LINER LT6600-2.5 Very low noise, differential amplifi er and 2.5mhz lowpass filter Datasheet

LT6600-2.5
Very Low Noise, Differential
Amplifier and 2.5MHz Lowpass Filter
FEATURES
DESCRIPTION
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The LT®6600-2.5 combines a fully differential ampliﬁer
with a 4th order 2.5MHz lowpass ﬁlter approximating a
Chebyshev frequency response. Most differential ampliﬁers require many precision external components to tail
or gain and bandwidth. In contrast, with the LT6600-2.5,
two external resistors program differential gain, and the
ﬁlter’s 2.5MHz cutoff frequency and passband ripple are
internally set. The LT6600-2.5 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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±0.6dB (Max) Ripple 4th Order Lowpass Filter with
2.5MHz Cutoff
Programmable Differential Gain via Two External
Resistors
Adjustable Output Common Mode Voltage
Operates and Speciﬁed with 3V, 5V, ±5V Supplies
86dB S/N with 3V Supply and 1VRMS Output
Low Distortion, 1VRMS, 800Ω Load
1MHz: 95dBc 2nd, 88dBc 3rd
Fully Differential Inputs and Outputs
Compatible with Popular Differential Ampliﬁer
Pinouts
SO-8 and DFN-12 Packages
Using a proprietary internal architecture, the LT6600-2.5
integrates an antialiasing ﬁlter and a differential ampliﬁer/driver without compromising distortion or low noise
performance. At unity gain the measured in band signalto-noise ratio is an impressive 86dB. 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.
APPLICATIONS
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High Speed ADC Antialiasing and DAC Smoothing in
Networking or Cellular Basestation Applications
High Speed Test and Measurement Equipment
Medical Imaging
Drop-in Replacement for Differential Ampliﬁers
The LT6600-2.5 also features low voltage operation. The
differential design provides outstanding performance for a
4VP-P signal level while the part operates with a single 3V
supply. The LT6600-2.5 is available in SO-8 and DFN-12
packages.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
For similar devices with higher cutoff frequency, refer
to the LT6600-5, LT6600-10, LT6600-15 and LT6600-20
data sheets.
TYPICAL APPLICATION
(S8 Pin Numbers Shown)
DAC Output Filter
DAC Output Spectrum
LT6600-2.5 Output Spectrum
0
0
5V
–20
7
52.3Ω
LTC1668
2
IOUT B
CLK
1
8
1580Ω
–
3
+
4
–
6
5
VOUT+
VOUT–
0.1μF
–5V 50MHz
–5V
–30
–30
LT6600-2.5
+
–20
BASEBAND SIGNAL
660025 TA01a
–40
DAC OUTPUT IMAGE
–50
(dBm)
1580Ω
LADCOM
IOUT A
(dBm)
52.3Ω
16-BIT 4kHz to 2.5MHz
DISCRETE MULTI-TONE
SIGNAL AT 50MSPS
–10
–10
0.1μF
5V
–40
–50
–60
–60
–70
–70
–80
–80
–90
–90
0
12 24 36 48 60 72 84 96 108 120
0
12 24 36 48 60 72 84 96 108 120
FREQUENCY (MHz)
FREQUENCY (MHz)
660025 TA01b
660025 TA01c
660025fe
1
LT6600-2.5
ABSOLUTE MAXIMUM RATINGS
(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
PIN CONFIGURATION
TOP VIEW
IN–
TOP VIEW
12 IN+
1
IN–
11 NC
NC
2
VOCM
3
V+
4
NC
5
8 V–
OUT+
6
7 OUT–
12
10 VMID
9 V–
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
DF PACKAGE
12-LEAD (4mm × 4mm) PLASTIC DFN
TJMAX = 150°C, θJA = 43°C/W, θJC = 4°C/W
EXPOSED PAD (PIN 13) IS V–, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
SPECIFIED TEMPERATURE RANGE
LT6600CS8-2.5#PBF
LT6600CS8-2.5#TRPBF
660025
8-Lead Plastic SO
0°C to 70°C
LT6600IS8-2.5#PBF
LT6600IS8-2.5#TRPBF
600I25
8-Lead Plastic SO
–40°C to 85°C
LT6600CDF-2.5#PBF
LT6600CDF-2.5#TRPBF
60025
12-Lead (4mm × 4mm) Plastic DFN
0°C to 70°C
LT6600IDF-2.5#PBF
LT6600IDF-2.5#TRPBF
60025
12-Lead (4mm × 4mm) Plastic DFN
–40°C to 85°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
SPECIFIED TEMPERATURE RANGE
LT6600CS8-2.5
LT6600CS8-2.5#TR
660025
8-Lead Plastic SO
0°C to 70°C
LT6600IS8-2.5
LT6600IS8-2.5#TR
600I25
8-Lead Plastic SO
–40°C to 85°C
Consult LTC Marketing for parts speciﬁed with wider operating temperature ranges.
*The temperature grade is identiﬁed by a label on the shipping container for the DFN Package.
Consult LTC Marketing for information on nonstandard lead based ﬁnish parts.
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 = 1580Ω, and RLOAD = 1k.
PARAMETER
CONDITIONS
Filter Gain, VS = 3V
VIN = 2VP-P, fIN = DC to 260kHz
RIN = 1580Ω
VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz)
l
VIN = 2VP-P, fIN = 1.9MHz (Gain Relative to 260kHz)
l
VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz)
l
MIN
TYP
MAX
UNITS
–0.5
0.1
0.4
dB
–0.15
0
0.1
dB
–0.2
0.2
0.6
dB
–0.6
0.1
0.5
dB
660025fe
2
LT6600-2.5
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 = 1580Ω, and RLOAD = 1k.
PARAMETER
Filter Gain, VS = 5V
RIN = 1580Ω
CONDITIONS
VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz)
l
VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz)
l
VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz)
l
VIN = 2VP-P, fIN = DC to 260kHz
MIN
TYP
MAX
–2.1
–0.9
0
dB
–34
–31
dB
–51
–0.5
–0.1
UNITS
dB
0.4
dB
VIN = 2VP-P, fIN = 700kHz (Gain Relative to 260kHz)
l
–0.15
0
0.1
dB
VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz)
l
–0.2
0.2
0.6
dB
VIN = 2VP-P, fIN = 2.2MHz (Gain Relative to 260kHz)
l
–0.6
0.1
0.5
dB
VIN = 2VP-P, fIN = 2.5MHz (Gain Relative to 260kHz)
l
–2.1
–0.9
0
dB
VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz)
l
–34
–31
dB
VIN = 2VP-P, fIN = 12.5MHz (Gain Relative to 260kHz)
l
–51
dB
Filter Gain, VS = ±5V
VIN = 2VP-P, fIN = DC to 260kHz
–0.6
–0.1
0.4
dB
Filter Gain, RIN = 402Ω
VIN = 0.5VP-P, fIN = DC to 260kHz, VS = 3V
VIN = 0.5VP-P, fIN = DC to 260kHz, VS = 5V
VIN = 0.5VP-P, fIN = DC to 260kHz, VS = ±5V
11.3
11.3
11.2
11.8
11.8
11.7
12.3
12.3
12.2
dB
dB
dB
Filter Gain Temperature Coefﬁcient (Note 2) fIN = 260kHz, VIN = 2VP-P
780
ppm/°C
Noise
Noise BW = 10kHz to 2.5MHz
51
μVRMS
Distortion (Note 4)
1MHz, 1VRMS, RL = 800Ω
2nd Harmonic
3rd Harmonic
95
88
dBc
dBc
Differential Output Swing
Measured Between Pins 4 and 5
VS = 5V
VS = 3V
Input Bias Current
Average of Pin 1 and Pin 8
Input Referred Differential Offset
RIN = 1580Ω, Differential Gain = 1V/V
VS = 3V
VS = 5V
VS = ±5V
l
l
l
5
5
5
25
30
35
mV
mV
mV
RIN = 402Ω, Differential Gain = 4V/V
VS = 3V
VS = 5V
VS = ±5V
l
l
l
3
3
3
13
16
20
mV
mV
mV
0.0
0.0
–2.5
1.5
3.0
1.0
V
V
V
1.5
3.0
2.0
V
V
V
45
45
35
mV
mV
mV
l
l
8.8
5.1
9.3
5.5
VP-P DIFF
VP-P DIFF
l
–35
–15
μA
Differential Offset Drift
10
Input Common Mode Voltage (Note 3)
Differential Input = 500mVP-P,
RIN ≥ 402Ω
VS = 3V
VS = 5V
VS = ±5V
l
l
l
Output Common Mode Voltage (Note 5)
Differential Input = 2VP-P,
Pin 7 = Open
VS = 3V
VS = 5V
VS = ±5V
l
l
l
1.0
1.5
–1.0
VS = 3V
VS = 5V
VS = ±5V
l
l
l
–25
–30
–55
Output Common Mode Offset
(with Respect to Pin 2)
Common Mode Rejection Ratio
63
Voltage at VMID (Pin 7)
VS = 5V (S8)
VS = 5V (DFN)
VS = 3V
Power Supply Current
VOCM = VMID = VS /2
l
l
dB
2.46
2.45
2.51
2.51
1.5
2.55
2.56
V
V
V
7.7
kΩ
l
4.3
5.7
VS = 5V
VS = 3V
l
l
–15
–10
–3
–3
VS = 3V, VS = 5V
VS = 3V, VS = 5V
VS = ±5V
l
l
VMID Input Resistance
VOCM Bias Current
10
5
–10
μV/°C
26
28
μA
μA
30
33
36
mA
mA
mA
660025fe
3
LT6600-2.5
ELECTRICAL CHARACTERISTICS
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
≥ 402Ω. For ±5V supplies, the minimum input common mode voltage is
guaranteed by design to reach –5V.
Note 4: Distortion is measured differentially using a single-ended stimulus.
The input common mode voltage, the voltage at VOCM, and the voltage at
VMID are equal to one half of the total power supply voltage.
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-2.5 is guaranteed functional over the operating
temperature range of –40°C to 85°C.
Note 7: The LT6600C-2.5 is guaranteed to meet speciﬁed performance
from 0°C to 70°C and is designed, characterized and expected to meet
speciﬁed performance from –40°C and 85°C, but is not tested or QA
sampled at these temperatures. The LT6600I-2.5 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.
TYPICAL PERFORMANCE CHARACTERISTICS
Amplitude Response
12
11
300
–12
–1
280
10
280
–24
–2
260
9
260
–3
240
8
240
7
220
6
200
GAIN (dB)
–36
–48
–72
–4
220
–5
200
–6
180
VS = 5V
RIN = 1580Ω 160
GAIN = 1
–8
140
TA = 25°C
–9
120
0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0
FREQUENCY (MHz)
–7
–84
–96
100k
1M
10M
FREQUENCY (Hz)
50M
660025 G01
GAIN (dB)
300
180
VS = 5V
RIN = 402Ω 160
GAIN = 4
140
3
TA = 25°C
120
2
0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0
FREQUENCY (MHz)
5
4
660025 G03
660025 G02
Output Impedance
vs Frequency (OUT+ or OUT–)
CMRR
100
PSRR
90
110
VIN = 1VP-P
VS = 5V
100 R = 1580Ω
IN
GAIN = 1
90
V+ TO
DIFFERENTIAL OUT
VS = 3V
80
70
60
PSRR (dB)
CMRR (dB)
10
GROUP DELAY (ns)
320
0
VS = ±2.5V
RIN = 1580Ω
GAIN = 1
GROUP DELAY (ns)
12
–60
OUTPUT IMPEDANCE (Ω)
Passband Gain and Group Delay
320
0
GAIN (dB)
Passband Gain and Group Delay
1
80
70
1
50
40
30
60
20
50
0.1
100k
1M
10M
FREQUENCY (Hz)
100M
660025 G04
10
0
40
1k
10k
100k
1M
FREQUENCY (Hz)
10M
100M
660025 G05
1k
10k
100k
1M
FREQUENCY (Hz)
10M
100M
660025 G06
660025fe
4
LT6600-2.5
TYPICAL PERFORMANCE CHARACTERISTICS
Distortion vs Frequency
–60
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
–80
–90
VIN = 2VP-P
VS = 3V
RL = 800Ω AT
EACH OUTPUT
–100
–110
1
FREQUENCY (MHz)
0.1
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
–70
DISTORTION (dB)
–70
DISTORTION (dB)
Distortion vs Frequency
–60
–80
–90
VIN = 2VP-P
VS = 5V
RL = 800Ω AT
EACH OUTPUT
–100
–110
1
FREQUENCY (MHz)
0.1
10
10
660025 G08
660025 G07
Distortion vs Signal Level
Distortion vs Frequency
–60
–80
2ND HARMONIC,
DIFFERENTIAL INPUT
3RD HARMONIC,
DIFFERENTIAL INPUT
2ND HARMONIC,
SINGLE-ENDED INPUT
3RD HARMONIC,
SINGLE-ENDED INPUT
–50
DISTORTION (dB)
–70
DISTORTION (dB)
–40
DIFFERENTIAL INPUT,
2ND HARMONIC
DIFFERENTIAL INPUT,
3RD HARMONIC
SINGLE-ENDED INPUT,
2ND HARMONIC
SINGLE-ENDED INPUT,
3RD HARMONIC
–90
–60
–70
–80
–90
VIN = 2VP-P
VS = ±5V
RL = 800Ω AT
EACH OUTPUT
–100
–110
0.1
1
FREQUENCY (MHz)
VS = 3V
F = 1MHz
RL = 800Ω AT
EACH OUTPUT
–100
–110
1
0
10
2
3
4
INPUT LEVEL (VP-P)
Distortion vs Signal Level
–60
–70
2ND HARMONIC,
DIFFERENTIAL INPUT
3RD HARMONIC,
DIFFERENTIAL INPUT
2ND HARMONIC,
SINGLE-ENDED INPUT
3RD HARMONIC,
SINGLE-ENDED INPUT
–50
DISTORTION (dB)
DISTORTION (dB)
Distortion vs Signal Level
–40
2ND HARMONIC,
DIFFERENTIAL INPUT
3RD HARMONIC,
DIFFERENTIAL INPUT
2ND HARMONIC,
SINGLE-ENDED INPUT
3RD HARMONIC,
SINGLE-ENDED INPUT
–50
6
660025 G10
660025 G09
–40
5
–80
–60
–70
–80
–90
–90
VS = 5V
F = 1MHz
RL = 800Ω AT
EACH OUTPUT
–100
–110
0
1
2
3
4
5
6
7
INPUT LEVEL (VP-P)
8
VS = ±5V
F = 1MHz
RL = 800Ω AT
EACH OUTPUT
–100
–110
9
660025 G11
0
1
2
3
4
5
6
7
INPUT LEVEL (VP-P)
8
9
660025 G12
660025fe
5
LT6600-2.5
TYPICAL PERFORMANCE CHARACTERISTICS
Distortion
vs Input Common Mode Level
–50
–60
–70
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–40
2VP-P 1MHz INPUT
RIN = 1580Ω
GAIN = 1
DISTORTION COMPONENT (dB)
DISTORTION COMPONENT (dB)
–40
Distortion
vs Input Common Mode Level
–80
–90
–100
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–50
–60
–70
–80
–90
–100
–110
2VP-P 1MHz INPUT, RIN = 402Ω, GAIN = 4
–110
–2
–1
0
1
2
–3
3
INPUT COMMON MODE VOLTAGE RELATIVE TO VMID (V)
–2
–1
0
1
2
–3
3
INPUT COMMON MODE VOLTAGE RELATIVE TO VMID (V)
660025 G13
660025 G14
Distortion
vs Output Common Mode Level
Supply Current
vs Total Supply Voltage
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
2ND HARMONIC,
VS = ±5V
3RD HARMONIC,
VS = ±5V
–50
–60
–70
–80
–90
–100
32
30
TOTAL SUPPLY CURRENT (mA)
DISTORTION COMPONENT (dB)
–40
TA = 85°C
28
TA = 25°C
26
24
22
TA = –40°C
20
18
2VP-P 1MHz INPUT, RIN = 1580Ω, GAIN = 1
–110
–1.5 –1.0 –0.5
0
1.0 1.5 2.0
VOLTAGE VOCM TO VMID (V)
0.5
16
2
2.5
3
4
6
8
5
7
9
TOTAL SUPPLY VOLTAGE (V)
10
660025 G16
660025 G15
Transient Response Gain = 1
VOUT+
50mV/DIV
DIFFERENTIAL
INPUT
200mV/DIV
500ns/DIV
660025 G17
660025fe
6
LT6600-2.5
PIN FUNCTIONS
(DFN/SO)
IN– and IN+ (Pins 1, 12/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 1580Ω/RIN.
NC (Pins 2, 5, 11/NA): No Connection
VOCM (Pin 3/Pin 2): DC Common Mode Reference Voltagefor the 2nd Filter Stage. Its value programs the common
mode voltage of the differential output of the ﬁlter. This
is a high impedance input, which can be driven from an
external voltage reference, or it can be tied to VMID on the
PC board. VOCM should be bypassed with a 0.01μF ceramic
capacitor unless it is connected to a ground plane.
V+ and V– (Pins 4, 8, 9/Pins 3, 6): Power Supply Pins. For
a single 3.3V or 5V supply (V– grounded) a quality 0.1μF
ceramic bypass capacitor is required from the positive
supply pin (V+) to the negative supply pin (V–). The bypass
should be as close as possible to the IC. For dual supply
applications, bypass V+ to ground and V– to ground with
a quality 0.1μF ceramic capacitor.
OUT+ and OUT– (Pins 6, 7/Pins 4, 5): Output Pins. These
are the ﬁlter differential outputs. Each pin can drive a 100Ω
and/or 50pF load to AC ground.
VMID (Pin 10/Pin 7): The VMID pin is internally biased at
mid-supply, see Block Diagram. For single supply operation, the VMID pin should be bypassed with a quality
0.01μF ceramic capacitor to V–. For dual supply operation,
VMID 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. VMID 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+
OUT–
V–
VMID
V+
11k
PROPRIETARY
LOWPASS
FILTER STAGE
1580Ω
11k
800Ω
V–
OP AMP
+
800Ω
+ –
–
VOCM
–
VOCM
+
– +
800Ω
800Ω
1580Ω
660025 BD
VIN–
IN–
VOCM
V+
OUT+
RIN
660025fe
7
LT6600-2.5
APPLICATIONS INFORMATION
Interfacing to the LT6600-2.5
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
is 2VP-P for frequencies below 2.5MHz. The common mode
output voltage is determined by the voltage at VOCM. Since
VOCM is shorted to VMID, 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 VMID.
Note: The referenced pin numbers correspond to the S8
package. See the Pin Functions for the equivalent DFN-12
package pin numbers.
The LT6600-2.5 requires two equal external resistors, RIN,
to set the differential gain to 1580Ω/RIN. The inputs to the
ﬁlter are the voltages VIN+ and VIN– presented to the see
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 LT6600-2.5 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 LT6600-2.5.
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 1580Ω gain setting resistor form a highpass ﬁlter, attenuating signals
below 1kHz. Larger values of coupling capacitors will
proportionally reduce this highpass 3dB frequency.
Figure 1 illustrates the LT6600-2.5 operating with a single
3.3V supply and unity passband gain; the input signal is
In Figure 3 the LT6600-2.5 is providing 12dB of gain. The
common mode output voltage is set to 2V.
3.3V
0.1μF
V
3
VIN
–
1580Ω
1
7
2
VIN
1
+
0.01μF
VIN
0
2
t
VIN–
8
+
V
3
3
–
+
4
VOUT+
LT6600-2.5
–5
+
1580Ω
VOUT–
6
2
VOUT+
1
VOUT–
t
0
660025 F01
Figure 1. (S8 Pin Numbers)
3.3V
0.1μF
V
0.1μF
2
1580Ω
1
7
1
VIN
0
+
0.1μF
t
VIN
2
0.01μF
8
+
–
+
4
LT6600-2.5
–
+
1580Ω
–1
V
3
5
3
VOUT+
VOUT–
6
VOUT+
2
VOUT–
1
0
660025 F02
t
Figure 2. (S8 Pin Numbers)
5V
0.1μF
V
3
VIN
–
402Ω
1
7
2
1
0
VIN+
VIN–
2
0.01μF
500mVP-P (DIFF)
VIN
t
8
+
–
+
4
LT6600-2.5
–
+
402Ω
+
–
V
3
5
3
VOUT+
VOUT+
2
VOUT–
6
2V
1
0
VOUT–
660025 F03
t
Figure 3. (S8 Pin Numbers)
660025fe
8
LT6600-2.5
APPLICATIONS INFORMATION
Use Figure 4 to determine the interface between the
LT6600-2.5 and a current output DAC. The gain, or “transimpedance,” is deﬁned as A = VOUT/IIN. To compute the
transimpedance, use the following equation:
A=
1580 • R1
(Ω)
(R1+ R2)
By setting R1 + R2 = 1580Ω, the gain equation reduces
to A = R1(Ω).
The voltage at the pins of the DAC is determined by R1,
R2, the voltage on VMID and the DAC output current.
Consider Figure 4 with R1 = 49.9Ω and R2 = 1540Ω. The
voltage at VMID, for VS = 3.3V, is 1.65V. The voltage at the
DAC pins is given by:
R1
R1• R2
+IIN •
R1+ R2 + 1580
R1+ R2
= 26mV +IIN • 48.3Ω
VDAC = VPIN7 •
IIN is IIN+ or IIN–. The transimpedance in this example is
49.6Ω.
Evaluating the LT6600-2.5
The low impedance levels and high frequency operation
of the LT6600-2.5 require some attention to the matching
networks between the LT6600-2.5 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 LT6600-2.5 with a network analyzer.
Figure 5 is a laboratory setup that can be used to characterize the LT6600-2.5 using single-ended instruments
with 50Ω source impedance and 50Ω input impedance.
For a 12dB gain conﬁguration the LT6600-2.5 requires a
402Ω source resistance yet the network analyzer output is
calibrated for a 50Ω load resistance. The 1:1 transformer,
53.6Ω and 388Ω resistors satisfy the two constraints
above. The transformer converts the single-ended source
into a differential stimulus. Similarly, the output of the
LT6600-2.5 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 LT6600-2.5
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.
Differential and Common Mode Voltage Ranges
The rail-to-rail output stage of the LT6600-2.5 can process
large differential signal levels. On a 3V supply, the output
signal can be 5.1VP-P. Similarly, a 5V supply can support
signals as large as 8.8VP-P. To prevent excessive power
dissipation in the internal circuitry, the user must limit
differential signal levels to 9VP-P.
The two ampliﬁers inside the LT6600-2.5 have independent control of their output common mode voltage (see
the Block Diagram section). The following guidelines will
optimize the performance of the ﬁlter.
2.5V
CURRENT
OUTPUT
DAC
0.1μF
IIN–
R1
R2
R1
1
7
0.01μF
NETWORK
ANALYZER
SOURCE
3
– +
4
VOUT+
50Ω
COILCRAFT
TTWB-1010
1:1 388Ω 1
7
53.6Ω
2
2 LT6600-2.5
IIN+
660025 F04
0.1μF
3.3V
8
R2
–
+
6
5
8
VOUT–
VOUT+ – VOUT–
IIN+ – IIN–
388Ω
=
3
–
+
4
COILCRAFT
TTWB-16A
4:1
402Ω
LT6600-2.5
–
+
6
402Ω
NETWORK
ANALYZER
INPUT
50Ω
5
0.1μF
660025 F05
1580 • R1
R1 + R2
–2.5V
Figure 4. (S8 Pin Numbers)
Figure 5. (S8 Pin Numbers)
660025fe
9
LT6600-2.5
APPLICATIONS INFORMATION
VMID can be allowed to ﬂoat, but it must be bypassed to
an AC ground with a 0.01μF capacitor or some instability
maybe observed. VMID 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 VMID. 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 VMID.
VOCM can be shorted to VMID for simplicity. If a different
common mode output voltage is required, connect VOCM
to a voltage source or resistor network. For 3V and 3.3V
supplies the voltage at VOCM must be less than or equal
to the mid-supply level. For example, voltage (VOCM) ≤
1.65V on a single 3.3V supply. For power supply voltages
higher than 3.3V the voltage at VOCM can be set above
mid-supply, as shown in Table 1. The voltage on VOCM
should not exceed 1V below the voltage on VMID. VOCM
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
4VP-P
1.4V ≤ VOCM ≤ 1.6V
2VP-P
1V ≤ VOCM ≤ 1.6V
1VP-P
0.75V ≤ VOCM ≤ 1.6V
8VP-P
2.4V ≤ VOCM ≤ 2.6V
4VP-P
1.5V ≤ VOCM ≤ 3.5V
5V
±5V
2VP-P
1V ≤ VOCM ≤ 3.75V
1VP-P
0.75V ≤ VOCM ≤ 3.75V
9VP-P
–2V ≤ VOCM ≤ 2V
4VP-P
–3.5V ≤ VOCM ≤ 3.5V
2VP-P
–3.75V ≤ VOCM ≤ 3.75V
1VP-P
–4.25V ≤ VOCM ≤ 3.75V
NOTE: VOCM is set by the voltage at this RIN. The voltage at VOCM should
not exceed 1V below the voltage at VMID. To achieve some of the output
common mode ranges shown in the table, the voltage at VMID 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-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
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 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. VMID sets the output
common mode voltage of the 1st differential ampliﬁer
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 1580Ω feedback
resistor and the external 402Ω input resistor. The resulting 1.25mA common mode DC current in each input
path,must be absorbed by the sources VIN+ and VIN–.
VOCM sets the common mode output voltage of the 2nd
differential ampliﬁer inside the LT6600-2.5, and therefore
sets the common mode output voltage of the ﬁlter. Since,
in the example of Figure 3, VOCM differs from VMID by 0.5V,
an additional 625μA (312μA per side) of DC current will
ﬂow in the resistors coupling the 1st differential ampliﬁer
output stage to ﬁlter output. Thus, a total of 3.125mA is
used to translate the common mode voltages.
A simple modiﬁcation to Figure 3 will reduce the DC
common mode currents by 36%. If VMID is shorted toVOCM
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.
660025fe
10
LT6600-2.5
APPLICATIONS INFORMATION
Noise
100
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 ﬂoor of the spectrum
analyzer and subtract the instrument noise from the ﬁlter
noise measurement.
2.5V
VIN
RIN
1
7
2
8
RIN
3
– +
4
COILCRAFT
TTWB-1010
25Ω
1:1
6
5
0.1μF
–2.5V
Figure 6. (S8 Pin Numbers)
Example: With the IC removed and the 25Ω resistorsgrounded, Figure 6, measure the total integrated noise (eS)
of the spectrum analyzer from 10kHz to 2.5MHz. 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 100kHz 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:
(eO )2 – (eS )2
A
Table 2 lists the typical input referred integrated noise for
various values of RIN.
Table 2. Noise Performance
RIN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 2.5MHz
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 5MHz
402Ω
18μVRMS
23μVRMS
2
806Ω
29μVRMS
39μVRMS
1
1580Ω
51μVRMS
73μVRMS
PASSBAND
GAIN (V/V)
4
20
40
10
20
INTEGRATED
SPECTRUM
ANALYZER
INPUT
660025 F06
eIN =
60
0
0.1
1
10
FREQUENCY (MHz)
660025 F07
50Ω
25Ω
–
30
Figure 7. Input Referred Noise, Gain = 1
LT6600-2.5
+
SPECTRAL DENSITY
0
0.01
0.1μF
80
40
INTEGRATED NOISE (μVRMS)
The noise performance of the LT6600-2.5 can be evaluated
with the circuit of Figure 6.
NOISE SPECTRAL DENSITY (nVRMS/√Hz)
50
Figure 7 is plot of the noise spectral density as a function
of frequency for an LT6600-2.5 with RIN = 1580Ω using
the ﬁxture 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 ampliﬁers combine high speed with largesignal 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 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
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, θJA, of
about 85°C/W. Without the extra metal trace connected to
660025fe
11
LT6600-2.5
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
(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
Junction temperature, TJ, is calculated from the ambienttemperature, 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:
TJ = TA + (PD • θJA) = TA + (VS • IS • θJA)
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, 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 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
35mm2 copper trace, the θJA is 100°C/W. The resulting
junction temperature is:
TJ = TA + (PD • θJA) = 85 + (5 • 0.0376 • 100) = 104°C
When using higher supply voltages or when driving small
impedances, more copper may be necessary to keep TJ
below 150°C.
where the supply current, IS, is a function of signal level, load
impedance, temperature and common mode voltages.
660025fe
12
LT6600-2.5
PACKAGE DESCRIPTION
DF Package
12-Lead Plastic DFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1733 Rev Ø)
2.50 REF
0.70 ±0.05
3.38 ±0.05
4.50 ± 0.05
3.10 ± 0.05
2.65 ± 0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ± 0.10
(4 SIDES)
2.50 REF
7
12
0.40 ± 0.10
3.38 ±0.10
2.65 ± 0.10
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 × 45°
CHAMFER
PIN 1
TOP MARK
(NOTE 6)
(DF12) DFN 0806 REV Ø
0.200 REF
6
R = 0.115
TYP
0.75 ± 0.05
1
0.25 ± 0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
0.00 – 0.05
NOTE:
1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220
VARIATION (WGGD-X)—TO BE APPROVED
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
660025fe
13
LT6600-2.5
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)
0°– 8° TYP
.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN
.053 – .069
(1.346 – 1.752)
.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)
2
3
4
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
SO8 0303
660025fe
14
LT6600-2.5
REVISION HISTORY
(Revision history begins at Rev E)
REV
DATE
DESCRIPTION
E
5/10
Updated Order Information section
PAGE NUMBER
2
660025fe
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.
15
LT6600-2.5
TYPICAL APPLICATION
5th Order Lowpass Filter (S8 Pin Numbers Shown)
V+
0.1μF
VIN–
R
R
1
7
C
VIN+
C=
GAIN =
2
8
R
R
–
3
+
+
–
6
VOUT+
VOUT–
5
0.1μF
1
2π • R • 2.5MHz
1580Ω
, MAXIMUM GAIN = 4
2R
4
LT6600
V–
660025 TA02a
Amplitude Response
10
Transient Response Gain = 1
VS = ±2.5V
GAIN = 1
R = 787Ω
TA = 25°C
0
–10
VOUT+
50mV/DIV
GAIN (dB)
–20
–30
DIFFERENTIAL
INPUT
200mV/DIV
–40
–50
–60
–70
–90
100k
660025 TA02c
500ns/DIV
–80
1M
FREQUENCY (Hz)
10M 20M
660025 TA02b
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660025fe
16 Linear Technology Corporation
LT 0510 REV E • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2003

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