LINER LT6600IS8-15

LT6600-15
Very Low Noise, Differential
Amplifier and 15MHz Lowpass Filter
FEATURES
DESCRIPTION
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The LT®6600-15 combines a fully differential amplifier with a
4th order 15MHz lowpass filter approximating a Chebyshev
frequency response. Most differential amplifiers require
many precision external components to tailor gain and
bandwidth. In contrast, with the LT6600-15, two external
resistors program differential gain, and the filter’s 15MHz
cutoff frequency and passband ripple are internally set.
The LT6600-15 also provides the necessary level shifting
to set its output common mode voltage to accommodate
the reference voltage requirements of A/Ds.
n
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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 15MHz
Cutoff
76dB S/N with 3V Supply and 2VP-P Output
Low Distortion, 2VP-P, 800Ω Load, VS = 3V
1MHz: 86dBc 2nd, 90dBc 3rd
10MHz: 63dBc 2nd, 69dBc 3rd
Fully Differential Inputs and Outputs
Compatible with Popular Differential Amplifier
Pinouts
SO-8 Package
APPLICATIONS
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High Speed ADC Antialiasing and DAC Smoothing in
Networking or Cellular Base Station Applications
High Speed Test and Measurement Equipment
Medical Imaging
Drop-in Replacement for Differential Amplifiers
Using a proprietary internal architecture, the LT6600-15
integrates an antialiasing 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 76dB. 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 LT6600-15 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.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
The LT6600-15 is packaged in an SO-8 and is pin compatible with stand alone differential amplifiers.
TYPICAL APPLICATION
An 8192 Point FFT Spectrum
0
–10
LTC2249
LT6600-15
3V
0.1μF
7
0.01μF
VIN
RIN
536Ω
2
8
5.6pF
3
–
VMID
VOCM
+
–20
3V
+
–
4
25Ω
25Ω
5
6
V+
+
5.6pF
–
5.6pF
DOUT
AIN
V–
VCM
AMPLITUDE (dB)
RIN
536Ω 1
INPUT 10.7MHz
2VP-P
fSAMPLE = 80MHz
–30
–40
–50
–60
–70
–80
–90
–100
2.2μF
–110
GAIN = 536Ω/RIN
660015 TA01a
–120
0
10
20
30
40
FREQUENCY (MHz)
660015 TA01b
660015fa
1
LT6600-15
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
Total Supply Voltage .................................................11V
Input Voltage (Note 8)...............................................±VS
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
TEMPERATURE RANGE
LT6600CS8-15#PBF
LT6600CS8-15#TRPBF
660015
8-Lead Plastic SO
–40°C to 85°C
LT6600IS8-15#PBF
LT6600IS8-15#TRPBF
600I15
8-Lead Plastic SO
–40°C to 85°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT6600CS8-15
LT6600CS8-15#TR
660015
8-Lead Plastic SO
–40°C to 85°C
LT6600IS8-15
LT6600IS8-15#TR
600I15
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 = 536Ω, and RLOAD = 1k.
PARAMETER
CONDITIONS
MIN
TYP
MAX
Filter Gain, VS = 3V
VIN = 2VP-P, fIN = DC to 260kHz
– 0.5
0.1
0.5
dB
Filter Gain, VS = 5V
UNITS
VIN = 2VP-P, fIN = 1.5MHz (Gain Relative to 260kHz)
l
–0.1
0
0.1
dB
VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz)
l
–0.3
0
0.4
dB
VIN = 2VP-P, fIN = 12MHz (Gain Relative to 260kHz)
l
–0.3
0.2
1.0
dB
VIN = 2VP-P, fIN = 15MHz (Gain Relative to 260kHz)
l
– 0.7
0
1.0
dB
VIN = 2VP-P, fIN = 45MHz (Gain Relative to 260kHz)
l
– 29
–25
dB
VIN = 2VP-P, fIN = 75MHz (Gain Relative to 260kHz)
l
–46
VIN = 2VP-P, fIN = DC to 260kHz
dB
– 0.5
0
0.5
dB
0
0.1
dB
VIN = 2VP-P, fIN = 1.5MHz (Gain Relative to 260kHz)
l
– 0.1
VIN = 2VP-P, fIN = 7.5MHz (Gain Relative to 260kHz)
l
–0.4
0
0.3
dB
VIN = 2VP-P, fIN = 12MHz (Gain Relative to 260kHz)
l
–0.4
0.1
0.9
dB
VIN = 2VP-P, fIN = 15MHz (Gain Relative to 260kHz)
l
–0.8
0
0.9
dB
VIN = 2VP-P, fIN = 45MHz (Gain Relative to 260kHz)
l
– 29
–25
dB
VIN = 2VP-P, fIN = 75MHz (Gain Relative to 260kHz)
l
– 46
dB
Filter Gain, VS = ±5V
VIN = 2VP-P, fIN = DC to 260kHz
– 0.6
–0.1
0.4
dB
Filter Gain, RIN = 133Ω
VOUT = 2VP-P, fIN = DC to 260kHz, VS = 3V
VOUT = 2VP-P, fIN = DC to 260kHz, VS = 5V
VOUT = 2VP-P, fIN = DC to 260kHz, VS = ±5V
11.5
11.5
11.4
12.0
12.0
11.9
12.5
12.5
12.4
dB
dB
dB
660015fa
2
LT6600-15
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 = 536Ω, and RLOAD = 1k.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Filter Gain Temperature Coefficient (Note 2)
fIN = 250kHz, VIN = 2VP-P
780
ppm/C
Noise
Noise BW = 10kHz to 15MHz
109
μVRMS
Distortion (Note 4)
1MHz, 2VP-P, RL = 800Ω, VS = 3V
2nd Harmonic
3rd Harmonic
86
90
dBc
dBc
10MHz, 2VP-P, RL = 800Ω, VS = 3V
2nd Harmonic
3rd Harmonic
63
69
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 = 536Ω
VS = 3V
VS = 5V
VS = ±5V
●
●
●
±5
±10
±10
±25
±30
±35
mV
mV
mV
RIN = 133Ω
VS = 3V
VS = 5V
VS = ±5V
●
●
●
±5
±5
±5
±15
±17
±20
mV
mV
mV
●
●
3.80
3.75
4.75
4.50
VP-P DIFF
VP-P DIFF
●
– 90
– 35
μA
Differential Offset Drift
10
μV/°C
Input Common Mode Voltage (Note 3)
Differential Input = 500mVP-P,
RIN = 133Ω
VS = 3V
VS = 5V
VS = ±5V
●
●
●
0.0
0.0
–2.5
1.5
3.0
1.0
V
V
V
Output Common Mode Voltage (Note 5)
Differential Input = 2VP-P,
Pin 7 at Mid-Supply
Common Mode Voltage at Pin 2
VS = 3V
VS = 5V
VS = ±5V
●
●
●
1.0
1.5
–1.0
1.5
3.0
2.0
V
V
V
VS = 3V
VS = 5V
VS = ±5V
●
●
●
–35
–40
–55
5
0
–10
40
40
35
mV
mV
mV
VS = 5V
VS = 3V
l
2.45
2.50
1.50
2.55
V
V
l
4.3
5.7
7.7
kΩ
VS = 5V
VS = 3V
●
●
–10
–10
–2
–2
VS = 3V, VS = 5V
VS = 3V
VS = 5V
VS = ±5V
●
●
●
Output Common Mode Offset
(with Respect to Pin 2)
Common Mode Rejection Ratio
64
Voltage at VMID (Pin 7)
VMID Input Resistance
VOCM Bias Current
VOCM = VMID= VS/2
Power Supply Current
Power Supply Voltage
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 ≥ 100Ω.
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.
35
l
38
3
dB
μA
μA
39
44
45
48
mA
mA
mA
mA
11
V
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-15 is guaranteed functional over the operating
temperature range –40°C to 85°C.
Note 7: The LT6600C-15 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-15 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.
660015fa
3
LT6600-15
TYPICAL PERFORMANCE CHARACTERISTICS
Amplitude Response
Passband Gain and Phase
10
1
0
0
GAIN
VS = 5V
GAIN = 1
TA = 25°C
180
0
–20
–3
45
GAIN (dB)
–30
VS = 5V
GAIN = 1
TA = 25°C
–60
0.1
1
10
FREQUENCY (MHz)
100
20
–90
–6
15
–7
–135
–7
10
–8
–180
–8
5
–9
–225
–9
–6
5
0
15
10
FREQUENCY (MHz)
20
45
25
2
20
0
15
–2
10
–4
5
5
15
10
FREQUENCY (MHz)
20
25
DELAY (ns)
30
DELAY
0
70
1
50
45
40
30
1
10
FREQUENCY (MHz)
1
10
FREQUENCY (MHz)
0.1
100
100
660015 G06
Distortion vs Frequency
–60
DISTORTION (dB)
60
PSRR (dB)
55
660015 G05
70
40
30
Distortion vs Signal Level
–70
–40
VIN = 2VP-P
VS = 3V
RL = 800Ω AT
EACH OUTPUT
GAIN = 1
TA = 25°C
–80
–90
3RD HARMONIC
VS = 3V
10MHz INPUT
RL = 800Ω AT
–50 EACH OUTPUT
GAIN = 1
–60 TA = 25°C
–70
2ND
HARMONIC
10MHz INPUT
–80
–90
VS = 3V
VIN = 200mVP-P
TA = 25°C
V+ TO DIFFOUT
–100
2ND
HARMONIC
1MHz INPUT
–100
–110
100
600015 G07
4
60
35
0
50
0
65
–50
1
10
FREQUENCY (MHz)
25
VIN = 1VP-P
VS = 5V
GAIN = 1
TA = 25°C
75
10
Power Supply Rejection Ratio
0
0.1
20
Common Mode Rejection Ratio
VS = 5V
GAIN = 1
TA = 25°C
0.1
0.1
80
10
15
10
FREQUENCY (MHz)
660015 G03
80
660015 G04
20
5
0
CMRR (dB)
40
4
–6
100
35
6
25
660015 G02
50
8
30
25
–45
OUTPUT IMPEDANCE (Ω)
GAIN
10
40
35
DELAY
Output Impedance
VS = 5V
GAIN = 4
TA = 25°C
45
–5
0
–5
Passband Gain and Delay
12
50
–4
–4
66005 G01
14
GAIN
–3
DISTORTION (dB)
–50
PHASE
GAIN (dB)
–2
VS = 5V
GAIN = 1
TA = 25°C
DELAY (ns)
–1
90
PHASE (DEG)
135
–2
–40
GAIN (dB)
1
–10
–1
GAIN (dB)
Passband Gain and Delay
225
0.1
1
10
FREQUENCY (MHz)
100
660015 G08
DIFFERENTIAL INPUT, 2ND HARMONIC
DIFFERENTIAL INPUT, 3RD HARMONIC
SINGLE-ENDED INPUT, 2ND HARMONIC
SINGLE-ENDED INPUT, 3RD HARMONIC
3RD
HARMONIC
1MHz INPUT
–110
0
1
2
3
4
5
INPUT LEVEL (VP-P)
660015 G09
660015fa
LT6600-15
TYPICAL PERFORMANCE CHARACTERISTICS
Distortion
vs Input Common Mode Level
–40
3RD
HARMONIC,
10MHz INPUT
–80
–90
2ND HARMONIC,
1MHz INPUT
–100
1
2
3
–70
–80
–90
–110
5
4
INPUT LEVEL (VP-P)
–2
–1
0
1
2
–3
3
INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–50
–60
–70
–80
–90
–100
GAIN = 4, RL = 800Ω AT EACH OUTPUT
TA = 25°C, 500mVP-P 1MHz INPUT
–3
3
–2
–1
0
1
2
INPUT COMMON MODE VOTLAGE RELATIVE TO PIN 7 (V)
66002 G11
660015 G10
660015 G12
Distortion
vs Output Common Mode
–40
DISTORTION COMPONENT (dB)
0
–60
–40
GAIN = 1
RL = 800Ω AT EACH
OUTPUT
TA = 25°C
2VP-P 1MHz INPUT
–100
3RD HARMONIC,
1MHz INPUT
–110
2ND HARMONIC,
VS = 3V
3RD HARMONIC,
VS = 3V
2ND HARMONIC,
VS = 5V
3RD HARMONIC,
VS = 5V
–50
DISTORTION COMPONENT (dB)
VS = ±5V
RL = 800Ω AT EACH OUTPUT
–50 GAIN = 1
TA = 25°C
–60
2ND HARMONIC,
10MHz INPUT
–70
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
2VP-P 1MHz INPUT
GAIN = 1,
RL = 800Ω AT EACH OUTPUT
TA = 25°C
–100
–110
–1.5 –1 –0.5
0 0.5 1 1.5 2
VOLTAGE PIN 2 TO PIN 7 (V)
2.5
660015 G13
Total Supply Current
vs Total Supply Voltage
Transient Response
50
OUT–
200mV/DIV
TOTAL SUPPLY CURRENT (mA)
DISTORTION (dB)
–40
Distortion
vs Input Common Mode Level
DISTORTION COMPONENT (dB)
Distortion vs Signal Level
45
TA = 85°C
40
TA = 25°C
OUT+
200mV/DIV
35
IN–
IN+
500mV/DIV
TA = –40°C
30
25
20
2
10
4
6
8
TOTAL SUPPLY VOLTAGE (V)
12
100ns/DIV
DIFFERENTIAL GAIN = 1
SINGLE-ENDED INPUT
DIFFERENTAL OUTPUT
660015 G15
660015 G14
660015fa
5
LT6600-15
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 536Ω/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.
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
536Ω
11k
200Ω
V–
OP AMP
+
200Ω
+ –
–
VOCM
–
VOCM
+
– +
200Ω
200Ω
536Ω
1
VIN–
RIN
IN–
2
3
4
VOCM
V+
OUT+
660015 BD
660015fa
6
LT6600-15
APPLICATIONS INFORMATION
output voltage is 1.65V, and the differential output voltage
is 2VP-P for frequencies below 15MHz. 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).
Interfacing to the LT6600-15
The LT6600-15 requires two equal external resistors, RIN,
to set the differential gain to 536Ω/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 LT6600-15 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
LT6600-15. 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 536Ω gain setting
resistor form a high pass filter, attenuating signals below
3kHz. Larger values of coupling capacitors will proportionally reduce this highpass 3dB frequency.
Figure 1 illustrates the LT6600-15 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
3.3V
0.1μF
V
3
–
536Ω
1
VIN
VIN+
1
0.01μF
+
VIN
0
t
VIN–
3
–
7
2
V
3
4
+
VOUT+
LT6600-15
2
8
536Ω
VOUT–
–5
+
6
2
VOUT+
1
VOUT–
t
0
660015 F01
Figure 1
3.3V
0.1μF
V
0.1μF
2
536Ω
1
7
1
VIN+
0
0.1μF
t
+
VIN
–1
2
0.01μF
8
V
3
–
+
4
LT6600-15
–
+
536Ω
5
3
VOUT+
2
VOUT–
1
6
VOUT+
VOUT–
0
660015 F02
Figure 2
62pF
5V
0.1μF
V
–
3
133Ω
1
VIN
7
2
1
0
VIN+
VIN–
2
0.01μF
500mVP-P (DIFF)
8
+
VIN
–
+
–
+
4
LT6600-15
–
+
133Ω
t
V
3
6
2V
5
3
VOUT+
VOUT+
2
VOUT–
1
0
VOUT–
660015 F03
t
62pF
Figure 3
660015fa
7
LT6600-15
APPLICATIONS INFORMATION
In Figure 3 the LT6600-15 is providing 12dB of gain. The
gain resistor has an optional 62pF in parallel to improve
the passband flatness near 15MHz. The common mode
output voltage is set to 2V.
Use Figure 4 to determine the interface between the
LT6600-15 and a current output DAC. The gain, or “transimpedance,” is defined as A = VOUT/IIN. To compute the
transimpedance, use the following equation:
A=
536 • R1
(Ω)
(R1+ R2)
By setting R1 + R2 = 536Ω, the gain equation reduces
to A = R1(Ω).
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 = 49.9Ω and R2 = 487Ω. The
voltage at Pin 7 is 1.65V. The voltage at the DAC pins is
given by:
R1
R1• R2
+ IIN •
R1+ R2 + 536
R1+ R2
= 77mV + IIN • 45.3Ω
VDAC = VPIN7 •
IIN is IIN+ or IIN–. The transimpedance in this example is
49.8Ω.
Evaluating the LT6600-15
The low impedance levels and high frequency operation
of the LT6600-15 require some attention to the matching
networks between the LT6600-15 and other devices. The
previous examples assume an ideal (0Ω) source impedance
and a large (1kΩ) load resistance. Among practical ex-
amples where impedance must be considered is the evaluation of the LT6600-15 with a network analyzer. Figure 5
is a laboratory setup that can be used to characterize the
LT6600-15 using single-ended instruments with 50Ω
source impedance and 50Ω input impedance. For a unity
gain configuration the LT6600-15 requires a 536Ω source
resistance yet the network analyzer output is calibrated
for a 50Ω load resistance. The 1:1 transformer, 52.3Ω
and 523Ω resistors satisfy the two constraints above.
The transformer converts the single-ended source into a
differential stimulus. Similarly, the output of the LT6600-15
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 LT6600-15 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 differential amplifiers inside the LT6600-15 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 LT6600-15 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
2.5V
0.1μF
CURRENT
OUTPUT
DAC
3.3V
NETWORK
ANALYZER
SOURCE
0.1μF
IIN–
R1
R2
0.011μF
IIN+
R1
1
3
7
– +
2
LT6600-15
8
–
R2
COILCRAFT
TTWB-1010
1:1 523Ω 1
+
4
5
VOUT+
50Ω
7
52.3Ω
2
8
VOUT–
523Ω
3
–
+
4
COILCRAFT
TTWB-16A
4:1
402Ω
LT6600-15
–
+
6
5
0.1μF
NETWORK
ANALYZER
INPUT
50Ω
402Ω
660015 F05
6
660015 F04
–2.5V
Figure 4
Figure 5
660015fa
8
LT6600-15
APPLICATIONS INFORMATION
20
1dB COMPRESSION
POINTS
OUTPUT LEVEL (dBV)
0
The LT6600-15 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 Distortion vs
Input Common Mode Level in the Typical Performance
Characteristics section).
25°C
85°C
3RD HARMONIC
85°C
–20
–40
3RD HARMONIC
25°C
–60
2ND
HARMONIC
85°C
–80
2ND HARMONIC, 25°C
–100
0
1
4
3
5
2
1MHz INPUT LEVEL (VP-P)
6
7
660015 F06
Figure 6. Output Level vs Input Level,
Differential 1MHz Input, Gain = 1
supply rails, the input/output behavior of the IC shown
in Figure 6 is relatively independent of the power supply
voltage.
The two amplifiers inside the LT6600-15 have independent control of their output common mode voltage (see
the Block Diagram section). The following guidelines will
optimize the performance of the filter.
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.
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the
LT6600-15 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 LT6600-15 (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 536Ω feedback resistor and the
external 133Ω input resistor. The resulting 3.7mA 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
LT6600-15, and therefore sets the common mode output
voltage of the filter. 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 flow in the resistors coupling
the 1st differential amplifier output stage to filter output.
Thus, a total of 9.9mA is used to translate the common
mode voltages.
A simple modification to Figure 3 will reduce the DC common mode currents by 40%. 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 6mA. Of
course, by AC coupling the inputs of Figure 3, the common
mode DC current can be reduced to 2.5mA.
660015fa
9
LT6600-15
APPLICATIONS INFORMATION
Noise
2.5V
0.1μF
The noise performance of the LT6600-15 can be evaluated
with the circuit of Figure 7.
RIN
VIN
7
Given the low noise output of the LT6600-15 and the
6dB attenuation of the transformer coupling network, it
is necessary to measure the noise floor of the spectrum
analyzer and subtract the instrument noise from the filter
noise measurement.
eIN =
8
4
LT6600-15
–
+
6
RIN
50Ω
25Ω
5
0.1μF
660015 F07
–2.5V
Figure 7
45
NOISE DENSITY (nVRMS/√Hz)
40
35
30
25
180
NOISE DENSITY,
GAIN = 1x
NOISE DENSITY,
GAIN = 4x
INTEGRATED NOISE,
GAIN = 1x
INTEGRATED NOISE,
GAIN = 4x
160
140
120
100
20
80
15
60
10
40
5
20
0
0.01
(eO )2 – (eS )2
A
0.1
1
10
FREQUENCY (MHz)
0
100
660015 F08
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 LT6600-15 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 15MHz
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 30MHz
4
133Ω
36μVRMS
51μVRMS
2
267Ω
62μVRMS
92μVRMS
1
536Ω
109μVRMS
169μVRMS
PASSBAND
GAIN (V/V)
2
– +
SPECTRUM
ANALYZER
INPUT
COILCRAFT
TTWB-1010
25Ω
1:1
INTEGRATED NOISE (μV)
Example: With the IC removed and the 25Ω resistors
grounded, Figure 7, measure the total integrated noise (eS)
of the spectrum analyzer from 10kHz to 15MHz. 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:
1
3
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
Figure 8. Input Referred Noise, Gain = 1
noise power added together, the resulting calculated noise
level will be higher than the true differential noise.
Power Dissipation
The LT6600-15 amplifiers combine high speed with largesignal currents in a small package. There is a need to
ensure that the die junction temperature does not exceed
150°C. The LT6600-15 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
LT6600-15 (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 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.
660015fa
10
LT6600-15
APPLICATIONS INFORMATION
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 a DC differential input
voltage of 250mV, a differential output voltage of 1V, no
load resistance and an ambient temperature of 85°C, the
supply current (current into Pin 3) measures 50mA. Assuming a PC board layout with a 35mm2 copper trace, the
θJA is 100°C/W. The resulting junction temperature is:
Table 2. LT6600-15 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 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:
TJ = TA + (PD • θJA) = TA + (VS • IS • θJA)
TJ = TA + (PD • θJA) = 85 + (5 • 0.05 • 100) = 110°C
where the supply current, IS, is a function of signal
level, load impedance, temperature and common mode
voltages.
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)
.050 BSC
.189 – .197
(4.801 – 5.004)
NOTE 3
.045 ±.005
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
660015fa
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
LT6600-15
TYPICAL APPLICATION
Dual Matched I and Q Lowpass Filter and ADC
(Typical Phase Matching ±1 Degree)
3V
0.1μF
VCMA
3V
2.2μF
0.1μF
RIN
536Ω
1
7
I
0.1μF
2
8
5.6pF
3
–
+
4
25Ω
LT6600-15
–
+
RIN
536Ω
INA
5.6pF
25Ω
5
5.6pF
6
LTC2299
3V
0.1μF
RIN
536Ω
1
7
Q
0.1μF
2
8
RIN
536Ω
5.6pF
3
–
+
4
25Ω
LT6600-15
–
+
5.6pF
25Ω
INB
5
5.6pF
6
GAIN = 536Ω/RIN
VCMB
2.2μF
660015 TA02
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®
LTC 1565-31
660015fa
12 Linear Technology Corporation
LT 0408 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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© LINEAR TECHNOLOGY CORPORATION 2005