 7 + 2V. Pin 2 is a high
impedance input.
The LT6600-20 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 Distortion
vs Input Common Mode Level in the Typical Performance
Characteristics section).
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the
LT6600-20 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. Pin 7 sets the output
common mode voltage of the 1st differential amplifi er inside
the LT6600-20 (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 402 feedback resistor and the
external 100 input resistor. The resulting 5mA common
mode DC current in each input path, must be absorbed by
the sources V
IN
+
and V
IN
–
. Pin 2 sets the common mode
output voltage of the 2nd differential amplifi er inside the
LT6600-20, and therefore sets the common mode output
voltage of the fi lter. Since, in the example of Figure 3, Pin 2
differs from Pin 7 by 0.5V, an additional 2.5mA (1.25mA
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 12.5mA 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 Pin 7 is shorted to Pin 2 the
common mode output voltage of both op amp stages will
be 2V and the resulting DC current will be 8mA. Of course,
by AC coupling the inputs of Figure 3, the common mode
DC current can be reduced to 2.5mA.
Figure 6. Output Level vs Input Level,
Differential 1MHz Input, Gain = 1
1MHz INPUT LEVEL (V
P-P
)
0
20
0
–20
–40
–60
–80
–100
–120
35
66002 F06
12
467
OUTPUT LEVEL (dBV)
3RD HARMONIC
85°C
1dB PASSBAND GAIN
COMPRESSION POINTS
1MHz 25°C
1MHz 85°C
3RD HARMONIC
25°C
2ND HARMONIC
25°C
2ND HARMONIC
85°C
P1-P3P4-P6P7-P9P10-P12

LT6600CS8-20#TRPBF

Mfr. #:
Buy LT6600CS8-20#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
Differential Amplifiers Very L N, Diff Amp & 20MHz Lpass Filt
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