LTC2238/LTC2237/LTC2236
10-Bit, 65/40/25Msps
Low Noise 3V ADCs
DESCRIPTIO
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FEATURES
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Sample Rate: 65Msps/40Msps/25Msps
Single 3V Supply (2.7V to 3.4V)
Low Power: 205mW/120mW/75mW
61.8dB SNR
85dB 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: LTC2251 (10-Bit), LTC2253 (12-Bit)
105Msps: LTC2250 (10-Bit), LTC2252 (12-Bit)
80Msps: LTC2239 (10-Bit), LTC2229 (12-Bit)
65Msps: LTC2238 (10-Bit), LTC2228 (12-Bit)
40Msps: LTC2237 (10-Bit), LTC2227 (12-Bit)
25Msps: LTC2236 (10-Bit), LTC2226 (12-Bit)
32-Pin (5mm × 5mm) QFN Package
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APPLICATIO S
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The LTC®2238/LTC2237/LTC2236 are 10-bit 65Msps/
40Msps/25Msps, low power 3V A/D converters designed
for digitizing high frequency, wide dynamic range signals.
The LTC2238/LTC2237/LTC2236 are perfect for demanding imaging and communications applications with AC
performance that includes 61.8dB SNR and 85dB SFDR
for signals at the Nyquist frequency.
DC specs include ±0.1LSB INL (typ), ±0.05LSB DNL (typ)
and ±0.5LSB INL, ±0.5LSB DNL over temperature. The
transition noise is a low 0.07LSBRMS.
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.
Wireless and Wired Broadband Communication
Imaging Systems
Ultrasound
Spectral Analysis
Portable Instrumentation
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TYPICAL APPLICATIO
REFH
REFL
LTC2238: SNR vs Input Frequency,
–1dB, 2V Range, 65Msps
FLEXIBLE
REFERENCE
62.5
OVDD
INPUT
S/H
–
10-BIT
PIPELINED
ADC CORE
CORRECTION
LOGIC
D9
•
•
•
D0
OUTPUT
DRIVERS
SNR (dBFS)
+
ANALOG
INPUT
61.5
60.5
59.5
OGND
58.5
CLOCK/DUTY
CYCLE
CONTROL
57.5
223876 TA01
CLK
0
100
50
150
INPUT FREQUENCY (MHz)
200
223876 G09
223876fa
1
LTC2238/LTC2237/LTC2236
U
W
U
PACKAGE/ORDER I FOR ATIO
U
OVDD = VDD (Notes 1, 2)
D7
D8
D9
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
LTC2238C, LTC2237C, LTC2236C ........... 0°C to 70°C
LTC2238I, LTC2237I, LTC2236I ..........–40°C to 85°C
Storage Temperature Range ..................–65°C to 125°C
32 31 30 29 28 27 26 25
AIN+ 1
24 D6
AIN– 2
23 D5
REFH 3
22 D4
REFH 4
21 OVDD
33
REFL 5
20 OGND
REFL 6
19 D3
VDD 7
18 D2
GND 8
17 D1
D0
NC
NC
OE
CLK
SHDN
9 10 11 12 13 14 15 16
NC
W
AXI U RATI GS
NC
W W
ABSOLUTE
UH PACKAGE
32-LEAD (5mm × 5mm) PLASTIC QFN
TJMAX = 125°C, θJA = 34°C/W
EXPOSED PAD (PIN 33) IS GND
MUST BE SOLDERED TO PCB
ORDER PART NUMBER
QFN PART MARKING*
2238
2238
2237
2237
2236
2236
LTC2238CUH
LTC2238IUH
LTC2237CUH
LTC2237IUH
LTC2236CUH
LTC2236IUH
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
●
10
LTC2238
TYP
MAX
MIN
LTC2237
TYP
MAX
10
MIN
LTC2236
TYP
MAX
10
UNITS
Bits
Integral
Linearity Error
Differential Analog Input
(Note 5)
●
–0.5
±0.1
0.5
–0.5
±0.1
0.5
–0.5
±0.1
0.5
LSB
Differential
Linearity Error
Differential Analog Input
●
–0.5
±0.05
0.5
–0.5
±0.05
0.5
–0.5
±0.05
0.5
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
%FS
±10
±10
±10
µV/°C
Internal Reference
±30
±30
±30
ppm/°C
External Reference
±5
±5
±5
ppm/°C
0.07
0.07
0.07
LSBRMS
Offset Drift
Full-Scale Drift
Transition Noise
SENSE = 1V
223876fa
2
LTC2238/LTC2237/LTC2236
U
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
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
IMD
Intermodulation
Distortion
MIN
61.8
60
60
LTC2236
TYP
MAX
UNITS
61.8
dB
61.8
dB
61.8
dB
61.8
dB
70MHz Input
61.7
61.7
61.6
140MHz Input
61.6
61.6
61.6
dB
85
85
85
dB
85
dB
5MHz Input
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
69
69
69
85
dB
dB
85
dB
85
85
85
dB
140MHz Input
80
80
80
dB
5MHz Input
85
85
85
dB
85
dB
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
75
75
74
5MHz Input
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
85
dB
85
dB
85
140MHz Input
Signal-to-Noise
Plus Distortion
Ratio
LTC2237
TYP
MAX
60
70MHz Input
S/(N+D)
MIN
61.8
70MHz Input
Spurious Free
Dynamic Range
4th Harmonic
or Higher
LTC2238
TYP
MAX
85
85
85
85
85
dB
61.8
61.8
61.8
dB
61.8
dB
60
60
60
dB
61.6
dB
61.8
dB
70MHz Input
61.7
61.6
61.6
dB
140MHz Input
61.6
61.6
61.5
dB
fIN1 = 28.2MHz,
fIN2 = 26.8MHz
80
80
80
dB
223876fa
3
LTC2238/LTC2237/LTC2236
U U
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I TER AL REFERE CE CHARACTERISTICS
(Note 4)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VCM Output Voltage
IOUT = 0
1.475
1.500
1.525
V
±25
VCM Output Tempco
ppm/°C
VCM Line Regulation
2.7V < VDD < 3.4V
3
mV/V
VCM Output Resistance
–1mA < IOUT < 1mA
4
Ω
U
U
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
0.4
V
V
OVDD = 2.5V
VOH
High Level Output Voltage
IO = –200µA
2.49
V
VOL
Low Level Output Voltage
IO = 1.6mA
0.09
V
OVDD = 1.8V
VOH
High Level Output Voltage
IO = –200µA
1.79
V
VOL
Low Level Output Voltage
IO = 1.6mA
0.09
V
223876fa
4
LTC2238/LTC2237/LTC2236
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
LTC2238
TYP
MAX
MIN
LTC2237
TYP
MAX
MIN
LTC2236
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
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)
CONDITIONS
MIN
LTC2238
TYP
MAX
MIN
LTC2237
TYP
MAX
PARAMETER
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)
●
2.7
5.4
2.7
5.4
2.7
5.4
ns
Data Access Time
After OE↓
CL = 5pF (Note 7)
●
4.3
10
4.3
10
4.3
10
ns
●
3.3
8.5
3.3
8.5
3.3
8.5
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
Pipeline
Latency
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 (LTC2238), 40MHz (LTC2237), or
25MHz (LTC2236), input range = 2VP-P with differential drive, unless
otherwise noted.
5
MIN
LTC2236
TYP
MAX
SYMBOL
40
1
12.5
12.5
500
500
18.9
5
12.5
12.5
500
500
18.9
5
0
1.4
5
UNITS
25
MHz
20
20
500
500
ns
ns
20
20
500
500
ns
ns
0
1.4
5
ns
ns
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 and 11 1111 1111.
Note 7: Guaranteed by design, not subject to test.
Note 8: VDD = 3V, fSAMPLE = 65MHz (LTC2238), 40MHz (LTC2237), or
25MHz (LTC2236), input range = 1VP-P with differential drive.
Note 9: Recommended operating conditions.
223876fa
5
LTC2238/LTC2237/LTC2236
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2238: Typical INL,
2V Range, 65Msps
LTC2238: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
65Msps
LTC2238: Typical DNL,
2V Range, 65Msps
0
1.00
1.00
0.75
0.50
0.50
0.25
0
–0.25
–20
–30
AMPLITUDE (dB)
0.75
DNL ERROR (LSB)
INL ERROR (LSB)
–10
0.25
0
–0.25
–0.50
–0.50
–0.75
–0.75
–1.00
–1.00
–40
–50
–60
–70
–80
–90
–100
0
256
512
1024
768
–110
–120
0
256
512
223876 G03
0
–10
–20
–20
–20
–30
–30
–30
–40
–40
–40
–60
–70
–80
AMPLITUDE (dB)
0
–10
AMPLITUDE (dB)
0
–50
–50
–60
–70
–80
–50
–60
–70
–80
–90
–90
–90
–100
–100
–100
–110
–110
–110
–120
–120
20
15
10
25
FREQUENCY (MHz)
–120
0
30
5
20
15
10
25
FREQUENCY (MHz)
5
20
15
10
25
FREQUENCY (MHz)
223876 G05
0
70000
30
223876 G06
LTC2238: Grounded Input
Histogram, 65Msps
LTC2238: SNR vs Input Frequency,
–1dB, 2V Range, 65Msps
62.5
65520
–10
60000
–20
61.5
–30
–40
–50
–60
–70
SNR (dBFS)
50000
COUNT
AMPLITUDE (dB)
0
30
223876 G04
LTC2238: 8192 Point 2-Tone FFT,
fIN = 28.2MHz and 26.8MHz,
–1dB, 2V Range, 65Msps
30
LTC2238: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
65Msps
LTC2238: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
65Msps
–10
5
20
15
10
25
FREQUENCY (MHz)
223876 G02
223876 G01
LTC2238: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
65Msps
0
5
CODE
CODE
AMPLITUDE (dB)
0
1024
768
40000
30000
60.5
59.5
–80
20000
–90
58.5
–100
10000
–110
–120
0
5
20
15
10
25
FREQUENCY (MHz)
30
223876 G07
0
0
0
511
512
CODE
513
223876 G08
57.5
0
100
50
150
INPUT FREQUENCY (MHz)
200
223876 G09
223876fa
6
LTC2238/LTC2237/LTC2236
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2238: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
LTC2238: SFDR vs Input Frequency,
–1dB, 2V Range, 65Msps
100
LTC2238: SNR vs Input Level,
fIN = 30MHz, 2V Range, 65Msps
100
80
70
SFDR
95
SFDR (dBFS)
90
85
80
75
80
70
SNR
dBFS
60
SNR (dBc AND dBFS)
SNR AND SFDR (dBFS)
90
50
dBc
40
30
20
60
70
10
65
50
100
50
200
150
INPUT FREQUENCY (MHz)
0
0
–60
0 10 20 30 40 50 60 70 80 90 100 110
SAMPLE RATE (Msps)
223876 G10
–50
–40
–20
–30
INPUT LEVEL (dBFS)
223876 G12
223876 G11
LTC2238: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2238: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
LTC2238: SFDR vs Input Level,
fIN = 30MHz, 2V Range, 65Msps
120
0
–10
80
6
75
5
110
dBFS
90
4
70
dBc
60
50
80dBc SFDR
REFERENCE LINE
40
30
20
IOVDD (mA)
70
80
IVDD (mA)
SFDR (dBc AND dBFS)
100
2V RANGE
65
1V RANGE
3
60
2
55
1
10
0
–60
0
50
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
0
0
10
20 30 40 50 60
SAMPLE RATE (Msps)
0.75
0.75
0.50
0.50
0
–0.25
–0.25
–0.75
–0.75
–1.00
–1.00
768
1024
–30
–40
–50
–60
–70
–80
–90
–100
–110
0
256
512
768
1024
CODE
223876 G16
80
–20
0
–0.50
512
70
0
–10
0.25
–0.50
CODE
20 30 40 50 60
SAMPLE RATE (Msps)
LTC2237: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
40Msps
AMPLITUDE (dB)
1.00
DNL ERROR (LSB)
INL ERROR (LSB)
1.00
0.25
10
223876 G15
LTC2237: Typical DNL,
2V Range, 40Msps
LTC2237: Typical INL,
2V Range, 40Msps
256
0
80
223876 G14
223876 G13
0
70
–120
0
5
10
15
20
FREQUENCY (MHz)
223876 G17
223876 G18
223876fa
7
LTC2238/LTC2237/LTC2236
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2237: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
40Msps
LTC2237: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
40Msps
0
–10
0
–10
–20
–20
–30
–40
–30
–40
–50
–60
–70
–80
AMPLITUDE (dB)
–20
–30
–40
AMPLITUDE (dB)
AMPLITUDE (dB)
0
–10
–50
–60
–70
–80
–60
–70
–80
–90
–90
–100
–100
–100
–110
–110
–110
–120
–120
–120
0
5
10
15
20
0
5
10
20
15
0
10
5
FREQUENCY (MHz)
223876 G20
LTC2237: 8192 Point 2-Tone FFT,
fIN = 21.6MHz and 23.6MHz,
–1dB, 2V Range, 40Msps
223876 G21
LTC2237: SNR vs Input Frequency,
–1dB, 2V Range, 40Msps
LTC2237: Grounded Input
Histogram, 40Msps
0
–10
70000
–20
60000
–30
–40
50000
–50
40000
20
15
FREQUENCY (MHz)
223876 G19
62.5
65520
–60
–70
SNR (dBFS)
COUNT
61.5
30000
60.5
59.5
–80
20000
–90
58.5
–100
10000
–110
–120
5
10
15
0
0
0
0
20
510
FREQUENCY (MHz)
511
CODE
57.5
512
223876 G22
LTC2237: SNR vs Input Level,
fIN = 5MHz, 2V Range, 40Msps
80
100
SFDR
95
70
dBFS
80
75
SNR (dBc AND dBFS)
SNR AND SFDR (dBFS)
90
85
200
223876 G24
LTC2237: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
100
90
100
50
150
INPUT FREQUENCY (MHz)
0
223876 G23
LTC2237: SFDR vs Input Frequency,
–1dB, 2V Range, 40Msps
SFDR (dBFS)
–50
–90
FREQUENCY (MHz)
AMPLITUDE (dB)
LTC2237: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
40Msps
80
70
SNR
60
50
dBc
40
30
20
60
70
10
65
0
50
100
150
INPUT FREQUENCY (MHz)
200
223876 G25
50
0
10
20 30 40 50 60
SAMPLE RATE (Msps)
70
80
223876 G26
0
–60
–50
–20
–40
–30
INPUT LEVEL (dBFS)
–10
0
223876 G27
223876fa
8
LTC2238/LTC2237/LTC2236
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2237: SFDR vs Input Level,
fIN = 5MHz, 2V Range, 40Msps
LTC2237: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2237: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
120
50
4
45
3
110
90
dBFS
70
dBc
60
80dBc SFDR
REFERENCE LINE
50
2V RANGE
IOVDD (mA)
80
IVDD (mA)
SFDR (dBc AND dBFS)
100
40
1V RANGE
2
40
35
30
1
20
10
0
–60
30
–50
–20
–40
–30
INPUT LEVEL (dBFS)
–10
0
10
0
30
40
20
SAMPLE RATE (Msps)
0
50
10
0
30
40
20
SAMPLE RATE (Msps)
223876 G29
223876 G28
223876 G30
LTC2236: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
25Msps
LTC2236: Typical DNL,
2V Range, 25Msps
LTC2236: Typical INL,
2V Range, 25Msps
0
1.00
1.00
50
0.75
0.50
0.50
0.25
0
–0.25
–20
–30
AMPLITUDE (dB)
0.75
DNL ERROR (LSB)
INL ERROR (LSB)
–10
0.25
0
–0.25
–0.50
–0.50
–0.75
–0.75
–1.00
–1.00
–40
–50
–60
–70
–80
–90
–100
0
256
512
1024
768
–110
–120
0
256
512
LTC2236: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
25Msps
0
–10
–20
–20
–20
–30
–30
–30
–40
–40
–40
–70
–80
AMPLITUDE (dB)
0
–10
AMPLITUDE (dB)
0
–10
–60
–50
–60
–70
–80
–50
–60
–70
–80
–90
–90
–90
–100
–100
–100
–110
–110
–110
–120
–120
2
8
6
4
10
FREQUENCY (MHz)
12
223876 G34
12
223876 G33
LTC2236: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
25Msps
LTC2236: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
25Msps
–50
8
6
4
10
FREQUENCY (MHz)
223876 G32
223876 G31
0
2
CODE
CODE
AMPLITUDE (dB)
0
1024
768
–120
0
2
8
6
4
10
FREQUENCY (MHz)
12
223876 G35
0
2
8
6
4
10
FREQUENCY (MHz)
12
223876 G36
223876fa
9
LTC2238/LTC2237/LTC2236
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2236: 8192 Point 2-Tone FFT,
fIN = 10.9MHz and 13.8MHz,
–1dB, 2V Range, 25Msps
LTC2236: Grounded Input
Histogram, 25Msps
0
70000
62.5
65520
–10
60000
–20
61.5
–30
–40
–50
–60
–70
SNR (dBFS)
50000
COUNT
AMPLITUDE (dB)
LTC2236: SNR vs Input Frequency,
–1dB, 2V Range, 25Msps
40000
30000
59.5
–80
20000
–90
58.5
–100
10000
–110
–120
0
8
6
4
10
FREQUENCY (MHz)
2
0
0
0
12
511
512
CODE
57.5
513
223876 G37
100
LTC2236: SNR vs Input Level,
fIN = 5MHz, 2V Range, 25Msps
100
80
SFDR
95
70
80
75
SNR (dBc AND dBFS)
SNR AND SFDR (dBFS)
90
85
200
223876 G39
LTC2236: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
90
100
50
150
INPUT FREQUENCY (MHz)
0
223876 G38
LTC2236: SFDR vs Input
Frequency, –1dB, 2V Range,
25Msps
SFDR (dBFS)
60.5
80
70
SNR
dBFS
60
50
dBc
40
30
20
60
70
10
65
50
100
50
200
150
INPUT FREQUENCY (MHz)
0
0
5
0
–60
10 15 20 25 30 35 40 45 50
SAMPLE RATE (Msps)
223876 G40
–50
–40
–20
–30
INPUT LEVEL (dBFS)
LTC2236: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2236: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
35
120
0
223876 G42
223876 G41
LTC2236: SFDR vs Input Level,
fIN = 5MHz, 2V Range, 25Msps
–10
3
110
30
dBFS
80
70
60
dBc
50
80dBc SFDR
REFERENCE LINE
40
2
2V RANGE
IOVDD (mA)
90
IVDD (mA)
SFDR (dBc AND dBFS)
100
25
1V RANGE
1
20
30
20
10
0
–60
15
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
0
223876 G43
0
5
25
10
15
20
SAMPLE RATE (Msps)
30
35
223876 G44
0
0
5
25
10
15
20
SAMPLE RATE (Msps)
30
35
223876 G45
223876fa
10
LTC2238/LTC2237/LTC2236
U
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PI FU CTIO S
AIN+ (Pin 1): Positive Differential Analog Input.
NC (Pins 12, 13, 14, 15): Do Not Connect These Pins.
AIN- (Pin 2): Negative Differential Analog Input.
D0-D9 (Pins 16, 17, 18, 19, 22, 23, 24, 25, 26, 27):
Digital Outputs. D9 is the MSB.
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.
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.
223876fa
11
LTC2238/LTC2237/LTC2236
W
FUNCTIONAL BLOCK DIAGRA
U
U
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
D9
CLOCK/DUTY
CYCLE
CONTROL
DIFF
REF
AMP
CONTROL
LOGIC
OUTPUT
DRIVERS
•
•
•
D0
REFH
0.1µF
223876 F01
REFL
OGND
CLK
MODE
SHDN
OE
2.2µF
1µF
1µF
Figure 1. Functional Block Diagram
223876fa
12
LTC2238/LTC2237/LTC2236
WU
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TI I G DIAGRA
tAP
ANALOG
INPUT
N+4
N+2
N
N+3
tH
N+5
N+1
tL
CLK
tD
D0-D9, OF
N–5
N–4
N–3
N–2
N–1
N
223876 TD01
223876fa
13
LTC2238/LTC2237/LTC2236
U
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APPLICATIO S I FOR ATIO
DYNAMIC PERFORMANCE
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.
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
input tone to the RMS value of the largest 3rd order
intermodulation product.
Signal-to-Noise Ratio
Spurious Free Dynamic Range (SFDR)
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.
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.
Signal-to-Noise Plus Distortion Ratio
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.
Input Bandwidth
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.
Aperture Delay Time
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)
223876fa
14
LTC2238/LTC2237/LTC2236
U
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APPLICATIO S I FOR ATIO
CONVERTER OPERATION
SAMPLE/HOLD OPERATION AND INPUT DRIVE
As shown in Figure 1, the LTC2238/LTC2237/LTC2236 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 LTC2238/LTC2237/LTC2236 has two
phases of operation, determined by the state of the CLK
input pin.
Sample/Hold Operation
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
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.
Figure 2 shows an equivalent circuit for the LTC2238/
LTC2237/LTC2236 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.
LTC2238/LTC2237/LTC2236
VDD
CSAMPLE
4pF
15Ω
AIN+
CPARASITIC
1pF
VDD
AIN–
CSAMPLE
4pF
15Ω
CPARASITIC
1pF
VDD
CLK
223876 F02
Figure 2. Equivalent Input Circuit
223876fa
15
LTC2238/LTC2237/LTC2236
U
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APPLICATIO S I FOR ATIO
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.
Input Drive Impedance
As with all high performance, high speed ADCs, the
dynamic performance of the LTC2238/LTC2237/LTC2236
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.
Input Drive Circuits
Figure 3 shows the LTC2238/LTC2237/LTC2236 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
2.2µF
0.1µF
ANALOG
INPUT
T1
1:1
25Ω
25Ω
AIN+
0.1µF
LTC2238
LTC2237
LTC2236
12pF
25Ω
T1 = MA/COM ETC1-1T 25Ω
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
AIN–
223876 F03
Figure 3. Single-Ended to Differential Conversion Using a Transformer
223876fa
16
LTC2238/LTC2237/LTC2236
U
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APPLICATIO S I FOR ATIO
VCM
HIGH SPEED
DIFFERENTIAL
25Ω
AMPLIFIER
ANALOG
INPUT
+
–
2.2µF
2.2µF
AIN+
+
CM
VCM
0.1µF
LTC2238
LTC2237
LTC2236
25Ω
–
25Ω
0.1µF
T1
12pF
0.1µF
AIN–
AIN+
12Ω
ANALOG
INPUT
LTC2238
LTC2237
LTC2236
8pF
25Ω
AIN–
12Ω
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
223876 F04
223876 F06
Figure 4. Differential Drive with an Amplifier
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.
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
VCM
2.2µF
0.1µF
AIN+
ANALOG
INPUT
25Ω
0.1µF
T1
0.1µF
25Ω
AIN–
VCM
1k
0.1µF
ANALOG
INPUT
1k
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2.2µF
25Ω
AIN+
LTC2238
LTC2237
LTC2236
LTC2238
LTC2237
LTC2236
223876 F07
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
12pF
25Ω
VCM
AIN–
2.2µF
0.1µF
223876 F05
0.1µF
6.8nH
ANALOG
INPUT
25Ω
Figure 5. Single-Ended Drive
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.
AIN+
0.1µF
LTC2238
LTC2237
LTC2236
T1
0.1µF
25Ω
6.8nH
AIN–
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
223876 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
223876fa
17
LTC2238/LTC2237/LTC2236
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APPLICATIO S I FOR ATIO
Reference Operation
Figure 9 shows the LTC2238/LTC2237/LTC2236 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.
LTC2238/LTC2237/LTC2236
4Ω
VCM
1.5V
1.5V BANDGAP
REFERENCE
2.2µF
1V
0.5V
RANGE
DETECT
AND
CONTROL
TIE TO VDD FOR 2V RANGE;
TIE TO VCM FOR 1V RANGE;
RANGE = 2 • VSENSE FOR
0.5V < VSENSE < 1V
SENSE
BUFFER
INTERNAL ADC
HIGH REFERENCE
1µF
REFH
2.2µF
0.1µF
DIFF AMP
1µF
REFL
INTERNAL ADC
LOW REFERENCE
223876 F09
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
pins are needed to reduce package inductance. Bypass
capacitors must be connected as shown in Figure 9.
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.
Figure 9. Equivalent Reference Circuit
1.5V
VCM
2.2µF
12k
0.75V
12k
SENSE
LTC2238
LTC2237
LTC2236
1µF
223876 F10
Figure 10. 1.5V Range ADC
223876fa
18
LTC2238/LTC2237/LTC2236
U
W
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APPLICATIO S I FOR ATIO
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 0.6dB. See the Typical Performance Characteristics section.
Driving the Clock Input
The CLK input can be driven directly with a CMOS or TTL
level signal. A sinusoidal clock can also be used along with
a low-jitter squaring circuit before the CLK pin (see
Figure 11).
The noise performance of the LTC2238/LTC2237/LTC2236
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.
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.
The transformer shown 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.
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
CLEAN
SUPPLY
4.7µF
4.7µF
FERRITE
BEAD
0.1µF
CLK
100Ω
223876 F12
Figure 12. CLK Drive Using an LVDS or PECL to CMOS Converter
ETC1-1T
0.1µF
SINUSOIDAL
CLOCK
INPUT
1k
CLK
50Ω
1k
CLK
5pF-30pF
LTC2238
LTC2237
LTC2236
NC7SVU04
Figure 11. Sinusoidal Single-Ended CLK Drive
LTC2238/
LTC2237/
LTC2236
DIFFERENTIAL
CLOCK
INPUT
223876 F13
0.1µF
223876 F11
LTC2238/
LTC2237/
LTC2236
IF LVDS USE FIN1002 OR FIN1018.
FOR PECL, USE AZ1000ELT21 OR SIMILAR
FERRITE
BEAD
0.1µF
CLEAN
SUPPLY
FERRITE
BEAD
VCM
Figure 13. LVDS or PECL CLK Drive Using a Transformer
223876fa
19
LTC2238/LTC2237/LTC2236
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APPLICATIO S I FOR ATIO
Maximum and Minimum Conversion Rates
DIGITAL OUTPUTS
The maximum conversion rate for the LTC2238/LTC2237/
LTC2236 is 65Msps (LTC2238), 40Msps (LTC2237), and
25Msps (LTC2236). 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 (LTC2238), 11.8ns
(LTC2237), and 18.9ns (LTC2236) for the ADC internal
circuitry to have enough settling time for proper operation.
Table 1 shows the relationship between the analog input
voltage, the digital data bits, and the overflow bit.
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 LTC2238/LTC2237/LTC2236 sample
rate is determined by droop of the sample-and-hold circuits. The pipelined architecture of this ADC relies on
storing analog signals on small valued capacitors. Junction leakage will discharge the capacitors. The specified
minimum operating frequency for the LTC2238/LTC2237/
LTC2236 is 1Msps.
LTC2228/27/26
OVDD
VDD
0.5V
TO 3.6V
VDD
0.1µF
OVDD
DATA
FROM
LATCH
PREDRIVER
LOGIC
43Ω
TYPICAL
DATA
OUTPUT
Table 1. Output Codes vs Input Voltage
AIN+ – AIN–
(2V Range)
OF
D9 – D0
(Offset Binary)
D9 – D0
(2’s Complement)
>+1.000000V
+0.998047V
+0.996094V
1
0
0
11 1111 1111
11 1111 1111
11 1111 1110
01 1111 1111
01 1111 1111
01 1111 1110
+0.001953V
0.000000V
–0.001953V
–0.003906V
0
0
0
0
10 0000 0001
10 0000 0000
01 1111 1111
01 1111 1110
00 0000 0001
00 0000 0000
11 1111 1111
11 1111 1110
–0.998047V
–1.000000V
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