LTC2248/LTC2247/LTC2246
14-Bit, 65/40/25Msps
Low Power 3V ADCs
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FEATURES
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DESCRIPTIO
The LTC®2248/LTC2247/LTC2246 are 14-bit 65Msps/
40Msps/25Msps, low power 3V A/D converters designed
for digitizing high frequency, wide dynamic range signals.
The LTC2248/LTC2247/LTC2246 are perfect for demanding imaging and communications applications with AC
performance that includes 74.3dB SNR and 90dB SFDR
for signals at the Nyquist frequency.
Sample Rate: 65Msps/40Msps/25Msps
Single 3V Supply (2.7V to 3.4V)
Low Power: 205mW/120mW/75mW
74.3dB SNR
90dB SFDR
No Missing Codes
Flexible Input: 1VP-P to 2VP-P Range
575MHz Full Power Bandwidth S/H
Clock Duty Cycle Stabilizer
Shutdown and Nap Modes
Pin Compatible Family
125Msps: LTC2253 (12-Bit), LTC2255 (14-Bit)
105Msps: LTC2252 (12-Bit), LTC2254 (14-Bit)
80Msps: LTC2229 (12-Bit), LTC2249 (14-Bit)
65Msps: LTC2228 (12-Bit), LTC2248 (14-Bit)
40Msps: LTC2227 (12-Bit), LTC2247 (14-Bit)
25Msps: LTC2226 (12-Bit), LTC2246 (14-Bit)
10Msps: LTC2225 (12-Bit), LTC2245 (14-Bit)
32-Pin (5mm × 5mm) QFN Package
DC specs include ±1LSB INL (typ), ±0.5LSB DNL (typ) and
no missing codes over temperature. The transition noise
is a low 1LSBRMS.
A single 3V supply allows low power operation. A separate
output supply allows the outputs to drive 0.5V to 3.6V
logic.
A single-ended CLK input controls converter operation. An
optional clock duty cycle stabilizer allows high performance at full speed for a wide range of clock duty cycles.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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APPLICATIO S
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Wireless and Wired Broadband Communication
Imaging Systems
Ultrasound
Spectral Analysis
Portable Instrumentation
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TYPICAL APPLICATIO
REFH
REFL
LTC2248: SNR vs Input Frequency,
–1dB, 2V Range, 65Msps
FLEXIBLE
REFERENCE
75
OVDD
ANALOG
INPUT
INPUT
S/H
–
14-BIT
PIPELINED
ADC CORE
CORRECTION
LOGIC
D13
•
•
•
D0
OUTPUT
DRIVERS
74
SNR (dBFS)
+
73
72
OGND
71
CLOCK/DUTY
CYCLE
CONTROL
70
2249 TA01a
CLK
0
100
50
150
INPUT FREQUENCY (MHZ)
200
2249 TAO1b
224876fa
1
LTC2248/LTC2247/LTC2246
U
W
U
PACKAGE/ORDER I FOR ATIO
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OVDD = VDD (Notes 1, 2)
D11
D12
D13
OF
MODE
SENSE
VDD
VCM
TOP VIEW
Supply Voltage (VDD) ................................................. 4V
Digital Output Ground Voltage (OGND) ....... –0.3V to 1V
Analog Input Voltage (Note 3) ..... –0.3V to (VDD + 0.3V)
Digital Input Voltage .................... –0.3V to (VDD + 0.3V)
Digital Output Voltage ................ –0.3V to (OVDD + 0.3V)
Power Dissipation ............................................ 1500mW
Operating Temperature Range
LTC2248C, LTC2247C, LTC2246C ........... 0°C to 70°C
LTC2248I, LTC2247I, LTC2246I ..........–40°C to 85°C
Storage Temperature Range ..................–65°C to 125°C
32 31 30 29 28 27 26 25
AIN+ 1
24 D10
AIN– 2
23 D9
REFH 3
22 D8
REFH 4
21 OVDD
33
REFL 5
20 OGND
REFL 6
19 D7
VDD 7
18 D6
GND 8
17 D5
D4
D3
D2
OE
CLK
SHDN
9 10 11 12 13 14 15 16
D1
W
AXI U RATI GS
D0
W W
ABSOLUTE
UH PACKAGE
32-LEAD (5mm × 5mm) PLASTIC QFN
TJMAX = 125°C, θJA = 34°C/W
EXPOSED PAD IS GND (PIN 33)
MUST BE SOLDERED TO PCB
ORDER PART NUMBER
QFN PART MARKING*
2248
2248
2247
2247
2246
2246
LTC2248CUH
LTC2248IUH
LTC2247CUH
LTC2247IUH
LTC2246CUH
LTC2246IUH
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
*The temperature grade is identified by a label on the shipping container.
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CO VERTER CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 4)
PARAMETER
CONDITIONS
Resolution
(No Missing Codes)
MIN
●
14
LTC2248
TYP
MAX
MIN
LTC2247
TYP
MAX
14
MIN
LTC2246
TYP
MAX
14
UNITS
Bits
Integral
Linearity Error
Differential Analog Input
(Note 5)
●
–4
±1
4
–4
±1
4
–4
±1
4
LSB
Differential
Linearity Error
Differential Analog Input
●
–1
±0.5
1
–1
±0.5
1
–1
±0.5
1
LSB
Offset Error
(Note 6)
●
–12
±2
12
–12
±2
12
–12
±2
12
mV
Gain Error
External Reference
●
–2.5
±0.5
2.5
–2.5
±0.5
2.5
–2.5
±0.5
2.5
Offset Drift
%FS
±10
±10
±10
µV/°C
ppm/°C
Full-Scale Drift
Internal Reference
±30
±30
±30
External Reference
±5
±5
±5
ppm/°C
Transition Noise
SENSE = 1V
1
1
1
LSBRMS
224876fa
2
LTC2248/LTC2247/LTC2246
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U
A ALOG I PUT
The ● denotes the specifications which apply over the full operating temperature range, otherwise
specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
CONDITIONS
+–
–)
MIN
TYP
MAX
UNITS
±0.5V to ±1V
VIN
Analog Input Range (AIN
2.7V < VDD < 3.4V (Note 7)
●
VIN,CM
Analog Input Common Mode (AIN+ + AIN–)/2
Differential Input (Note 7)
Single Ended Input (Note 7)
●
●
1
0.5
IIN
Analog Input Leakage Current
0V < AIN+, AIN– < VDD
●
ISENSE
SENSE Input Leakage
0V < SENSE < 1V
IMODE
MODE Pin Leakage
tAP
Sample-and-Hold Acquisition Delay Time
tJITTER
Sample-and-Hold Acquisition Delay Time Jitter
0.2
psRMS
CMRR
Analog Input Common Mode Rejection Ratio
80
dB
575
MHz
AIN
1.5
1.5
V
1.9
2
V
V
–1
1
µA
●
–3
3
µA
●
–3
3
µA
0
Full Power Bandwidth
Figure 8 Test Circuit
ns
W U
DY A IC ACCURACY
The ● denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C. AIN = –1dBFS. (Note 4)
SYMBOL
PARAMETER
CONDITIONS
SNR
Signal-to-Noise Ratio
5MHz Input
SFDR
Spurious Free
Dynamic Range
2nd or 3rd
Harmonic
MIN
Spurious Free
Dynamic Range
4th Harmonic
or Higher
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
IMD
Intermodulation
Distortion
MIN
74.4
72.9
72.5
LTC2246
TYP
MAX
UNITS
74.5
dB
74.2
dB
74.4
dB
74.3
dB
70MHz Input
74.3
73.9
140MHz Input
73.9
73.3
73
dB
90
90
90
dB
90
dB
5MHz Input
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
73.4
76
76
76
90
dB
dB
90
dB
85
85
85
dB
140MHz Input
80
80
80
dB
5MHz Input
95
95
95
dB
95
dB
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
84
84
84
5MHz Input
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
95
dB
95
dB
95
140MHz Input
Signal-to-Noise
Plus Distortion
Ratio
LTC2247
TYP
MAX
72.9
70MHz Input
S/(N+D)
MIN
74.3
70MHz Input
SFDR
LTC2248
TYP
MAX
95
95
90
90
90
dB
74.3
74.4
74.5
dB
74.2
dB
72.2
72.2
72
dB
74.3
dB
74.2
dB
70MHz Input
74.1
73.6
73.4
dB
140MHz Input
71.9
71.9
71.8
dB
fIN1 = 28.2MHz
fIN2 = 26.8MHz
90
90
90
dB
224876fa
3
LTC2248/LTC2247/LTC2246
U U
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I TER AL REFERE CE CHARACTERISTICS
(Note 4)
PARAMETER
CONDITIONS
MIN
TYP
MAX
VCM Output Voltage
IOUT = 0
1.475
1.500
1.525
±25
VCM Output Tempco
UNITS
V
ppm/°C
VCM Line Regulation
2.7V < VDD < 3.4V
3
mV/V
VCM Output Resistance
–1mA < IOUT < 1mA
4
Ω
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DIGITAL I PUTS A D DIGITAL OUTPUTS
The ● denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
LOGIC INPUTS (CLK, OE, SHDN)
VIH
High Level Input Voltage
VDD = 3V
●
VIL
Low Level Input Voltage
VDD = 3V
●
IIN
Input Current
VIN = 0V to VDD
●
CIN
Input Capacitance
(Note 7)
3
pF
COZ
Hi-Z Output Capacitance
OE = High (Note 7)
3
pF
ISOURCE
Output Source Current
VOUT = 0V
50
mA
ISINK
Output Sink Current
VOUT = 3V
50
mA
VOH
High Level Output Voltage
IO = –10µA
IO = –200µA
●
IO = 10µA
IO = 1.6mA
●
2
V
–10
0.8
V
10
µA
LOGIC OUTPUTS
OVDD = 3V
VOL
Low Level Output Voltage
2.7
2.995
2.99
0.005
0.09
V
V
V
V
0.4
OVDD = 2.5V
VOH
High Level Output Voltage
IO = –200µA
2.49
V
VOL
Low Level Output Voltage
IO = 1.6mA
0.09
V
VOH
High Level Output Voltage
IO = –200µA
1.79
V
VOL
Low Level Output Voltage
IO = 1.6mA
0.09
V
OVDD = 1.8V
U W
POWER REQUIRE E TS
The ● denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 8)
MIN
LTC2248
TYP
MAX
MIN
LTC2247
TYP
MAX
MIN
LTC2246
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
UNITS
VDD
Analog Supply
Voltage
(Note 9)
●
2.7
3
3.4
2.7
3
3.4
2.7
3
3.4
V
OVDD
Output Supply
Voltage
(Note 9)
●
0.5
3
3.6
0.5
3
3.6
0.5
3
3.6
V
IVDD
Supply Current
●
68.3
80
40
48
25
30
mA
PDISS
Power Dissipation
●
205
240
120
144
75
90
mW
PSHDN
Shutdown Power
SHDN = H,
OE = H, No CLK
2
2
2
mW
PNAP
Nap Mode Power
SHDN = H,
OE = L, No CLK
15
15
15
mW
224876fa
4
LTC2248/LTC2247/LTC2246
WU
TI I G CHARACTERISTICS
The ● denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
CONDITIONS
MIN
fs
Sampling Frequency (Note 9)
●
1
tL
CLK Low Time
Duty Cycle Stabilizer Off
Duty Cycle Stabilizer On
(Note 7)
●
●
7.3
5
tH
CLK High Time
Duty Cycle Stabilizer Off
Duty Cycle Stabilizer On
(Note 7)
●
●
7.3
5
tAP
Sample-and-Hold
Aperture Delay
tD
CLK to DATA delay
CL = 5pF (Note 7)
●
Data Access Time
After OE↓
CL = 5pF (Note 7)
LTC2248
TYP
MAX
MIN
65
1
7.7
7.7
500
500
11.8
5
7.7
7.7
500
500
11.8
5
0
BUS Relinquish Time (Note 7)
1.4
5.4
●
4.3
●
3.3
1.4
40
1
12.5
12.5
500
500
18.9
5
12.5
12.5
500
500
18.9
5
LTC2246
TYP
MAX
UNITS
25
MHz
20
20
500
500
ns
ns
20
20
500
500
ns
ns
0
2.7
5.4
10
4.3
8.5
3.3
5
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: All voltage values are with respect to ground with GND and OGND
wired together (unless otherwise noted).
Note 3: When these pin voltages are taken below GND or above VDD, they
will be clamped by internal diodes. This product can handle input currents
of greater than 100mA below GND or above VDD without latchup.
Note 4: VDD = 3V, fSAMPLE = 65MHz (LTC2248), 40MHz (LTC2247), or
25MHz (LTC2246), input range = 2VP-P with differential drive, unless
otherwise noted.
MIN
0
2.7
Pipeline
Latency
LTC2247
TYP
MAX
1.4
ns
2.7
5.4
ns
10
4.3
10
ns
8.5
3.3
8.5
ns
5
5
Cycles
Note 5: Integral nonlinearity is defined as the deviation of a code from a
straight line passing through the actual endpoints of the transfer curve.
The deviation is measured from the center of the quantization band.
Note 6: Offset error is the offset voltage measured from –0.5 LSB when
the output code flickers between 00 0000 0000 0000 and
11 1111 1111 1111.
Note 7: Guaranteed by design, not subject to test.
Note 8: VDD = 3V, fSAMPLE = 65MHz (LTC2248), 40MHz (LTC2247), or
25MHz (LTC2246), input range = 1VP-P with differential drive.
Note 9: Recommended operating conditions.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2248: Typical DNL,
2V Range, 65Msps
2.0
1.00
1.5
0.75
1.0
0.50
DNL ERROR (LSB)
INL ERROR (LSB)
LTC2248: Typical INL,
2V Range, 65Msps
0.5
0
–0.5
0.25
0
–0.25
–1.0
–0.50
–1.5
–0.75
–2.0
–1.00
0
4096
8192
CODE
12288
16384
2248 G01
0
4096
8192
CODE
12288
16384
2248 G02
224876fa
5
LTC2248/LTC2247/LTC2246
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2248: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
65Msps
0
0
–10
–10
–20
–20
–30
–30
AMPLITUDE (dB)
AMPLITUDE (dB)
LTC2248: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
65Msps
–40
–50
–60
–70
–80
–50
–60
–70
–80
–90
–90
–100
–100
–110
–110
–120
–120
0
5
30
10
15
20
25
FREQUENCY (MHz)
LTC2248: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
65Msps
0
5
2248 G03
10
15
20
25
FREQUENCY (MHz)
0
0
–10
–10
–20
–20
–20
–30
–30
–30
–50
–60
–70
–80
AMPLITUDE (dB)
0
–10
–40
–40
–50
–60
–70
–80
–50
–60
–70
–80
–90
–90
–100
–100
–100
–110
–110
–110
–120
–120
5
10
15
20
25
FREQUENCY (MHz)
30
0
2248 G05
LTC2248: Grounded Input
Histogram, 65Msps
5
10
15
20
25
FREQUENCY (MHz)
–120
30
0
5
10
15
20
25
FREQUENCY (MHz)
2248 G06
LTC2248: SNR vs Input Frequency,
–1dB, 2V Range, 65Msps
30
2248 G06a
LTC2248: SFDR vs Input Frequency,
–1dB, 2V Range, 65Msps
100
75
25000
2248 G04
–40
–90
0
30
LTC2248: 8192 Point 2-Tone FFT,
fIN = 28.2MHz and 26.8MHz,
–1dB, 2V Range, 65Msps
LTC2248: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
65Msps
AMPLITUDE (dB)
AMPLITUDE (dB)
–40
21824
20412
95
74
20000
15000
10224
10000
9042
SFDR (dBFS)
SNR (dBFS)
COUNT
90
73
72
85
80
75
71
5000
2116
172
0
70
1596
121
8196 8197 8198 8199 8200 8201 8202 8203
CODE
2248 G08
70
65
0
100
50
150
INPUT FREQUENCY (MHz)
200
2248 G09
0
50
100
150
INPUT FREQUENCY (MHz)
200
2248 G10
224876fa
6
LTC2248/LTC2247/LTC2246
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2248: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
LTC2248: SNR and SFDR vs
Clock Duty Cycle, 65Msps
110
100
80
SFDR: DCS ON
90
80
SNR
70
60
SNR (dBc AND dBFS)
SNR AND SFDR (dBFS)
SFDR
dBFS
70
95
100
SNR AND SFDR (dBFS)
LTC2248: SNR vs Input Level,
fIN = 30MHz, 2V Range, 65Msps
90
SFDR: DCS OFF
85
80
50
dBc
40
30
20
75 SNR: DCS ON
10
SNR: DCS OFF
60
70
0 10 20 30 40 50 60 70 80 90 100 110
SAMPLE RATE (Msps)
2248 G11
30
35
40 45 50 55 60
CLOCK DUTY CYCLE (%)
65
0
–60
70
120
– 40
–30
–20
INPUT LEVEL (dBFS)
–10
0
2248 G13
LTC2248: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2248: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
LTC2248: SFDR vs Input Level,
fIN = 30MHz, 2V Range, 65Msps
–50
2247 G12
80
6
75
5
110
dBFS
90
70
dBc
70
90dBc SFDR
REFERENCE LINE
60
50
40
4
IOVDD (mA)
80
IVDD (mA)
SFDR (dBc AND dBFS)
100
1V RANGE
65
2V RANGE
3
60
2
55
1
30
20
–60
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
50
0
2248 G14
LTC2247: Typical INL,
2V Range, 40Msps
10
20 30 40 50 60
SAMPLE RATE (Msps)
70
0
80
0.50
0.5
0
–0.5
–1.0
–30
–0.25
–1.00
0
4096
8192
CODE
12288
16384
2247 G01
–40
–50
–60
–70
–80
–90
–0.50
–2.0
80
2248 G16
–20
0
–0.75
70
0
0.25
–1.5
20 30 40 50 60
SAMPLE RATE (Msps)
–10
AMPLITUDE (dB)
1.0
DNL ERROR (LSB)
0.75
10
LTC2247: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
40Msps
1.00
1.5
0
2248 G15
LTC2247: Typical DNL,
2V Range, 40Msps
2.0
INL ERROR (LSB)
0
–100
–110
–120
0
4096
8192
CODE
12288
16384
2247 G02
0
5
10
15
FREQUENCY (MHz)
20
2247 G03
224876fa
7
LTC2248/LTC2247/LTC2246
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2247: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
40Msps
LTC2247: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
40Msps
0
0
–10
–10
–20
–20
–20
–30
–30
–30
–40
–50
–60
–70
–80
AMPLITUDE (dB)
0
–10
AMPLITUDE (dB)
AMPLITUDE (dB)
LTC2247: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
40Msps
–40
–50
–60
–70
–80
–40
–50
–60
–70
–80
–90
–90
–90
–100
–100
–100
–110
–110
–110
–120
–120
0
5
10
15
FREQUENCY (MHz)
–120
0
20
5
2247 G04
LTC2247: 8192 Point 2-Tone FFT,
fIN = 21.6MHz and 23.6MHz,
–1dB, 2V Range, 40Msps
10
15
FREQUENCY (MHz)
0
20
LTC2247: Grounded Input
Histogram, 40Msps
24558
25000
74
–50
–60
–70
14833
15000
15714
73
72
10000
–80
–90
4641
5000
–100
–110
30
0
0
5
10
15
FREQUENCY (MHz)
20
4520
640
71
546 36
70
8184 8185 8186 818781888189 8190 81918192
CODE
2247 G08
2247 G07
LTC2247: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
LTC2247: SFDR vs Input Frequency,
–1dB, 2V Range, 40Msps
100
80
80
75
SNR (dBc AND dBFS)
SNR AND SFDR (dBFS)
85
200
2247 G09
dBFS
70
SFDR
100
90
100
50
150
INPUT FREQUENCY (MHz)
0
LTC2247: SNR vs Input Level,
fIN = 5MHz, 2V Range, 40Msps
110
95
SFDR (dBFS)
SNR (dBFS)
20000
–40
COUNT
AMPLITUDE (dB)
–30
–120
20
2247 G06
75
–10
–20
10
15
FREQUENCY (MHz)
LTC2247: SNR vs Input Frequency,
–1dB, 2V Range, 40Msps
30000
0
5
2247 G05
90
80
SNR
60
50
dBc
40
30
20
70
70
10
65
0
50
100
150
INPUT FREQUENCY (MHz)
200
2247 G10
60
0
40
20
60
SAMPLE RATE (Msps)
80
2247 G11
0
–60
–50
– 40
–30
–20
INPUT LEVEL (dBFS)
–10
0
2247 G12
224876fa
8
LTC2248/LTC2247/LTC2246
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC2247: SFDR vs Input Level,
fIN = 5MHz, 2V Range, 40Msps
LTC2247: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2247: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
120
50
4
45
3
110
80
IVDD (mA)
SNR (dBc AND dBFS)
90
dBc
70
90dBc SFDR
REFERENCE LINE
60
IOVDD (mA)
dBFS
100
2V RANGE
40
2
1V RANGE
50
1
35
40
30
20
–60
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
30
0
10
0
30
40
20
SAMPLE RATE (Msps)
2247 G13
1.0
0.50
–0.5
0
–10
–20
–30
AMPLITUDE (dB)
0.75
DNL ERROR (LSB)
INL ERROR (LSB)
1.5
0.25
0
–0.25
–40
–50
–60
–70
–80
–0.50
–1.0
–1.5
–0.75
–2.0
–1.00
50
2247 G15
1.00
2.0
0
30
40
20
SAMPLE RATE (Msps)
LTC2246: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
25Msps
LTC2246: Typical DNL,
2V Range, 25Msps
0.5
10
0
2247 G14
LTC2246: Typical INL,
2V Range, 25Msps
–90
–100
–110
0
4096
8192
CODE
12288
16384
4096
0
8192
CODE
12288
2246 G01
16384
–120
–20
–20
–20
–30
–30
–30
–70
–80
AMPLITUDE (dB)
0
–10
AMPLITUDE (dB)
0
–10
–60
–40
–50
–60
–70
–80
–50
–60
–70
–80
–90
–90
–100
–100
–100
–110
–110
–110
–120
–120
2
4
6
8
10
FREQUENCY (MHz)
12
2246 G04
0
2
4
6
8
10
FREQUENCY (MHz)
12
–40
–90
0
4
6
8
10
FREQUENCY (MHz)
2246 G03
0
–50
2
LTC2246: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
25Msps
–10
–40
0
2246 G02
LTC2246: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
25Msps
LTC2246: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
25Msps
AMPLITUDE (dB)
0
50
12
2246 G05
–120
0
2
4
6
8
10
FREQUENCY (MHz)
12
2246 G06
224876fa
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LTC2248/LTC2247/LTC2246
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC2246: 8192 Point 2-Tone FFT,
fIN = 10.9MHz and 13.8MHz,
–1dB, 2V Range, 25Msps
LTC2246: Grounded Input
Histogram, 25Msps
25000
0
75
22016
–10
–20
20000
74
18803
–30
AMPLITUDE (dB)
LTC2246: SNR vs Input Frequency,
–1dB, 2V Range, 25Msps
COUNT
–50
–60
–70
15000
13373
10000
–80
–90
5000
72
0
2
4
6
8
10
FREQUENCY (MHz)
0
12
2246 G07
71
3227
–110
43
853
278
70
8179 8180 8181 8182 8183 8184 8185 8186
CODE
2246 G08
LTC2246: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
LTC2246: SFDR vs Input
Frequency, –1dB, 2V Range,
25Msps
100
100
50
150
INPUT FREQUENCY (MHz)
0
80
110
dBFS
70
85
80
75
SFDR
SNR (dBc AND dBFS)
SNR AND SFDR (dBFS)
100
90
200
2246 G09
LTC2246: SNR vs Input Level,
fIN = 5MHz, 2V Range, 25Msps
95
SFDR (dBFS)
73
6919
–100
–120
SNR (dBFS)
–40
90
80
SNR
60
50
dBc
40
30
20
70
70
10
65
50
100
60
200
150
INPUT FREQUENCY (MHz)
0
10
0
2246 G10
20
30
40
SAMPLE RATE (Msps)
0
–60
50
35
120
–30
–40
–20
INPUT LEVEL (dBFS)
–10
0
2246 G12
LTC2246: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2246: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
LTC2246: SFDR vs Input Level,
fIN = 5MHz, 2V Range, 25Msps
–50
2246 G11
3
110
dBFS
30
90
dBc
70
90dBc SFDR
REFERENCE LINE
60
IOVDD (mA)
2
80
IVDD (mA)
SFDR (dBc AND dBFS)
100
2V RANGE
25
1V RANGE
1
50
20
40
30
20
–60
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
15
0
2246 G13
0
5
25
20
15
10
SAMPLE RATE (Msps)
30
35
2246 G14
0
0
5
25
20
15
10
SAMPLE RATE (Msps)
30
35
2246 G15
224876fa
10
LTC2248/LTC2247/LTC2246
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AIN+ (Pin 1): Positive Differential Analog Input.
AIN- (Pin 2): Negative Differential Analog Input.
REFH (Pins 3, 4): ADC High Reference. Short together and
bypass to pins 5, 6 with a 0.1µF ceramic chip capacitor as
close to the pin as possible. Also bypass to pins 5, 6 with
an additional 2.2µF ceramic chip capacitor and to ground
with a 1µF ceramic chip capacitor.
REFL (Pins 5, 6): ADC Low Reference. Short together and
bypass to pins 3, 4 with a 0.1µF ceramic chip capacitor as
close to the pin as possible. Also bypass to pins 3, 4 with
an additional 2.2µF ceramic chip capacitor and to ground
with a 1µF ceramic chip capacitor.
VDD (Pins 7, 32): 3V Supply. Bypass to GND with 0.1µF
ceramic chip capacitors.
GND (Pin 8): ADC Power Ground.
CLK (Pin 9): Clock Input. The input sample starts on the
positive edge.
SHDN (Pin 10): Shutdown Mode Selection Pin. Connecting SHDN to GND and OE to GND results in normal
operation with the outputs enabled. Connecting SHDN to
GND and OE to VDD results in normal operation with the
outputs at high impedance. Connecting SHDN to VDD and
OE to GND results in nap mode with the outputs at high
impedance. Connecting SHDN to VDD and OE to VDD
results in sleep mode with the outputs at high impedance.
OE (Pin 11): Output Enable Pin. Refer to SHDN pin
function.
D0 – D13 (Pins 12, 13, 14, 15, 16, 17, 18, 19, 22, 23, 24,
25, 26, 27): Digital Outputs. D13 is the MSB.
OGND (Pin 20): Output Driver Ground.
OVDD (Pin 21): Positive Supply for the Output Drivers.
Bypass to ground with 0.1µF ceramic chip capacitor.
OF (Pin 28): Over/Under Flow Output. High when an over
or under flow has occurred.
MODE (Pin 29): Output Format and Clock Duty Cycle
Stabilizer Selection Pin. Connecting MODE to GND selects
offset binary output format and turns the clock duty cycle
stabilizer off. 1/3 VDD selects offset binary output format
and turns the clock duty cycle stabilizer on. 2/3 VDD selects
2’s complement output format and turns the clock duty
cycle stabilizer on. VDD selects 2’s complement output
format and turns the clock duty cycle stabilizer off.
SENSE (Pin 30): Reference Programming Pin. Connecting
SENSE to VCM selects the internal reference and a ±0.5V
input range. VDD selects the internal reference and a ±1V
input range. An external reference greater than 0.5V and
less than 1V applied to SENSE selects an input range of
±VSENSE. ±1V is the largest valid input range.
VCM (Pin 31): 1.5V Output and Input Common Mode Bias.
Bypass to ground with 2.2µF ceramic chip capacitor.
GND (Exposed Pad) (Pin 33): ADC Power Ground. The
exposed pad on the bottom of the package needs to be
soldered to ground.
224876fa
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LTC2248/LTC2247/LTC2246
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FUNCTIONAL BLOCK DIAGRA
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AIN+
AIN–
VCM
INPUT
S/H
FIRST PIPELINED
ADC STAGE
SECOND PIPELINED
ADC STAGE
THIRD PIPELINED
ADC STAGE
FOURTH PIPELINED
ADC STAGE
FIFTH PIPELINED
ADC STAGE
1.5V
REFERENCE
SIXTH PIPELINED
ADC STAGE
SHIFT REGISTER
AND CORRECTION
2.2µF
RANGE
SELECT
REFH
SENSE
REFL
INTERNAL CLOCK SIGNALS
OVDD
REF
BUF
OF
D13
CLOCK/DUTY
CYCLE
CONTROL
DIFF
REF
AMP
CONTROL
LOGIC
OUTPUT
DRIVERS
•
•
•
D0
REFH
224876 F01
REFL
0.1µF
OGND
M0DE
CLK
SHDN
OE
2.2µF
1µF
1µF
Figure 1. Functional Block Diagram
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TI I G DIAGRA
Timing Diagram
tAP
ANALOG
INPUT
N+4
N+2
N
N+3
tH
N+5
N+1
tL
CLK
tD
D0-D13, OF
N–5
N–4
N–3
N–2
N–1
N
224876 TD01
224876fa
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LTC2248/LTC2247/LTC2246
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DYNAMIC PERFORMANCE
input tone to the RMS value of the largest 3rd order
intermodulation product.
Signal-to-Noise Plus Distortion Ratio
The signal-to-noise plus distortion ratio [S/(N + D)] is the
ratio between the RMS amplitude of the fundamental input
frequency and the RMS amplitude of all other frequency
components at the ADC output. The output is band limited
to frequencies above DC to below half the sampling
frequency.
Spurious Free Dynamic Range (SFDR)
Signal-to-Noise Ratio
Input Bandwidth
The signal-to-noise ratio (SNR) is the ratio between the
RMS amplitude of the fundamental input frequency and
the RMS amplitude of all other frequency components
except the first five harmonics and DC.
The input bandwidth is that input frequency at which the
amplitude of the reconstructed fundamental is reduced by
3dB for a full scale input signal.
Spurious free dynamic range is the peak harmonic or
spurious noise that is the largest spectral component
excluding the input signal and DC. This value is expressed
in decibels relative to the RMS value of a full scale input
signal.
Aperture Delay Time
Total Harmonic Distortion
Total harmonic distortion is the ratio of the RMS sum of all
harmonics of the input signal to the fundamental itself. The
out-of-band harmonics alias into the frequency band
between DC and half the sampling frequency. THD is
expressed as:
THD = 20Log (√(V22 + V32 + V42 + . . . Vn2)/V1)
where V1 is the RMS amplitude of the fundamental frequency and V2 through Vn are the amplitudes of the
second through nth harmonics. The THD calculated in this
data sheet uses all the harmonics up to the fifth.
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
produce intermodulation distortion (IMD) in addition to
THD. IMD is the change in one sinusoidal input caused by
the presence of another sinusoidal input at a different
frequency.
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer function can create distortion products at the sum and difference frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3,
etc. The 3rd order intermodulation products are 2fa + fb,
2fb + fa, 2fa – fb and 2fb – fa. The intermodulation
distortion is defined as the ratio of the RMS value of either
The time from when CLK reaches mid-supply to the instant
that the input signal is held by the sample and hold circuit.
Aperture Delay Jitter
The variation in the aperture delay time from conversion to
conversion. This random variation will result in noise
when sampling an AC input. The signal to noise ratio due
to the jitter alone will be:
SNRJITTER = –20log (2π • fIN • tJITTER)
CONVERTER OPERATION
As shown in Figure 1, the LTC2248/LTC2247/LTC2246 is
a CMOS pipelined multistep converter. The converter has
six pipelined ADC stages; a sampled analog input will
result in a digitized value five cycles later (see the Timing
Diagram section). For optimal AC performance the analog
inputs should be driven differentially. For cost sensitive
applications, the analog inputs can be driven single-ended
with slightly worse harmonic distortion. The CLK input is
single-ended. The LTC2248/LTC2247/LTC2246 has two
phases of operation, determined by the state of the CLK
input pin.
Each pipelined stage shown in Figure 1 contains an ADC,
a reconstruction DAC and an interstage residue amplifier.
In operation, the ADC quantizes the input to the stage and
the quantized value is subtracted from the input by the
224876fa
13
LTC2248/LTC2247/LTC2246
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APPLICATIO S I FOR ATIO
DAC to produce a residue. The residue is amplified and
output by the residue amplifier. Successive stages operate
out of phase so that when the odd stages are outputting
their residue, the even stages are acquiring that residue
and vice versa.
When CLK is low, the analog input is sampled differentially
directly onto the input sample-and-hold capacitors, inside
the “Input S/H” shown in the block diagram. At the instant
that CLK transitions from low to high, the sampled input is
held. While CLK is high, the held input voltage is buffered
by the S/H amplifier which drives the first pipelined ADC
stage. The first stage acquires the output of the S/H during
this high phase of CLK. When CLK goes back low, the first
stage produces its residue which is acquired by the
second stage. At the same time, the input S/H goes back
to acquiring the analog input. When CLK goes back high,
the second stage produces its residue which is acquired
by the third stage. An identical process is repeated for the
third, fourth and fifth stages, resulting in a fifth stage
residue that is sent to the sixth stage ADC for final
evaluation.
Each ADC stage following the first has additional range to
accommodate flash and amplifier offset errors. Results
from all of the ADC stages are digitally synchronized such
that the results can be properly combined in the correction
logic before being sent to the output buffer.
SAMPLE/HOLD OPERATION AND INPUT DRIVE
Sample/Hold Operation
Figure 2 shows an equivalent circuit for the LTC2248/
LTC2247/LTC2246 CMOS differential sample-and-hold.
The analog inputs are connected to the sampling capacitors (CSAMPLE) through NMOS transistors. The capacitors
shown attached to each input (CPARASITIC) are the summation of all other capacitance associated with each input.
During the sample phase when CLK is low, the transistors
connect the analog inputs to the sampling capacitors and
they charge to and track the differential input voltage.
When CLK transitions from low to high, the sampled input
voltage is held on the sampling capacitors. During the hold
phase when CLK is high, the sampling capacitors are
disconnected from the input and the held voltage is passed
to the ADC core for processing. As CLK transitions from
high to low, the inputs are reconnected to the sampling
capacitors to acquire a new sample. Since the sampling
capacitors still hold the previous sample, a charging glitch
proportional to the change in voltage between samples will
be seen at this time. If the change between the last sample
and the new sample is small, the charging glitch seen at
the input will be small. If the input change is large, such as
the change seen with input frequencies near Nyquist, then
a larger charging glitch will be seen.
Single-Ended Input
For cost sensitive applications, the analog inputs can be
driven single-ended. With a single-ended input the harmonic distortion and INL will degrade, but the SNR and
DNL will remain unchanged. For a single-ended input, AIN+
should be driven with the input signal and AIN– should be
connected to 1.5V or VCM.
Common Mode Bias
For optimal performance the analog inputs should be
driven differentially. Each input should swing ±0.5V for
the 2V range or ±0.25V for the 1V range, around a
common mode voltage of 1.5V. The VCM output pin (Pin
31) may be used to provide the common mode bias level.
VCM can be tied directly to the center tap of a transformer
to set the DC input level or as a reference level to an op amp
differential driver circuit. The VCM pin must be bypassed to
ground close to the ADC with a 2.2µF or greater capacitor.
LTC2248/47/46
VDD
CSAMPLE
4pF
15Ω
AIN+
CPARASITIC
1pF
VDD
AIN–
CSAMPLE
4pF
15Ω
CPARASITIC
1pF
VDD
CLK
224876 F02
Figure 2. Equivalent Input Circuit
224876fa
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LTC2248/LTC2247/LTC2246
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Input Drive Impedance
As with all high performance, high speed ADCs, the
dynamic performance of the LTC2248/LTC2247/LTC2246
can be influenced by the input drive circuitry, particularly
the second and third harmonics. Source impedance and
reactance can influence SFDR. At the falling edge of CLK,
the sample-and-hold circuit will connect the 4pF sampling
capacitor to the input pin and start the sampling period.
The sampling period ends when CLK rises, holding the
sampled input on the sampling capacitor. Ideally the input
circuitry should be fast enough to fully charge
the sampling capacitor during the sampling period
1/(2FENCODE); however, this is not always possible and the
incomplete settling may degrade the SFDR. The sampling
glitch has been designed to be as linear as possible to
minimize the effects of incomplete settling.
For the best performance, it is recommended to have a
source impedance of 100Ω or less for each input. The
source impedance should be matched for the differential
inputs. Poor matching will result in higher even order
harmonics, especially the second.
VCM
2.2µF
0.1µF
ANALOG
INPUT
T1
1:1
AIN+
25Ω
LTC2248/47/46
0.1µF
25Ω
12pF
25Ω
AIN–
T1 = MA/COM ETC1-1T 25Ω
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
224876 F03
Figure 3. Single-Ended to Differential Conversion
Using a Transformer
VCM
HIGH SPEED
DIFFERENTIAL
25Ω
AMPLIFIER
ANALOG
INPUT
+
AIN+
LTC2248/47/46
+
CM
–
2.2µF
12pF
–
25Ω
AIN–
224876 F04
Figure 4. Differential Drive with an Amplifier
Input Drive Circuits
Figure 3 shows the LTC2248/LTC2247/LTC2246 being
driven by an RF transformer with a center tapped secondary. The secondary center tap is DC biased with VCM,
setting the ADC input signal at its optimum DC level.
Terminating on the transformer secondary is desirable, as
this provides a common mode path for charging glitches
caused by the sample and hold. Figure 3 shows a 1:1 turns
ratio transformer. Other turns ratios can be used if the
source impedance seen by the ADC does not exceed 100Ω
for each ADC input. A disadvantage of using a transformer
is the loss of low frequency response. Most small RF
transformers have poor performance at frequencies below 1MHz.
Figure 4 demonstrates the use of a differential amplifier to
convert a single ended input signal into a differential input
signal. The advantage of this method is that it provides low
frequency input response; however, the limited gain bandwidth of most op amps will limit the SFDR at high input
frequencies.
VCM
1k
0.1µF
ANALOG
INPUT
1k
2.2µF
25Ω
AIN+
LTC2248/47/46
12pF
25Ω
AIN–
0.1µF
224876 F05
Figure 5. Single-Ended Drive
Figure 5 shows a single-ended input circuit. The impedance seen by the analog inputs should be matched. This
circuit is not recommended if low distortion is required.
The 25Ω resistors and 12pF capacitor on the analog inputs
serve two purposes: isolating the drive circuitry from the
sample-and-hold charging glitches and limiting the
wideband noise at the converter input.
224876fa
15
LTC2248/LTC2247/LTC2246
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For input frequencies above 70MHz, the input circuits of
Figure 6, 7 and 8 are recommended. The balun transformer gives better high frequency response than a flux
coupled center tapped transformer. The coupling capacitors allow the analog inputs to be DC biased at 1.5V. In
Figure 8, the series inductors are impedance matching
elements that maximize the ADC bandwidth.
VCM
2.2µF
0.1µF
12Ω
ANALOG
INPUT
AIN+
LTC2248/47/46
25Ω
0.1µF
T1
0.1µF
8pF
25Ω
12Ω
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
AIN–
224876 F06
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
VCM
2.2µF
0.1µF
AIN+
ANALOG
INPUT
25Ω
LTC2248/47/46
Reference Operation
Figure 9 shows the LTC2248/LTC2247/LTC2246 reference circuitry consisting of a 1.5V bandgap reference, a
difference amplifier and switching and control circuit. The
internal voltage reference can be configured for two pin
selectable input ranges of 2V (±1V differential) or 1V
(±0.5V differential). Tying the SENSE pin to VDD selects
the 2V range; tying the SENSE pin to VCM selects the 1V
range.
The 1.5V bandgap reference serves two functions: its
output provides a DC bias point for setting the common
mode voltage of any external input circuitry; additionally,
the reference is used with a difference amplifier to generate the differential reference levels needed by the internal
ADC circuitry. An external bypass capacitor is required for
the 1.5V reference output, VCM. This provides a high
frequency low impedance path to ground for internal and
external circuitry.
The difference amplifier generates the high and low reference for the ADC. High speed switching circuits are
connected to these outputs and they must be externally
bypassed. Each output has two pins. The multiple output
0.1µF
T1
0.1µF
LTC2248/47/46
25Ω
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
1.5V
AIN–
VCM
2.2µF
2.2µF
6.8nH
25Ω
AIN+
LTC2248/47/46
0.5V
1V
TIE TO VDD FOR 2V RANGE;
TIE TO VCM FOR 1V RANGE;
RANGE = 2 • VSENSE FOR
0.5V < VSENSE < VCM • 1.1
1.5
VCM
ANALOG
INPUT
1.5V BANDGAP
REFERENCE
224876 F07
RANGE
DETECT
AND
CONTROL
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
0.1µF
4Ω
SENSE
BUFFER
INTERNAL ADC
HIGH REFERENCE
1µF
REFH
0.1µF
T1
0.1µF
25Ω
2.2µF
6.8nH
0.1µF
DIFF AMP
–
AIN
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
224876 F08
1µF
REFL
INTERNAL ADC
LOW REFERENCE
224876 F09
Figure 9. Equivalent Reference Circuit
224876fa
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LTC2248/LTC2247/LTC2246
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pins are needed to reduce package inductance. Bypass
capacitors must be connected as shown in Figure 9.
a low-jitter squaring circuit before the CLK pin (see
Figure 11).
Other voltage ranges in-between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 10. An external reference can be used by applying
its output directly or through a resistor divider to SENSE.
It is not recommended to drive the SENSE pin with a logic
device. The SENSE pin should be tied to the appropriate
level as close to the converter as possible. If the SENSE pin
is driven externally, it should be bypassed to ground as
close to the device as possible with a 1µF ceramic capacitor.
The noise performance of the LTC2248/LTC2247/LTC2246
can depend on the clock signal quality as much as on the
analog input. Any noise present on the clock signal will
result in additional aperture jitter that will be RMS summed
with the inherent ADC aperture jitter.
Input Range
The input range can be set based on the application. The
2V input range will provide the best signal-to-noise performance while maintaining excellent SFDR. The 1V input
range will have better SFDR performance, but the SNR will
degrade by 5.8dB. See the Typical Performance Characteristics section.
In applications where jitter is critical, such as when digitizing high input frequencies, use as large an amplitude as
possible. Also, if the ADC is clocked with a sinusoidal
signal, filter the CLK signal to reduce wideband noise and
distortion products generated by the source.
Figures 12 and 13 show alternatives for converting a
differential clock to the single-ended CLK input. The use of
a transformer provides no incremental contribution to
phase noise. The LVDS or PECL to CMOS translators
provide little degradation below 70MHz, but at 140MHz
will degrade the SNR compared to the transformer solution. The nature of the received signals also has a large
Driving the Clock Input
CLEAN
SUPPLY
4.7µF
The CLK input can be driven directly with a CMOS or TTL
level signal. A sinusoidal clock can also be used along with
FERRITE
BEAD
0.1µF
1.5V
VCM
CLK
2.2µF
100Ω
12k
0.75V
SENSE
LTC2248/
LTC2247/
LTC2246
LTC2248/47/46
1µF
12k
224876 F12
IF LVDS USE FIN1002 OR FIN1018.
FOR PECL, USE AZ1000ELT21 OR SIMILAR
224876 F10
Figure 12. CLK Drive Using an LVDS or PECL to CMOS Converter
Figure 10. 1.5V Range ADC
CLEAN
SUPPLY
4.7µF
FERRITE
BEAD
ETC1-1T
0.1µF
SINUSOIDAL
CLOCK
INPUT
0.1µF
5pF-30pF
1k
CLK
CLK
LTC2248/47/46
LTC2248/
LTC2247/
LTC2246
DIFFERENTIAL
CLOCK
INPUT
224876 F13
50Ω
1k
NC7SVU04
0.1µF
224876 F11
Figure 11. Sinusoidal Single-Ended CLK Drive
FERRITE
BEAD
VCM
Figure 13. LVDS or PECL CLK Drive Using a Transformer
224876fa
17
LTC2248/LTC2247/LTC2246
U
W
U U
APPLICATIO S I FOR ATIO
bearing on how much SNR degradation will be experienced. For high crest factor signals such as WCDMA or
OFDM, where the nominal power level must be at least 6dB
to 8dB below full scale, the use of these translators will
have a lesser impact.
storing analog signals on small valued capacitors. Junction leakage will discharge the capacitors. The specified
minimum operating frequency for the LTC2248/LTC2247/
LTC2246 is 1Msps.
The transformer in the example may be terminated with
the appropriate termination for the signaling in use. The
use of a transformer with a 1:4 impedance ratio may be
desirable in cases where lower voltage differential signals
are considered. The center tap may be bypassed to ground
through a capacitor close to the ADC if the differential
signals originate on a different plane. The use of a capacitor at the input may result in peaking, and depending on
transmission line length may require a 10Ω to 20Ω ohm
series resistor to act as both a low pass filter for high
frequency noise that may be induced into the clock line by
neighboring digital signals, as well as a damping mechanism for reflections.
DIGITAL OUTPUTS
Maximum and Minimum Conversion Rates
The maximum conversion rate for the LTC2248/LTC2247/
LTC2246 is 65Msps (LTC2248), 40Msps (LTC2247), and
25Msps (LTC2246). For the ADC to operate properly, the
CLK signal should have a 50% (±5%) duty cycle. Each half
cycle must have at least 7.3ns (LTC2248), 11.8ns
(LTC2247), and 18.9ns (LTC2246) for the ADC internal
circuitry to have enough settling time for proper operation.
An optional clock duty cycle stabilizer circuit can be used
if the input clock has a non 50% duty cycle. This circuit
uses the rising edge of the CLK pin to sample the analog
input. The falling edge of CLK is ignored and the internal
falling edge is generated by a phase-locked loop. The input
clock duty cycle can vary from 40% to 60% and the clock
duty cycle stabilizer will maintain a constant 50% internal
duty cycle. If the clock is turned off for a long period of
time, the duty cycle stabilizer circuit will require a hundred
clock cycles for the PLL to lock onto the input clock. To use
the clock duty cycle stabilizer, the MODE pin should be
connected to 1/3VDD or 2/3VDD using external resistors.
The lower limit of the LTC2248/LTC2247/LTC2246 sample
rate is determined by droop of the sample-and-hold circuits. The pipelined architecture of this ADC relies on
Table 1 shows the relationship between the analog input
voltage, the digital data bits, and the overflow bit.
Table 1. Output Codes vs Input Voltage
AIN+ – AIN–
(2V Range)
OF
D13 – D0
(Offset Binary)
D13 – D0
(2’s Complement)
>+1.000000V
+0.999878V
+0.999756V
1
0
0
11 1111 1111 1111
11 1111 1111 1111
11 1111 1111 1110
01 1111 1111 1111
01 1111 1111 1111
01 1111 1111 1110
+0.000122V
0.000000V
–0.000122V
–0.000244V
0
0
0
0
10 0000 0000 0001
10 0000 0000 0000
01 1111 1111 1111
01 1111 1111 1110
00 0000 0000 0001
00 0000 0000 0000
11 1111 1111 1111
11 1111 1111 1110
–0.999878V
–1.000000V
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