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LT6600IS8-2.5#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
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 0.01F 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 625A (312A 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 625A.
LT6600-2.5
11
660025fe
APPLICATIONS INFORMATION
Noise
The noise performance of the LT6600-2.5 can be evaluated
with the circuit of Figure 6.
Given the low noise output of the LT6600-2.5 and the 6dB
attenuation of the transformer coupling network, it will
be necessary to measure the noise fl oor of the spectrum
analyzer and subtract the instrument noise from the fi lter
noise measurement.
Figure 7 is plot of the noise spectral density as a function
of frequency for an LT6600-2.5 with R
IN
= 1580 using
the fi xture of Figure 6 (the instrument noise has been
subtracted from the results).
The noise at each output is comprised of a differential
component and a common mode component. Using a
transformer or combiner to convert the differential outputs
to single-ended signal rejects the common mode noise and
gives a true measure of the S/N achievable in the system.
Conversely, if each output is measured individually and the
noise power added together, the resulting calculated noise
level will be higher than the true differential noise.
Power Dissipation
The LT6600-2.5 amplifi ers combine high speed with large-
signal currents in a small package. There is a need to
ensure that the die’s junction temperature does not exceed
150°C. The LT6600-2.5 S8 package has Pin 6 fused to the
lead frame to enhance thermal conduction when connect-
ing to a ground plane or a large metal trace. Metal trace
and plated through-holes can be used to spread the heat
generated by the device to the backside of the PC board.
For example, on a 3/32" FR-4 board with 2oz copper, a
totalof 660 square millimeters connected to Pin 6 of the
LT6600-2.5 S8 (330 square millimeters on each side of
the PC board) will result in a thermal resistance, θ
JA
, of
about 85°C/W. Without the extra metal trace connected to
Figure 7. Input Referred Noise, Gain = 1
FREQUENCY (MHz)
0.01
0
30
40
50
0.1 1 10
660025 F07
20
10
0
60
80
100
40
20
NOISE SPECTRAL DENSITY (nV
RMS
/√Hz)
INTEGRATED NOISE (µV
RMS
)
SPECTRAL DENSITY
INTEGRATED
Figure 6. (S8 Pin Numbers)
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
LT6600-2.5
3
4
1
7
2
8
5
6
R
IN
R
IN
25Ω
25Ω
660025 F06
SPECTRUM
ANALYZER
INPUT
50Ω
V
IN
COILCRAFT
TTWB-1010
1:1
Example: With the IC removed and the 25 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
input resistors grounded, measure the total integrated noise
out of the fi lter (e
O
). With the signal source connected, set
the frequency to 100kHz and adjust the amplitude until
V
IN
measures 100mV
P-P
. Measure the output amplitude,
V
OUT
, and compute the passband gain A = V
OUT
/V
IN
. Now
compute the input referred integrated noise (e
IN
) as:
e
IN
=
(e
O
)
2
–(e
S
)
2
A
Table 2 lists the typical input referred integrated noise for
various values of R
IN
.
Table 2. Noise Performance
PASSBAND
GAIN (V/V) R
IN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 2.5MHz
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 5MHz
4 402 18V
RMS
23V
RMS
2 806 29V
RMS
39V
RMS
1 1580 51V
RMS
73V
RMS
LT6600-2.5
12
660025fe
APPLICATIONS INFORMATION
the V
–
pin to provide a heat sink, the thermal resistance
will be around 105°C/W. Table 3 can be used as a guide
when considering thermal resistance.
Table 3. LT6600-2.5 SO-8 Package Thermal Resistance
COPPER AREA
TOPSIDE
(mm
2
)
BACKSIDE
(mm
2
)
BOARD AREA
(mm
2
)
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
1100 1100 2500 65°C/W
330 330 2500 85°C/W
35 35 2500 95°C/W
35 0 2500 100°C/W
0 0 2500 105°C/W
Junction temperature, T
J
, is calculated from the ambient-
temperature, T
A
, and power dissipation, P
D
. The power
dissipation is the product of supply voltage, V
S
, and
supply current, I
S
. Therefore, the junction temperature
is given by:
T
J
= T
A
+ (P
D
• θ
JA
) = T
A
+ (V
S
• I
S
• θ
JA
)
where the supply current, I
S
, is a function of signal level, load
impedance, temperature and common mode voltages.
For a given supply voltage, the worst-case power dissipation
occurs when the differential input signal is maximum, the
common mode currents are maximum (see Applications
Information regarding Common Mode DC Currents), the
load impedance is small and the ambient temperature is
maximum. To compute the junction temperature, measure
the supply current under these worst-case conditions, es-
timate the thermal resistance from Table 2, then apply the
equation for T
J
. For example, using the circuit in Figure 3
with DC differential input voltage of 1V, a differential
output voltage of 4V, no load resistance and an ambient
temperature of 85°C, the supply current (current into V
+
)
measures 37.6mA. Assuming a PC board layout with a
35mm
2
copper trace, the θ
JA
is 100°C/W. The resulting
junction temperature is:
T
J
= T
A
+ (P
D
• θ
JA
) = 85 + (5 • 0.0376 • 100) = 104°C
When using higher supply voltages or when driving small
impedances, more copper may be necessary to keep T
J
below 150°C.
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LT6600IS8-2.5#PBF

Mfr. #:
Buy LT6600IS8-2.5#PBF
Manufacturer:
Analog Devices Inc.
Description:
Differential Amplifiers Very L N, Diff Amp & 2.5MHz Lpass Filt
Lifecycle:
New from this manufacturer.
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