LINER LT6600-5 Very low noise, differential amplifi er and 5mhz lowpass filter Datasheet

LT6600-5
Very Low Noise, Differential
Amplifier and 5MHz Lowpass Filter
FEATURES
DESCRIPTION
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The LT®6600-5 combines a fully differential ampliﬁer with a
4th order 5MHz lowpass ﬁlter approximating a Chebyshev
frequency response. Most differential ampliﬁers require
many precision external components to tailor gain and
bandwidth. In contrast, with the LT6600-5, two external
resistors program differential gain, and the ﬁlter’s 5MHz
cutoff frequency and passband ripple are internally set.
The LT6600-5 also provides the necessary level shifting
to set its output common mode voltage to accommodate
the reference voltage requirements of A/Ds.
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Programmable Differential Gain via Two External
Resistors
Adjustable Output Common Mode Voltage
Operates and Speciﬁed with 3V, 5V, ±5V Supplies
0.5dB Ripple 4th Order Lowpass Filter with 5MHz
Cutoff
82dB S/N with 3V Supply and 2VP-P Output
Low Distortion, 2VP-P, 800Ω Load
1MHz: 93dBc 2nd, 96dBc 3rd
Fully Differential Inputs and Outputs
Compatible with Popular Differential Ampliﬁer
Pinouts
Available in an SO-8 Package
APPLICATIONS
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High Speed ADC Anti-Aliasing and DAC Smoothing in
Networking or Cellular Base Station Applications
High Speed Test and Measurement Equipment
Medical Imaging
Drop-in Replacement for Differential Ampliﬁers
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Using a proprietary internal architecture, the LT6600-5
integrates an anti-aliasing ﬁlter and a differential ampliﬁer/driver without compromising distortion or low noise
performance. At unity gain the measured in band signal-to-noise ratio is an impressive 82dB. At higher gains
the input referred noise decreases so the part can process
smaller input differential signals without signiﬁcantly
degrading the output signal-to-noise ratio.
The LT6600-5 also features low voltage operation. The
differential design provides outstanding performance for
a 2VP-P signal level while the part operates with a single
3V supply.
For similar devices with other cutoff frequencies, refer to
the LT6600-20, LT6600-10 and LT6600-2.5.
TYPICAL APPLICATION
Dual, Matched, 5MHz Lowpass Filter
5MHz Phase Distribution
(50 Units)
3V 0.1μF
0.01μF
IIN
30
1
7
2
8
RIN
3
– +
4
LT6600-5
+ –
QOUT
5
6
GAIN =
VOCM
(1V-1.5V)
3V 0.1μF
RIN
0.01μF
QIN
1
7
2
8
RIN
3
– +
4
LT6600-5
+ –
6
806Ω
RIN
PERCENTAGE OF UNITS (%)
RIN
25
20
15
10
5
IOUT
0
5
–135 –134.5 –134 –133.5 –133 –132.5 –132 –131.5
5MHz PHASE (DEG)
66005 TA01
66005fb
1
LT6600-5
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
Total Supply Voltage .................................................11V
Input Current (Note 8)..........................................±10mA
Operating Temperature Range (Note 6).... –40°C to 85°C
Speciﬁed Temperature Range (Note 7) .... –40°C to 85°C
Junction Temperature ........................................... 150°C
Storage Temperature Range...................– 65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
TOP VIEW
IN– 1
8
IN+
VOCM 2
7
VMID
V+ 3
6
V–
OUT+ 4
5
OUT–
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 150°C, θJA = 100°C/W
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
SPECIFIED TEMPERATURE RANGE
LT6600CS8-5#PBF
LT6600CS8-5#TRPBF
66005
8-Lead Plastic SO
–40°C to 85°C
LT6600IS8-5#PBF
LT6600IS8-5#TRPBF
6600I5
8-Lead Plastic SO
–40°C to 85°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
SPECIFIED TEMPERATURE RANGE
LT6600CS8-5
LT6600CS8-5#TR
66005
8-Lead Plastic SO
–40°C to 85°C
LT6600IS8-5
LT6600IS8-5#TR
6600I5
8-Lead Plastic SO
–40°C to 85°C
Consult LTC Marketing for parts speciﬁed with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel speciﬁcations, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the speciﬁcations which apply over the full operating temperature
range, otherwise speciﬁcations are at TA = 25°C. Unless otherwise speciﬁed VS = 5V (V+ = 5V, V – = 0V), RIN = 806Ω, and RLOAD = 1k.
PARAMETER
CONDITIONS
MIN
TYP
MAX
Filter Gain, VS = 3V
VIN = 2VP-P, fIN = DC to 260kHz
– 0.5
0
0.5
dB
Filter Gain, VS = 5V
Filter Gain, VS = ±5V
UNITS
VIN = 2VP-P, fIN = 500k (Gain Relative to 260kHz)
l
–0.15
0
0.1
dB
VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz)
l
–0.4
– 0.1
0.3
dB
VIN = 2VP-P, fIN = 4MHz (Gain Relative to 260kHz)
l
– 0.7
– 0.1
0.6
dB
VIN = 2VP-P, fIN = 5MHz (Gain Relative to 260kHz)
l
–1.1
–0.2
0.8
dB
VIN = 2VP-P, fIN = 15MHz (Gain Relative to 260kHz)
l
– 28
–25
dB
VIN = 2VP-P, fIN = 25MHz (Gain Relative to 260kHz)
l
–44
VIN = 2VP-P, fIN = DC to 260kHz
dB
– 0.5
0
0.5
dB
VIN = 2VP-P, fIN = 500k (Gain Relative to 260kHz)
l
– 0.15
0
0.1
dB
VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz)
l
–0.4
– 0.1
0.3
dB
VIN = 2VP-P, fIN = 4MHz (Gain Relative to 260kHz)
l
– 0.7
–0.1
0.6
dB
VIN = 2VP-P, fIN = 5MHz (Gain Relative to 260kHz)
l
– 1.1
–0.2
0.8
dB
VIN = 2VP-P, fIN = 15MHz (Gain Relative to 260kHz)
l
– 28
–25
VIN = 2VP-P, fIN = 25MHz (Gain Relative to 260kHz)
l
– 44
VIN = 2VP-P, fIN = DC to 260kHz
– 0.6
–0.1
dB
dB
0.4
dB
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2
LT6600-5
ELECTRICAL CHARACTERISTICS
The l denotes the speciﬁcations which apply over the full operating temperature
range, otherwise speciﬁcations are at TA = 25°C. Unless otherwise speciﬁed VS = 5V (V+ = 5V, V – = 0V), RIN = 806Ω, and RLOAD = 1k.
PARAMETER
CONDITIONS
Filter Gain, RIN = 229Ω
VIN = 0.5VP-P, fIN = DC to 260kHz
VS = 3V
VS = 5V
VS = ±5V
MIN
TYP
MAX
UNITS
10.4
10.3
10.1
10.9
10.8
10.7
11.5
11.4
11.3
dB
dB
dB
Filter Gain Temperature Coefﬁcient (Note 2) fIN = 260kHz, VIN = 2VP-P
780
ppm/C
45
μVRMS
Noise
Noise BW = 10kHz to 5MHz, RIN = 806Ω
Distortion (Note 4)
1MHz, 2VP-P, RL = 800Ω
2nd Harmonic
3rd Harmonic
93
96
dBc
dBc
5MHz, 2VP-P, RL = 800Ω
2nd Harmonic
3rd Harmonic
66
73
dBc
dBc
Differential Output Swing
Measured Between Pins 4 and 5
Pin 7 Shorted to Pin 2
VS = 5V
VS = 3V
Input Bias Current
Average of Pin 1 and Pin 8
Input Referred Differential Offset
RIN = 806Ω
VS = 3V
VS = 5V
VS = ±5V
l
l
l
5
10
8
25
30
35
mV
mV
mV
RIN = 229Ω
VS = 3V
VS = 5V
VS = ±5V
l
l
l
5
5
5
13
16
20
mV
mV
mV
l
l
3.85
3.85
4.8
4.8
VP-P DIFF
VP-P DIFF
l
–70
–30
μA
Differential Offset Drift
10
μV/°C
Input Common Mode Voltage (Note 3)
Differential Input = 500mVP-P,
RIN = 229Ω
VS = 3V
VS = 5V
VS = ±5V
l
l
l
0.0
0.0
–2.5
1.5
3.0
1.0
V
V
V
Output Common Mode Voltage (Note 5)
Differential Output = 2VP-P,
Pin 7 = Open
VS = 3V
VS = 5V
VS = ±5V
l
l
l
1.0
1.5
–2.5
1.5
3.0
2.0
V
V
V
VS = 3V
VS = 5V
VS = ±5V
l
l
l
–25
–30
–55
50
45
35
mV
mV
mV
VS = 5
VS = 3
l
Output Common Mode Offset
(with Respect to Pin 2)
Common Mode Rejection Ratio
61
Voltage at VMID (Pin 7)
VOCM = VMID = VS/2
Power Supply Current
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: This is the temperature coefﬁcient of the internal feedback
resistors assuming a temperature independent external resistor (RIN).
Note 3: The input common mode voltage is the average of the voltages
applied to the external resistors (RIN). Speciﬁcation guaranteed for
RIN ≥ 229Ω.
Note 4: Distortion is measured differentially using a differential stimulus.
The input common mode voltage, the voltage at Pin 2, and the voltage at
Pin 7 are equal to one half of the total power supply voltage.
dB
2.46
2.51
1.5
2.55
V
V
7.7
kΩ
l
4.3
5.5
VS = 5
VS = 3
l
l
–15
–10
–3
–3
VS = 3V, VS = 5
VS = 3V, VS = 5
VS = ±5V
l
l
VMID Input Resistance
VOCM Bias Current
5
0
–5
28
30
μA
μA
31
34
38
mA
mA
mA
Note 5: Output common mode voltage is the average of the voltages at
Pins 4 and 5. The output common mode voltage is equal to the voltage
applied to Pin 2.
Note 6: The LT6600C is guaranteed functional over the operating
temperature range –40°C to 85°C.
Note 7: The LT6600C is guaranteed to meet 0°C to 70°C speciﬁcations and
is designed, characterized and expected to meet the extended temperature
limits, but is not tested at –40°C and 85°C. The LT6600I is guaranteed to
meet speciﬁed performance from –40°C to 85°C.
Note 8: The inputs are protected by back-to-back diodes. If the differential
input voltage exceeds 1.4V, the input current should be limited to less than
10mA.
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LT6600-5
TYPICAL PERFORMANCE CHARACTERISTICS
Amplitude Response
Passband Gain and Delay
110
12
–1
100
11
100
–2
90
10
90
–3
80
–10
–40
DELAY
–4
70
120
GAIN
9
80
DELAY
8
70
–5
60
7
60
–50
–6
50
6
50
–60
–7
40
5
40
–70
–8 GAIN = 1
TA = 25°C
–9
0 1 2
30
4 GAIN = 4
TA = 25°C
3
0 1 2 3 4 5 6 7
FREQUENCY (MHz)
30
–80
0.1
1
10
FREQUENCY (MHz)
100
3 4 5 6 7
FREQUENCY (MHz)
8
66005 G01
20
10
Common Mode Rejection Ratio
90
VS = 5V
GAIN = 1
TA = 25°C
80
10
CMRR (dB)
1
VS = 5V
GAIN = 1
VIN = 1VP-P
TA = 25°C
70
60
60
50
40
30
20
40
30
0.01
100
0.1
1
10
FREQUENCY (MHz)
66005 G04
–80
–90
–100
–110
–70
–80
0.1
1
FREQUENCY (MHz)
VS = 3V
RL = 800Ω
–50 T = 25°C
A
–90
–110
10
66005 G07
3RD HARMONIC,
5MHz INPUT
–60
–70
2ND HARMONIC,
5MHz INPUT
–80
3RD HARMONIC,
1MHz INPUT
–90
–100
VS = ±5V, VIN = 2VP-P
RL = 800Ω, TA = 25°C
0.1
1
FREQUENCY (MHz)
100
Distortion vs Signal Level
–100
VS = 3V, VIN = 2VP-P
RL = 800Ω, TA = 25°C
1
10
FREQUENCY (MHz)
–40
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
–60
DISTORTION (dB)
–70
0.1
66005 G06
Distortion vs Frequency
–50
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
–60
0
0.01
100
66005 G05
Distortion vs Frequency
–50
VS = 3V
VIN = 200mVP-P
TA = 25°C
V+ TO DIFFOUT
10
DISTORTION (dB)
1
10
FREQUENCY (MHz)
10
Power Supply Rejection Ratio
50
0.1
9
80
70
0.1
20
8
66005 G03
PSRR (dB)
OUTPUT IMPEDANCE (Ω)
9
66005 G02
Output Impedance vs Frequency
100
DISTORTION (dB)
110
DELAY (ns)
GAIN (dB)
–20
–30
GAIN
0
DELAY (ns)
13
GAIN (dB)
VS = 5V
GAIN = 1
TA = 25°C
0
GAIN (dB)
Passband Gain and Delay
120
1
10
2ND HARMONIC,
1MHz INPUT
–110
10
66005 G08
0
1
2
3
INPUT LEVEL (VP-P)
4
5
66005 G09
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4
LT6600-5
TYPICAL PERFORMANCE CHARACTERISTICS
Distortion vs Input Common Mode
–40
2ND HARMONIC
5MHz INPUT
–70
3RD HARMONIC
1MHz INPUT
–80
2ND HARMONIC
1MHz INPUT
–90
–100
VS = ±5V
RL = 800Ω, TA = 25°C
–110
0
1
2
3
5
4
INPUT LEVEL (VP-P)
66005 G10
–60
–70
–80
–90
–70
–80
–90
Distortion vs Temperature
20
OUT–
200mV/DIV
34
1dB PASSBAND GAIN
COMPRESSION POINTS
0
30
OUT+
200mV/DIV
TA = 25°C
28
26
TA = –40°C
24
IN–
500mV/DIV
IN+
22
3RD HARMONIC
TA = 85°C
–20
3RD HARMONIC
TA = 25°C
–40
–60
–80
2ND HARMONIC
TA = 85°C
–100
2ND HARMONIC
TA = 25°C
–120
20
2
6
10
4
8
TOTAL SUPPLY VOLTAGE (V)
12
1MHz TA = 25°C
1MHz TA = 85°C
TA = 85°C
32
0
100ns/DIV
1
4
3
5
2
1MHz INPUT LEVEL (VP-P)
6
7
66005 G15
66005 G14
66005 G13
Distortion
vs Output Common Mode
Input Referred Noise
–40
45
GAIN = 4
PIN 7 = VS/2
–50 TA = 25°C
0.5VP-P 1MHz INPUT
–60 RL = 800Ω
2ND HARMONIC, VS = 3V
3RD HARMONIC, VS = 3V
2ND HARMONIC, VS = 5V
3RD HARMONIC, VS = 5V
2ND HARMONIC, VS = ±5V
3RD HARMONIC, VS = ±5V
–70
–80
–90
–100
–110
–1.5 –1.0 –0.5
0 0.5 1.0 1.5 2.0
VOLTAGE PIN 2 TO PIN 7 (V)
2.5
66005 G16
NOISE DENSITY (nV/√Hz)
40
35
90
INTEGRATED NOISE, GAIN = 1X
INTEGRATED NOISE, GAIN = 4X
NOISE DENSITY, GAIN = 1X
NOISE DENSITY, GAIN = 4X
80
70
30
60
25
50
20
40
15
30
10
20
5
10
0
0.01
0.1
10
INTEGRATED NOISE (μV)
DISTORTION COMPONENT (dB)
–60
Transient Response, Differential
Gain = 1, Single-Ended Input,
Differential Output
36
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–50
–100 GAIN = 4, PIN 7 = VS/2
2VP-P 1MHz INPUT
RL = 800Ω, TA = 25°C
–110
2
3
–3
–1
0
1
–2
INPUT COMMON MODE VOLTAGE
RELATIVE TO PIN 7 (V)
66005 G12
–100 GAIN = 1, PIN 7 = VS/2
2VP-P 1MHz INPUT
RL = 800Ω, TA = 25°C
–110
–1
0
1
2
3
–3
–2
INPUT COMMON MODE VOLTAGE
RELATIVE TO PIN 7 (V)
66005 G11
Power Supply Current
vs Power Supply Voltage
POWER SUPPLY CURRENT (mA)
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–50
OUTPUT LEVEL (dBV)
DISTORTION (dB)
–60
DISTORTION COMPONENT (dB)
3RD HARMONIC
5MHz INPUT
–50
Distortion vs Input Common Mode
–40
DISTORTION COMPONENT (dB)
Distortion vs Signal Level
–40
0
100
FREQUENCY (MHz)
66005 G17
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5
LT6600-5
PIN FUNCTIONS
IN – and IN+ (Pins 1, 8): Input Pins. Signals can be applied to either or both input pins through identical external
resistors, RIN. The DC gain from differential inputs to the
differential outputs is 806Ω/RIN.
VOCM (Pin 2): Is the DC Common Mode Reference Voltage
for the 2nd Filter Stage. Its value programs the common
mode voltage of the differential output of the ﬁlter. Pin 2 is a
high impedance input, which can be driven from an external
voltage reference, or Pin 2 can be tied to Pin 7 on the PC
board. Pin 2 should be bypassed with a 0.01μF ceramic
capacitor unless it is connected to a ground plane.
V+ and V – (Pins 3, 6): Power Supply Pins. For a single
3.3V or 5V supply (Pin 6 grounded) a quality 0.1μF ceramic
bypass capacitor is required from the positive supply pin
(Pin 3) to the negative supply pin (Pin 6). The bypass
should be as close as possible to the IC. For dual supply
applications, bypass Pin 3 to ground and Pin 6 to ground
with a quality 0.1μF ceramic capacitor.
OUT+ and OUT– (Pins 4, 5): Output Pins. Pins 4 and 5 are
the ﬁlter differential outputs. Each pin can drive a 100Ω
and/or 50pF load to AC ground.
VMID (Pin 7): The VMID pin is internally biased at midsupply, see block diagram. For single supply operation
the VMID pin should be bypassed with a quality 0.01μF
ceramic capacitor to Pin 6. For dual supply operation,
Pin 7 can be bypassed or connected to a high quality DC
ground. A ground plane should be used. A poor ground
will increase noise and distortion. Pin 7 sets the output
common mode voltage of the 1st stage of the ﬁlter. It has
a 5.5kΩ impedance, and it can be overridden with an
external low impedance voltage source.
BLOCK DIAGRAM
VIN+
RIN
IN+
VMID
8
7
V+
V–
OUT–
6
5
11k
PROPRIETARY
LOWPASS
FILTER STAGE
806Ω
11k
400Ω
V–
OP AMP
+
400Ω
+ –
–
VOCM
–
VOCM
+
– +
400Ω
400Ω
806Ω
1
VIN–
RIN
IN–
2
3
4
VOCM
V+
OUT+
66005 BD
66005fb
6
LT6600-5
APPLICATIONS INFORMATION
is 2VP-P for frequencies below 5MHz. 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
(refer to the Distortion vs Input Common Mode Level
graphs in the Typical Performance Characteristics).
Interfacing to the LT6600-5
The LT6600-5 requires 2 equal external resistors, RIN, to
set the differential gain to 806Ω/RIN. The inputs to the
ﬁlter are the voltages VIN+ and VIN– presented to these
external components, Figure 1. The difference between
VIN+ and VIN– is the differential input voltage. The average of VIN+ and VIN– is the common mode input voltage.
Similarly, the voltages VOUT+ and VOUT– appearing at Pins 4
and 5 of the LT6600-5 are the ﬁlter outputs. The difference between VOUT+ and VOUT– is the differential output
voltage. The average of VOUT+ and VOUT– is the common
mode output voltage.
Figure 2 shows how to AC couple signals into the LT6600-5.
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 806Ω gain setting resistor
form a high pass ﬁlter, attenuating signals below 2kHz.
Larger values of coupling capacitors will proportionally
reduce this highpass 3dB frequency.
Figure 1 illustrates the LT6600-5 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 2VP-P. The common mode
output voltage is 1.65V and the differential output voltage
In Figure 3 the LT6600-5 is providing 12dB of gain. The
gain resistor has an optional 62pF in parallel to improve
3.3V
0.1μF
V
3
VIN
–
806Ω
1
7
2
VIN+
1
0
+
4
VOUT+
LT6600-5
VOUT–
–5
+
806Ω
t
VIN–
8
+
3
–
2
0.01μF
VIN
V
3
6
2
VOUT+
1
VOUT–
t
0
66005 F01
Figure 1
3.3V
0.1μF
V
0.1μF
806Ω
2
1
–
7
1
VIN+
0
0.1μF
t
VIN
0.01μF
+
3
4
+
VOUT+
LT6600-5
2
8
–
+
806Ω
–1
V
3
2
VOUT–
5
1
6
VOUT+
VOUT–
0
66005 F02
Figure 2
62pF
5V
0.1μF
V
3
VIN
–
200Ω
1
7
2
1
0
VIN+
VIN–
2
0.01μF
500mVP-P (DIFF)
VIN
8
+
200Ω
t
+
–
V
3
–
+
4
LT6600-5
–
+
6
2V
5
3
VOUT+
VOUT+
2
VOUT–
1
0
VOUT–
66005 F03
t
0.01μF
62pF
Figure 3
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7
LT6600-5
APPLICATIONS INFORMATION
the passband ﬂatness near 5MHz. The common mode
output voltage is set to 2V.
Use Figure 4 to determine the interface between the
LT6600-5 and a current output DAC. The gain, or “transimpedance,” is deﬁned as A = VOUT/IIN Ω. To compute
the transimpedance, use the following equation:
A=
806 • R1
Ω
R1+ R2
By setting R1 + R2 = 806Ω, the gain equation reduces
to A = R1Ω.
The voltage at the pins of the DAC is determined by R1,
R2, the voltage on Pin 7 and the DAC output current
(IIN+ or IIN–). Consider Figure 4 with R1 = 49.9Ω and R2
= 750Ω. The voltage at Pin 7 is 1.65V. The voltage at the
DAC pins is given by:
R1
R1• R2
+IIN
R1+ R2 + 806
R1+ R2
= 51mV +IIN 46.8Ω
Figure 5 is a laboratory setup that can be used to characterize the LT6600-5 using single-ended instruments with 50Ω
source impedance and 50Ω input impedance. For a unity
gain conﬁguration the LT6600-5 requires a 806Ω source
resistance yet the network analyzer output is calibrated
for a 50Ω load resistance. The 1:1 transformer, 51.1Ω
and 787Ω resistors satisfy the two constraints above.
The transformer converts the single-ended source into a
differential stimulus. Similarly, the output the LT6600-5
will have lower distortion with larger load resistance yet
the analyzer input is typically 50Ω. The 4:1 turns (16:1
impedance) transformer and the two 402Ω resistors of
Figure 5, present the output of the LT6600-5 with a 1600Ω
differential load, or the equivalent of 800Ω to ground at
each output. The impedance seen by the network analyzer
input is still 50Ω, reducing reﬂections in the cabling between the transformer and analyzer input.
2.5V
VDAC = VPIN7 •
0.1μF
NETWORK
ANALYZER
SOURCE
IIN is IIN– or IIN+. The transimpedance in this example is
50.3Ω.
50Ω
COILCRAFT
TTWB-1010
1:1 787Ω 1
7
51.1Ω
2
8
787Ω
CURRENT
OUTPUT
DAC
3
–
+
4
COILCRAFT
TTWB-16A
4:1
402Ω
NETWORK
ANALYZER
INPUT
LT6600-5
–
+
6
402Ω
50Ω
5
0.1μF
66005 F05
3.3V
0.1μF
–2.5V
IIN–
R2
R1
1
7
3
–
+
4
0.01μF
2
LT6600-5
R2
8
–
IIN+
+
5
VOUT+
Differential and Common Mode Voltage Ranges
VOUT–
6
R1
Figure 5
66005 F04
Figure 4
Evaluating the LT6600-5
The low impedance levels and high frequency operation
of the LT6600-5 require some attention to the matching
networks between the LT6600-5 and other devices. The
previous examples assume an ideal (0Ω) source impedance and a large (1kΩ) load resistance. Among practical examples where impedance must be considered is
the evaluation of the LT6600-5 with a network analyzer.
The differential ampliﬁers inside the LT6600-5 contain
circuitry to limit the maximum peak-to-peak differential
voltage through the ﬁlter. 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
2VP-P and it becomes noticeable above 3.5VP-P. This is
illustrated in Figure 6; the LTC6600-5 was conﬁgured with
unity passband gain and the input of the ﬁlter was driven
with a 1MHz signal. Because this voltage limiting takes
place well before the output stage of the ﬁlter reaches the
supply rails, the input/output behavior of the IC shown
in Figure 6 is relatively independent of the power supply
voltage.
66005fb
8
LT6600-5
APPLICATIONS INFORMATION
20
1dB PASSBAND GAIN
COMPRESSION POINTS
OUTPUT LEVEL (dBV)
0
the power supply level and gain setting (see the Electrical
Characteristics section).
1MHz TA = 25°C
1MHz TA = 85°C
3RD HARMONIC
TA = 85°C
–20
Common Mode DC Currents
3RD HARMONIC
TA = 25°C
–40
–60
–80
2ND HARMONIC
TA = 85°C
–100
2ND HARMONIC
TA = 25°C
–120
0
1
4
3
5
2
1MHz INPUT LEVEL (VP-P)
Figure 6
6
7
66005 F06
The two ampliﬁers inside the LT6600-5 have independent
control of their output common mode voltage (see the
Block Diagram section). The following guidelines will
optimize the performance of the ﬁlter for single supply
operation.
Pin 7 must be bypassed to an AC ground with a 0.01μF or
higher 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 can be set above mid-supply.
The voltage on Pin 2 should not be more than 1V below
the voltage on Pin 7. The voltage on Pin 2 should not be
more than 2V above the voltage on Pin 7. Pin 2 is a high
impedance input.
The LT6600-5 was designed to process a variety of input
signals including signals centered around the mid-supply 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 VIN+ and VIN– in Figure 1) is determined by
In applications like Figure 1 and Figure 3 where the LT6600-5
not only provides lowpass ﬁltering 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 ampliﬁer inside the LT6600-5 (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 VIN+ and VIN–. Pin 2 sets the common mode
output voltage of the 2nd differential ampliﬁer inside the
LT6600-5, and therefore sets the common mode output
voltage of the ﬁlter. Since in the example, Figure 3, Pin 2
differs from Pin 7 by 0.5V, an additional 1.25mA (0.625mA
per side) of DC current will ﬂow in the resistors coupling
the 1st differential ampliﬁer output stage to ﬁlter output.
Thus, a total of 6.25mA is used to translate the common
mode voltages.
A simple modiﬁcation to Figure 3 will reduce the DC common 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 4mA. Of course,
by AC coupling the inputs of Figure 3 and shorting Pin 7
to Pin 2, the common mode DC current is eliminated.
Noise
The noise performance of the LT6600-5 can be evaluated
with the circuit of Figure 7.
Given the low noise output of the LT6600-5 and the 6dB
attenuation of the transformer coupling network, it will
be necessary to measure the noise ﬂoor of the spectrum
analyzer and subtract the instrument noise from the ﬁlter
noise measurement.
66005fb
9
LT6600-5
APPLICATIONS INFORMATION
45
2.5V
0.1μF
1
7
2
8
RIN
3
– +
4
LT6600-5
25Ω
–
+
NOISE DENSITY (nV/√Hz)
RIN
SPECTRUM
ANALYZER
INPUT
50Ω
5
0.1μF
6
66005 F07
–2.5V
35
70
30
60
25
50
20
40
15
30
10
20
5
10
0
0.01
Figure 7
80
0.1
10
INTEGRATED NOISE (μV)
VIN
40
COILCRAFT
TTWB-1010
25Ω
1:1
90
INTEGRATED NOISE, GAIN = 1X
INTEGRATED NOISE, GAIN = 4X
NOISE DENSITY, GAIN = 1X
NOISE DENSITY, GAIN = 4X
0
100
FREQUENCY (MHz)
66005 G08
Example: With the IC removed and the 25Ω resistors
grounded, measure the total integrated noise (eS) of the
spectrum analyzer from 10kHz to 5MHz. With the IC inserted, the signal source (VIN) disconnected, and the input
resistors grounded, measure the total integrated noise
out of the ﬁlter (eO). With the signal source connected,
set the frequency to 1MHz and adjust the amplitude until
VIN measures 100mVP-P. Measure the output amplitude,
VOUT, and compute the passband gain A = VOUT/VIN. Now
compute the input referred integrated noise (eIN) as:
eIN =
(eO )2 – (eS )2
A
Table 1 lists the typical input referred integrated noise for
various values of RIN.
Figure 8 is plot of the noise spectral density as a function
of frequency for an LT6600-5 with RIN = 806Ω and 200Ω
using the ﬁxture of Figure 7 (the instrument noise has
been subtracted from the results).
Table 1. Noise Performance
RIN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 10MHz
INPUT REFERRED
NOISE dBm/Hz
4
200Ω
24μVRMS
–149
2
402Ω
38μVRMS
–145
1
806Ω
69μVRMS
–140
PASSBAND
GAIN (V/V)
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.
Figure 8
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 LT6600-5 ampliﬁers combine high speed with largesignal currents in a small package. There is a need to
ensure that the dies’s junction temperature does not
exceed 150°C. The LT6600-5 package has Pin 6 fused
to the lead frame to enhance thermal conduction when
connecting 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 total of 660 square millimeters connected to Pin 6
of the LT6600-5 (330 square millimeters on each side of
the PC board) will result in a thermal resistance, θJA, of
about 85°C/W. Without extra metal trace connected to the
V – pin to provide a heat sink, the thermal resistance will
be around 105°C/W. Table 2 can be used as a guide when
considering thermal resistance.
Table 2. LT6600-5 SO-8 Package Thermal Resistance
COPPER AREA
TOPSIDE
(mm2)
BACKSIDE
(mm2)
BOARD AREA
(mm2)
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
66005fb
10
LT6600-5
APPLICATIONS INFORMATION
Junction temperature, TJ, is calculated from the ambient
temperature, TA, and power dissipation, PD. The power
dissipation is the product of supply voltage, VS, and
supply current, IS. Therefore, the junction temperature
is given by:
ent temperature is maximum. To compute the junction
temperature, measure the supply current under these
worst-case conditions, estimate the thermal resistance
from Table 2, then apply the equation for TJ. For example,
using the circuit in Figure 3 with DC differential input voltage of 250mV, a differential output voltage of 1V, 1kΩ load
resistance and an ambient temperature of 85°C, the supply
current (current into Pin 3) measures 32.2mA. Assuming
a PC board layout with a 35mm2 copper trace, the θJA is
100°C/W. The resulting junction temperature is:
TJ = TA + (PD • θJA) = TA + (VS • IS • θJA)
where the supply current, IS, 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 ambi-
TJ = TA + (PD • θJA) = 85 + (5 • 0.0322 • 100) = 101°C
When using higher supply voltages or when driving small
impedances, more copper may be necessary to keep TJ
below 150°C.
PACKAGE DESCRIPTION
S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
.189 – .197
(4.801 – 5.004)
NOTE 3
.045 ±.005
.050 BSC
8
.245
MIN
7
6
5
.160 ±.005
.150 – .157
(3.810 – 3.988)
NOTE 3
.228 – .244
(5.791 – 6.197)
.030 ±.005
TYP
1
RECOMMENDED SOLDER PAD LAYOUT
.010 – .020
× 45°
(0.254 – 0.508)
.008 – .010
(0.203 – 0.254)
3
4
.053 – .069
(1.346 – 1.752)
.004 – .010
(0.101 – 0.254)
0°– 8° TYP
.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN
2
.014 – .019
(0.355 – 0.483)
TYP
INCHES
(MILLIMETERS)
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
.050
(1.270)
BSC
SO8 0303
66005fb
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
LT6600-5
TYPICAL APPLICATION
Dual, Matched, 6th Order, 5MHz Lowpass Filter
Single-Ended Input (IIN and QIN) and Differential Output (IOUT and QOUT)
V+ 0.1μF
IIN
0.1μF
V+
806Ω
1
249Ω
249Ω
V
3
249Ω
V
+ 16
LT1568
15
INVA
INVB
14
SA
SB
13
OUTA OUTB
12
OUTA OUTB
11
GNDA GNDB
10
NC
EN
–
– 9
V
V
2
QIN
+
4
5
6
7
8
1
7
249Ω
249Ω
2
8
249Ω
806Ω
3
– +
4
LT6600-5
+ –
IOUT
5
6
0.1μF
V–
Q
I
GAIN = OUT OR OUT = 1
IIN
QIN
806Ω
0.1μF
V+ 0.1μF
1
7
V–
2
8
806Ω
3
– +
4
LT6600-5
+ –
6
QOUT
5
0.1μF
66005 TA02
V–
Amplitude Response
Transient Response
12
GAIN (dB) 20 LOG (IOUT/IIN) OR
20 LOG (QOUT/QIN)
0
OUTPUT
(IOUT OR QOUT)
200mV/DIV
–12
–24
–36
–48
–60
INPUT
(IIN OR QIN)
500mV/DIV
–72
–84
–96
–108
100
1
10
FREQUENCY (Hz)
100ns/DIV
40
66005 TA02c
66005 TA02b
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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LTC1566-1
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LT1567
Very Low Noise, High Frequency Filter Building Block
1.4nV/√Hz Op Amp, MSOP Package, Differential Output
LT1568
Very Low Noise, 4th Order Building Block
Lowpass and Bandpass Filter Designs Up to 10MHz,
Differential Outputs
LTC1569-7
Linear Phase, DC Accurate, Tunable 10th Order Lowpass
Filter
One External Resistor Sets Filter Cutoff Frequency, Differential Inputs
LT6600-2.5
Very Low Noise, Differential Ampliﬁer
and 2.5MHz Lowpass Filter
Adjustable Output Common Mode Voltage
LT6600-10
Very Low Noise, Differential Ampliﬁer
and 10MHz Lowpass Filter
Adjustable Output Common Mode Output Voltage
LT6600-20
Very Low Noise, Differential Ampliﬁer
and 20MHz Lowpass Filter
Adjustable Output Common Mode Voltage
®
66005fb
12 Linear Technology Corporation
LT 0409 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004

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