LTC2298/LTC2297/LTC2296
Dual 14-Bit, 65/40/25Msps
Low Power 3V ADCs
DESCRIPTIO
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FEATURES
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The LTC®2298/LTC2297/LTC2296 are 14-bit 65Msps/
40Msps/25Msps, low power dual 3V A/D converters designed for digitizing high frequency, wide dynamic range
signals. The LTC2298/LTC2297/LTC2296 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.
Integrated Dual 14-Bit ADCs
Sample Rate: 65Msps/40Msps/25Msps
Single 3V Supply (2.7V to 3.4V)
Low Power: 400mW/235mW/150mW
74.3dB SNR
90dB SFDR
110dB Channel Isolation at 100MHz
Multiplexed or Separate Data Bus
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
105Msps: LTC2282 (12-Bit), LTC2284 (14-Bit)
80Msps: LTC2294 (12-Bit), LTC2299 (14-Bit)
65Msps: LTC2293 (12-Bit), LTC2298 (14-Bit)
40Msps: LTC2292 (12-Bit), LTC2297 (14-Bit)
25Msps: LTC2291 (12-Bit), LTC2296 (14-Bit)
10Msps: LTC2290 (12-Bit), LTC2295 (14-Bit)
64-Pin (9mm × 9mm) QFN Package
DC specs include ±1.2LSB 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. An optional multiplexer allows both channels to
share a digital output bus.
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
Spectral Analysis
Portable Instrumentation
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TYPICAL APPLICATIO
+
ANALOG
INPUT A
INPUT
S/H
–
LTC2298: SNR vs Input Frequency,
–1dB, 2V Range, 65Msps
OVDD
14-BIT
PIPELINED
ADC CORE
OUTPUT
DRIVERS
75
D13A
••
•
D0A
74
CLK A
CLOCK/DUTY CYCLE
CONTROL
CLK B
CLOCK/DUTY CYCLE
CONTROL
MUX
SNR (dBFS)
OGND
73
72
71
OVDD
+
ANALOG
INPUT B
INPUT
S/H
–
14-BIT
PIPELINED
ADC CORE
OUTPUT
DRIVERS
D13B
••
•
D0B
70
0
100
50
150
INPUT FREQUENCY (MHz)
200
229876 TA01b
OGND
229876 TA01
229876fa
1
LTC2298/LTC2297/LTC2296
W W
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W
ABSOLUTE
AXI U RATI GS
OVDD = VDD (Notes 1, 2)
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
LTC2298C, LTC2297C, LTC2296C ........... 0°C to 70°C
LTC2298I, LTC2297I, LTC2296I ..........–40°C to 85°C
Storage Temperature Range ..................–65°C to 125°C
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W
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PACKAGE/ORDER I FOR ATIO
64 GND
63 VDD
62 SENSEA
61 VCMA
60 MODE
59 SHDNA
58 OEA
57 OFA
56 DA13
55 DA12
54 DA11
53 DA10
52 DA9
51 DA8
50 OGND
49 OVDD
TOP VIEW
AINA+ 1
AINA– 2
REFHA 3
REFHA 4
REFLA 5
REFLA 6
VDD 7
CLKA 8
CLKB 9
VDD 10
REFLB 11
REFLB 12
REFHB 13
REFHB 14
AINB– 15
AINB+ 16
GND 17
VDD 18
SENSEB 19
VCMB 20
MUX 21
SHDNB 22
OEB 23
DB0 24
DB1 25
DB2 26
DB3 27
DB4 28
DB5 29
DB6 30
OGND 31
OVDD 32
65
48 DA7
47 DA6
46 DA5
45 DA4
44 DA3
43 DA2
42 DA1
41 DA0
40 OFB
39 DB13
38 DB12
37 DB11
36 DB10
35 DB9
34 DB8
33 DB7
ORDER PART
NUMBER
QFN PART*
MARKING
LTC2298CUP
LTC2298IUP
LTC2297CUP
LTC2297IUP
LTC2296CUP
LTC2296IUP
LTC2298UP
LTC2297UP
LTC2296UP
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/
UP PACKAGE
64-LEAD (9mm × 9mm) PLASTIC QFN
TJMAX = 125°C, θJA = 20°C/W
EXPOSED PAD (PIN 65) IS GND AND MUST BE SOLDERED TO PCB
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
Resolution
(No Missing Codes)
Integral Linearity Error
Differential
Linearity Error
Offset Error
Gain Error
Offset Drift
Full-Scale Drift
Gain Matching
Offset Matching
Transition Noise
CONDITIONS
●
Differential Analog Input (Note 5)
Differential Analog Input
●
(Note 6)
External Reference
●
Internal Reference
External Reference
SENSE = 1V
●
●
MIN
14
LTC2298
TYP
MAX
MIN
14
LTC2297
TYP
MAX
MIN
14
LTC2296
TYP
MAX
–5
–1
±1.2
±0.5
5
1
–5
–1
±1.2
±0.5
5
1
–5
–1
±1.2
±0.5
5
1
–12
–2.5
±2
±0.5
±10
±30
±5
±0.3
±2
1
12
2.5
–12
–2.5
±2
±0.5
±10
±30
±5
±0.3
±2
1
12
2.5
–12
–2.5
±2
±0.5
±10
±30
±5
±0.3
±2
1
12
2.5
UNITS
Bits
LSB
LSB
mV
%FS
µV/°C
ppm/°C
ppm/°C
%FS
mV
LSBRMS
229876fa
2
LTC2298/LTC2297/LTC2296
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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
VIN
Analog Input Range (AIN+ –AIN–)
CONDITIONS
VIN,CM
Analog Input Common Mode (AIN+
+AIN
–)/2
MIN
2.7V < VDD < 3.4V (Note 7)
●
TYP
MAX
UNITS
±0.5V to ±1V
V
Differential Input (Note 7)
●
1
1.5
1.9
V
Single Ended Input (Note 7)
●
0.5
1.5
2
V
0V < AIN+, AIN–
●
–1
1
µA
IIN
Analog Input Leakage Current
< VDD
ISENSE
SENSEA, SENSEB Input Leakage
0V < SENSEA, SENSEB < 1V
●
–3
3
µA
IMODE
MODE Input Leakage Current
0V < MODE < VDD
●
–3
3
µA
tAP
Sample-and-Hold Acquisition Delay Time
0
ns
tJITTER
Sample-and-Hold Acquisition Delay Time Jitter
0.2
psRMS
CMRR
Analog Input Common Mode Rejection Ratio
80
dB
575
MHz
Full Power Bandwidth
Figure 8 Test Circuit
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
MIN
74.4
72.4
72.1
LTC2296
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
75
75
90
dB
dB
90
dB
85
85
85
dB
140MHz Input
80
80
80
dB
5MHz Input
90
90
90
dB
90
dB
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
80
80
78
5MHz Input
12.5MHz Input
●
20MHz Input
●
30MHz Input
●
90
dB
90
dB
90
140MHz Input
Signal-to-Noise
Plus Distortion
Ratio
LTC2297
TYP
MAX
72.7
70MHz Input
S/(N+D)
MIN
74.3
70MHz Input
SFDR
LTC2298
TYP
MAX
90
90
90
90
90
dB
74.3
74.4
74.5
dB
74.2
dB
72.2
71.9
71.6
dB
74.3
dB
74.2
dB
70MHz Input
74.1
73.6
73.4
140MHz Input
71.9
71.9
71.8
dB
90
90
90
dB
–110
–110
–110
Intermodulation
Distortion
fIN = Nyquist,
Nyquist + 1MHz
Crosstalk
fIN = Nyquist
dB
dB
229876fa
3
LTC2298/LTC2297/LTC2296
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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 < 3.3V
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, MUX)
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)
2
V
–10
0.8
V
10
µA
3
pF
LOGIC OUTPUTS
OVDD = 3V
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
●
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
VOH
High Level Output Voltage
IO = –200µA
1.79
V
VOL
Low Level Output Voltage
IO = 1.6mA
0.09
V
OVDD = 1.8V
229876fa
4
LTC2298/LTC2297/LTC2296
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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
LTC2298
TYP
MAX
MIN
LTC2297
TYP
MAX
MIN
LTC2296
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
Both ADCs at fS(MAX)
●
133
150
78
95
50
60
mA
PDISS
Power Dissipation
Both ADCs at fS(MAX)
●
400
450
235
285
150
180
mW
PSHDN
Shutdown Power
(Each Channel)
SHDN = H,
OE = H, No CLK
2
2
2
mW
PNAP
Nap Mode Power
(Each Channel)
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
LTC2298
TYP
MAX
MIN
LTC2297
TYP
MAX
PARAMETER
fs
Sampling Frequency (Note 9)
●
1
65
1
40
1
25
MHz
tL
CLK Low Time
Duty Cycle Stabilizer Off
Duty Cycle Stabilizer On
(Note 7)
●
●
7.3
5
7.7
7.7
500
500
11.8
5
12.5
12.5
500
500
18.9
5
20
20
500
500
ns
ns
tH
CLK High Time
Duty Cycle Stabilizer Off
Duty Cycle Stabilizer On
(Note 7)
●
●
7.3
5
7.7
7.7
500
500
11.8
5
12.5
12.5
500
500
18.9
5
20
20
500
500
ns
ns
tAP
Sample-and-Hold
Aperture Delay
tD
CLK to DATA Delay
CL = 5pF (Note 7)
●
1.4
2.7
5.4
1.4
2.7
5.4
1.4
2.7
5.4
ns
tMD
MUX to DATA Delay CL = 5pF (Note 7)
●
1.4
2.7
5.4
1.4
2.7
5.4
1.4
2.7
5.4
ns
Data Access Time
After OE↓
●
4.3
10
4.3
10
4.3
10
ns
●
3.3
8.5
3.3
8.5
3.3
8.5
0
CL = 5pF (Note 7)
BUS Relinquish Time (Note 7)
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 (LTC2298), 40MHz (LTC2297), or
25MHz (LTC2296), input range = 2VP-P with differential drive, unless
otherwise noted.
5
MIN
LTC2296
TYP
MAX
SYMBOL
0
5
0
5
UNITS
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 0000 and
11 1111 1111 1111.
Note 7: Guaranteed by design, not subject to test.
Note 8: VDD = 3V, fSAMPLE = 65MHz (LTC2298), 40MHz (LTC2297), or
25MHz (LTC2296), input range = 1VP-P with differential drive. The supply
current and power dissipation are the sum total for both channels with
both channels active.
Note 9: Recommended operating conditions.
229876fa
5
LTC2298/LTC2297/LTC2296
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC2298: Typical INL,
2V Range, 65Msps
LTC2298/LTC2297/LTC2296:
Crosstalk vs Input Frequency
INL ERROR (LSB)
CROSSTALK (dB)
–105
–110
–115
–120
–125
–130
2.0
1.00
1.5
0.75
1.0
0.50
DNL ERROR (LSB)
–100
LTC2298: Typical DNL,
2V Range, 65Msps
0.5
0
–0.5
20
–0.25
–0.50
–1.5
–0.75
100
40
60
80
INPUT FREQUENCY (MHz)
0
–1.0
–2.0
0
0.25
–1.00
0
4096
8192
CODE
12288
0
16384
4096
8192
CODE
2298 G01
12288
16384
2298 G02
229876 G01
0
–10
–20
–20
–20
–30
–30
–30
–40
–50
–60
–70
–80
AMPLITUDE (dB)
0
–10
AMPLITUDE (dB)
–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
20
25
FREQUENCY (MHz)
30
–20
–20
–30
–30
AMPLITUDE (dB)
0
–10
–40
–50
–60
–70
–80
10
15
20
25
FREQUENCY (MHz)
30
2298 G05
LTC2298: Grounded Input
Histogram, 65Msps
21824
20412
20000
–70
15000
10224
10000
9042
–80
–90
–110
–110
2298 G06
5
25000
–60
–100
30
0
2298 G04
–50
–90
10
15
20
25
FREQUENCY (MHz)
–120
30
–40
–100
5
10
15
20
25
FREQUENCY (MHz)
LTC2298: 8192 Point 2-Tone FFT,
fIN = 28.2MHz and 26.8MHz,
–1dB, 2V Range, 65Msps
0
0
5
2298 G03
–10
–120
0
COUNT
AMPLITUDE (dB)
0
–10
LTC2298: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
65Msps
AMPLITUDE (dB)
LTC2298: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
65Msps
LTC2298: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
65Msps
LTC2298: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
65Msps
–120
5000
2116
172
0
5
10
15
20
25
FREQUENCY (MHz)
30
2298 G06a
0
1596
121
8196 8197 8198 8199 8200 8201 8202 8203
CODE
2298 G08
229876fa
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LTC2298/LTC2297/LTC2296
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC2298: SNR vs Input Frequency,
–1dB, 2V Range, 65Msps
LTC2298: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
LTC2298: SFDR vs Input Frequency,
–1dB, 2V Range, 65Msps
100
75
110
95
100
SNR AND SFDR (dBFS)
74
SFDR (dBFS)
SNR (dBFS)
90
73
72
85
80
75
90
80
70
71
70
70
65
100
50
150
INPUT FREQUENCY (MHz)
0
60
200
80
SFDR: DCS ON
LTC2298: SFDR vs Input Level,
fIN = 30MHz, 2V Range, 65Msps
120
dBFS
110
70
SFDR: DCS OFF
85
80
60
SFDR (dBc AND dBFS)
SNR (dBc AND dBFS)
100
90
50
dBc
40
30
20
75 SNR: DCS ON
40 45 50 55 60
CLOCK DUTY CYCLE (%)
65
0
–60
70
2298 G12
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
0
2298 G13
80
dBc
70
90dBc SFDR
REFERENCE LINE
60
50
20
–60
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
0
2298 G14
LTC2298: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2298: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
155
12
145
10
135
8
1V RANGE
125
2V RANGE
6
115
4
105
2
95
90
30
IOVDD (mA)
35
IVDD (mA)
30
dBFS
40
10
SNR: DCS OFF
70
0 10 20 30 40 50 60 70 80 90 100 110
SAMPLE RATE (Msps)
2298 G11
2298 G10
LTC2298: SNR vs Input Level,
fIN = 30MHz, 2V Range, 65Msps
95
SNR AND SFDR (dBFS)
100
150
INPUT FREQUENCY (MHz)
2298 G09
LTC2298: SNR and SFDR vs
Clock Duty Cycle, 65Msps
100
50
0
200
0
10
20 30 40 50 60
SAMPLE RATE (Msps)
70
80
2298 G15
0
0
10
20 30 40 50 60
SAMPLE RATE (Msps)
70
80
2298 G16
229876fa
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LTC2298/LTC2297/LTC2296
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC2297: Typical INL,
2V Range, 40Msps
0
1.00
0.75
1.0
0.50
0.5
0
–0.5
–1.0
–10
–20
–30
AMPLITUDE (dB)
1.5
DNL ERROR (LSB)
0.25
0
–0.25
–1.5
–0.75
–2.0
–1.00
0
4096
8192
CODE
12288
16384
–50
–60
–70
–80
–100
–110
–120
4096
0
8192
CODE
2297 G01
12288
16384
0
0
–10
–10
–20
–20
–20
–30
–30
–30
AMPLITUDE (dB)
0
–50
–60
–70
–80
–40
–50
–60
–70
–80
–70
–80
–90
–100
–100
–110
–110
–110
–120
–120
20
–120
0
5
2297 G04
LTC2297: 8192 Point 2-Tone FFT,
fIN = 21.6MHz and 23.6MHz,
–1dB, 2V Range, 40Msps
10
15
FREQUENCY (MHz)
0
20
2297 G05
LTC2297: Grounded Input
Histogram, 40Msps
0
10
15
FREQUENCY (MHz)
20
2297 G06
75
24558
25000
74
–30
–40
5
LTC2297: SNR vs Input Frequency,
–1dB, 2V Range, 40Msps
30000
–10
–20
2297 G03
–60
–100
10
15
FREQUENCY (MHz)
20
–50
–90
5
10
15
FREQUENCY (MHz)
–40
–90
0
5
LTC2297: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
40Msps
–10
–40
0
2297 G02
LTC2297: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
40Msps
LTC2297: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
40Msps
–60
–70
–80
14833
15000
SNR (dBFS)
20000
–50
COUNT
AMPLITUDE (dB)
–40
–90
–0.50
AMPLITUDE (dB)
INL ERROR (LSB)
2.0
AMPLITUDE (dB)
LTC2297: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
40Msps
LTC2297: Typical DNL,
2V Range, 40Msps
15714
73
72
10000
–90
–100
4641
5000
–110
–120
0
5
10
15
FREQUENCY (MHz)
20
2297 G07
0
30
640
71
4520
546 36
8184 8185 8186 818781888189 8190 81918192
CODE
2297 G08
70
0
100
50
150
INPUT FREQUENCY (MHz)
200
2297 G09
229876fa
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LTC2298/LTC2297/LTC2296
U W
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2297: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
LTC2297: SFDR vs Input Frequency,
–1dB, 2V Range, 40Msps
110
80
95
SFDR
SNR AND SFDR (dBFS)
100
90
85
80
75
dBFS
70
SNR (dBc AND dBFS)
100
SFDR (dBFS)
LTC2297: SNR vs Input Level,
fIN = 5MHz, 2V Range, 40Msps
90
80
SNR
60
50
dBc
40
30
20
70
70
10
65
60
50
100
0
200
150
INPUT FREQUENCY (MHz)
0
40
20
60
SAMPLE RATE (Msps)
0
–60
80
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
2297 G12
2297 G11
2297 G10
LTC2297: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2297: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
LTC2297: SFDR vs Input Level,
fIN = 5MHz, 2V Range, 40Msps
120
0
100
8
90
6
110
80
IVDD (mA)
SNR (dBc AND dBFS)
90
dBc
70
90dBc SFDR
REFERENCE LINE
60
IOVDD (mA)
dBFS
100
2V RANGE
80
4
1V RANGE
50
2
70
40
30
20
–60
– 50
– 40
–30
–20
INPUT LEVEL (dBFS)
–10
60
0
0
10
30
40
20
SAMPLE RATE (Msps)
LTC2296: Typical INL,
2V Range, 25Msps
1.0
0.50
0
–20
–30
0.25
0
–0.25
–0.50
–1.0
–1.5
–0.75
–2.0
–1.00
0
4096
8192
CODE
12288
16384
2296 G01
50
–10
AMPLITUDE (dB)
0.75
DNL ERROR (LSB)
1.5
–0.5
30
40
20
SAMPLE RATE (Msps)
LTC2296: 8192 Point FFT,
fIN = 5MHz, –1dB, 2V Range,
25Msps
1.00
2.0
0
10
2297 G15
LTC2296: Typical DNL,
2V Range, 25Msps
0.5
0
2297 G14
2297 G13
INL ERROR (LSB)
0
50
–40
–50
–60
–70
–80
–90
–100
–110
–120
0
4096
8192
CODE
12288
16384
2296 G02
0
2
4
6
8
10
FREQUENCY (MHz)
12
2296 G03
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LTC2298/LTC2297/LTC2296
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0
–10
–20
–20
–20
–30
–30
–30
–40
–50
–60
–70
–80
AMPLITUDE (dB)
0
–10
AMPLITUDE (dB)
AMPLITUDE (dB)
0
–10
–40
–50
–60
–70
–80
–40
–50
–60
–70
–80
–90
–90
–90
–100
–100
–100
–110
–110
–110
–120
–120
0
2
4
6
8
10
FREQUENCY (MHz)
12
0
2
4
6
8
10
FREQUENCY (MHz)
–120
12
0
2
12
4
6
8
10
FREQUENCY (MHz)
2296 G05
2296 G04
LTC2296: 8192 Point 2-Tone FFT,
fIN = 10.9MHz and 13.8MHz,
–1dB, 2V Range, 25Msps
2296 G06
LTC2296: Grounded Input
Histogram, 25Msps
LTC2296: SNR vs Input Frequency,
–1dB, 2V Range, 25Msps
25000
0
75
22016
–10
–20
20000
74
18803
–30
–50
–60
–70
15000
SNR (dBFS)
–40
COUNT
AMPLITUDE (dB)
LTC2296: 8192 Point FFT,
fIN = 140MHz, –1dB, 2V Range,
25Msps
LTC2296: 8192 Point FFT,
fIN = 70MHz, –1dB, 2V Range,
25Msps
LTC2296: 8192 Point FFT,
fIN = 30MHz, –1dB, 2V Range,
25Msps
13373
10000
–80
73
72
6919
–90
5000
–100
–110
–120
0
2
4
6
8
10
FREQUENCY (MHz)
0
12
2296 G07
71
3227
43
853
278
70
8179 8180 8181 8182 8183 8184 8185 8186
CODE
2296 G08
LTC2296: SNR and SFDR vs
Sample Rate, 2V Range,
fIN = 5MHz, –1dB
LTC2296: SFDR vs Input
Frequency, –1dB, 2V Range,
25Msps
100
80
110
dBFS
70
85
80
75
SNR (dBc AND dBFS)
SNR AND SFDR (dBFS)
100
90
200
2296 G09
LTC2296: SNR vs Input Level,
fIN = 5MHz, 2V Range, 25Msps
95
SFDR (dBFS)
100
50
150
INPUT FREQUENCY (MHz)
0
SFDR
90
80
SNR
60
50
dBc
40
30
20
70
70
10
65
0
50
100
150
INPUT FREQUENCY (MHz)
200
2296 G10
60
0
10
20
30
40
SAMPLE RATE (Msps)
50
2296 G11
0
–60
–50
–30
–40
–20
INPUT LEVEL (dBFS)
–10
0
2296 G12
229876fa
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LTC2298/LTC2297/LTC2296
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC2296: IOVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB,
OVDD = 1.8V
LTC2296: IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
LTC2296: SFDR vs Input Level,
fIN = 5MHz, 2V Range, 25Msps
70
120
6
110
dBFS
60
90
dBc
70
90dBc SFDR
REFERENCE LINE
60
IOVDD (mA)
4
80
IVDD (mA)
SFDR (dBc AND dBFS)
100
2V RANGE
50
1V RANGE
2
50
40
40
30
30
–50
–40
–30
–20
INPUT LEVEL (dBFS)
–10
0
2296 G13
0
5
25
20
15
10
SAMPLE RATE (Msps)
30
35
2296 G14
0
0
5
25
20
15
10
SAMPLE RATE (Msps)
30
35
2296 G15
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U
PI FU CTIO S
AINA+ (Pin 1): Channel A Positive Differential Analog
Input.
AINA– (Pin 2): Channel A Negative Differential Analog
Input.
REFHA (Pins 3, 4): Channel A 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.
REFLA (Pins 5, 6): Channel A 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, 10, 18, 63): Analog 3V Supply. Bypass to
GND with 0.1µF ceramic chip capacitors.
CLKA (Pin 8): Channel A Clock Input. The input sample
starts on the positive edge.
CLKB (Pin 9): Channel B Clock Input. The input sample
starts on the positive edge.
REFLB (Pins 11, 12): Channel B Low Reference. Short
together and bypass to Pins 13, 14 with a 0.1µF ceramic
chip capacitor as close to the pin as possible. Also bypass
to Pins 13, 14 with an additional 2.2µF ceramic chip capacitor and to ground with a 1µF ceramic chip capacitor.
REFHB (Pins 13, 14): Channel B High Reference. Short
together and bypass to Pins 11, 12 with a 0.1µF ceramic
chip capacitor as close to the pin as possible. Also bypass
to Pins 11, 12 with an additional 2.2µF ceramic chip capacitor and to ground with a 1µF ceramic chip capacitor.
AINB– (Pin 15): Channel B Negative Differential Analog
Input.
AINB+ (Pin 16): Channel B Positive Differential Analog
Input.
GND (Pins 17, 64): ADC Power Ground.
SENSEB (Pin 19): Channel B Reference Programming Pin.
Connecting SENSEB to VCMB 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 SENSEB selects an input
range of ±VSENSEB. ±1V is the largest valid input range.
VCMB (Pin 20): Channel B 1.5V Output and Input Common
Mode Bias. Bypass to ground with 2.2µF ceramic chip
capacitor. Do not connect to VCMA.
229876fa
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LTC2298/LTC2297/LTC2296
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PI FU CTIO S
MUX (Pin 21): Digital Output Multiplexer Control. If MUX
is High, Channel A comes out on DA0-DA13, OFA; Channel B
comes out on DB0-DB13, OFB. If MUX is Low, the output
busses are swapped and Channel A comes out on DB0DB13, OFB; Channel B comes out on DA0-DA13, OFA. To
multiplex both channels onto a single output bus, connect
MUX, CLKA and CLKB together.
SHDNB (Pin 22): Channel B Shutdown Mode Selection
Pin. Connecting SHDNB to GND and OEB to GND results
in normal operation with the outputs enabled. Connecting
SHDNB to GND and OEB to VDD results in normal operation with the outputs at high impedance. Connecting
SHDNB to VDD and OEB to GND results in nap mode with
the outputs at high impedance. Connecting SHDNB to VDD
and OEB to VDD results in sleep mode with the outputs at
high impedance.
OEB (Pin 23): Channel B Output Enable Pin. Refer to
SHDNB pin function.
DB0 – DB13 (Pins 24 to 30, 33 to 39): Channel B Digital
Outputs. DB13 is the MSB.
OGND (Pins 31, 50): Output Driver Ground.
OVDD (Pins 32, 49): Positive Supply for the Output Drivers. Bypass to ground with 0.1µF ceramic chip capacitor.
OFB (Pin 40): Channel B Overflow/Underflow Output.
High when an overflow or underflow has occurred.
DA0 – DA13 (Pins 41 to 48, 51 to 56): Channel A Digital
Outputs. DA13 is the MSB.
OFA (Pin 57): Channel A Overflow/Underflow Output.
High when an overflow or underflow has occurred.
SHDNA (Pin 59): Channel A Shutdown Mode Selection
Pin. Connecting SHDNA to GND and OEA to GND results
in normal operation with the outputs enabled. Connecting
SHDNA to GND and OEA to VDD results in normal operation with the outputs at high impedance. Connecting
SHDNA to VDD and OEA to GND results in nap mode with
the outputs at high impedance. Connecting SHDNA to VDD
and OEA to VDD results in sleep mode with the outputs at
high impedance.
MODE (Pin 60): Output Format and Clock Duty Cycle
Stabilizer Selection Pin. Note that MODE controls both
channels. 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.
VCMA (Pin 61): Channel A 1.5V Output and Input Common
Mode Bias. Bypass to ground with 2.2µF ceramic chip
capacitor. Do not connect to VCMB.
SENSEA (Pin 62): Channel A Reference Programming Pin.
Connecting SENSEA to VCMA 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 SENSEA selects an input
range of ±VSENSEA. ±1V is the largest valid input range.
GND (Exposed Pad) (Pin 65): ADC Power Ground. The
Exposed Pad on the bottom of the package needs to be
soldered to ground.
OEA (Pin 58): Channel A Output Enable Pin. Refer to
SHDNA pin function.
229876fa
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LTC2298/LTC2297/LTC2296
W
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
0.1µF
229876 F01
REFL
OGND
CLK
MODE
SHDN
OE
2.2µF
1µF
1µF
Figure 1. Functional Block Diagram (Only One Channel is Shown)
229876fa
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LTC2298/LTC2297/LTC2296
W
UW
TI I G DIAGRA S
Dual Digital Output Bus Timing
(Only One Channel is Shown)
tAP
N+4
N+2
N
ANALOG
INPUT
N+1
tH
N+3
N+5
tL
CLK
tD
N–4
N–5
D0-D13, OF
N–3
N–2
N–1
N
229876 TD01
Multiplexed Digital Output Bus Timing
tAPA
ANALOG
INPUT A
A+4
A+2
A
A+1
A+3
tAPB
ANALOG
INPUT B
B+4
B+2
B
B+1
tH
tL
A–5
B–5
B+3
CLKA = CLKB = MUX
D0A-D13A, OFA
A–4
tD
D0B-D13B, OFB
B–5
B–4
A–3
B–3
A–2
B–2
B–3
A–3
B–2
A–2
A–1
t MD
A–5
B–4
A–4
B–1
229876 TD02
229876fa
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LTC2298/LTC2297/LTC2296
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APPLICATIO S I FOR ATIO
DYNAMIC PERFORMANCE
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.
Signal-to-Noise Ratio
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.
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.
Spurious Free Dynamic Range (SFDR)
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.
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.
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.
Aperture Delay Time
The time from when CLK reaches midsupply 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)
Intermodulation Distortion
Crosstalk
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.
Crosstalk is the coupling from one channel (being driven
by a full-scale signal) onto the other channel (being driven
by a –1dBFS signal).
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,
CONVERTER OPERATION
As shown in Figure 1, the LTC2298/LTC2297/LTC2296 are
dual CMOS pipelined multistep converters. The converters have 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
229876fa
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LTC2298/LTC2297/LTC2296
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APPLICATIO S I FOR ATIO
sensitive applications, the analog inputs can be driven
single-ended with slightly worse harmonic distortion. The
CLK input is single-ended. The LTC2298/LTC2297/
LTC2296 have 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
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 LTC2298/
LTC2297/LTC2296 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
LTC2298/LTC2297/LTC2296
VDD
AIN+
CPARASITIC
1pF
VDD
AIN–
CSAMPLE
4pF
15Ω
CSAMPLE
4pF
15Ω
CPARASITIC
1pF
VDD
CLK
229876 F02
Figure 2. Equivalent Input Circuit
229876fa
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APPLICATIO S I FOR ATIO
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 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.
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 LTC2298/LTC2297/LTC2296 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.
VCM
2.2µF
0.1µF
ANALOG
INPUT
T1
1:1
25Ω
25Ω
0.1µF
Input Drive Impedance
As with all high performance, high speed ADCs, the
dynamic performance of the LTC2298/LTC2297/LTC2296
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
AIN+
LTC2298
LTC2297
LTC2296
12pF
25Ω
T1 = MA/COM ETC1-1T 25Ω
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
AIN–
229876 F03
Figure 3. Single-Ended to Differential Conversion
Using a Transformer
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.
229876fa
17
LTC2298/LTC2297/LTC2296
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APPLICATIO S I FOR ATIO
VCM
HIGH SPEED
DIFFERENTIAL
25Ω
AMPLIFIER
ANALOG
INPUT
+
–
2.2µF
AIN+
+
CM
VCM
2.2µF
0.1µF
LTC2298
LTC2297
LTC2296
25Ω
25Ω
0.1µF
AIN–
AIN+
0.1µF
T1
12pF
–
12Ω
ANALOG
INPUT
LTC2298
LTC2297
LTC2296
8pF
25Ω
12Ω
AIN–
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
229876 F04
229876 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.
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
VCM
2.2µF
VCM
1k
0.1µF
ANALOG
INPUT
1k
25Ω
0.1µF
25Ω
AIN+
0.1µF
LTC2298
LTC2297
LTC2296
25Ω
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
AIN–
0.1µF
0.1µF
T1
LTC2298
LTC2297
LTC2296
12pF
25Ω
AIN+
ANALOG
INPUT
2.2µF
AIN–
229876 F07
229876 F05
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
Figure 5. Single-Ended Drive
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.
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
6.8nH
ANALOG
INPUT
25Ω
AIN+
0.1µF
LTC2298
LTC2297
LTC2296
T1
0.1µF
25Ω
6.8nH
–
AIN
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
229876 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
229876fa
18
LTC2298/LTC2297/LTC2296
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APPLICATIO S I FOR ATIO
Reference Operation
Figure 9 shows the LTC2298/LTC2297/LTC2296 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
pins are needed to reduce package inductance. Bypass
capacitors must be connected as shown in Figure 9. Each
ADC channel has an independent reference with its own
bypass capacitors. The two channels can be used with the
same or different input ranges.
Other voltage ranges 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.
For the best channel matching, connect an external reference
to SENSEA and SENSEB.
LTC2298/LTC2297/LTC2296
1.5V
VCM
4Ω
1.5V
1.5V BANDGAP
REFERENCE
2.2µF
2.2µF
12k
0.5V
1V
0.75V
TIE TO VDD FOR 2V RANGE;
TIE TO VCM FOR 1V RANGE;
RANGE = 2 • VSENSE FOR
0.5V < VSENSE < 1V
RANGE
DETECT
AND
CONTROL
12k
BUFFER
0.1µF
LTC2298
LTC2297
LTC2296
1µF
Figure 10. 1.5V Range ADC
INTERNAL ADC
HIGH REFERENCE
Input Range
REFH
2.2µF
SENSE
229876 F10
SENSE
1µF
VCM
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.
DIFF AMP
1µF
REFL
INTERNAL ADC
LOW REFERENCE
229876 F09
Figure 9. Equivalent Reference Circuit
Driving the Clock Input
The CLK inputs 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 (Figure 11).
229876fa
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LTC2298/LTC2297/LTC2296
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APPLICATIO S I FOR ATIO
CLEAN
SUPPLY
4.7µF
SINUSOIDAL
CLOCK
INPUT
0.1µF
4.7µF
FERRITE
BEAD
FERRITE
BEAD
0.1µF
0.1µF
1k
CLK
50Ω
1k
CLEAN
SUPPLY
LTC2298
LTC2297
LTC2296
CLK
100Ω
NC7SVU04
LTC2298
LTC2297
LTC2296
229876 F11
Figure 11. Sinusoidal Single-Ended CLK Drive
The noise performance of the LTC2298/LTC2297/LTC2296
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.
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.
It is recommended that CLKA and CLKB are shorted
together and driven by the same clock source. If a small
time delay is desired between when the two channels
sample the analog inputs, CLKA and CLKB can be driven
by two different signals. If this delay exceeds 1ns, the
performance of the part may degrade. CLKA and CLKB
should not be driven by asynchronous signals.
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
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.
229876 F12
IF LVDS USE FIN1002 OR FIN1018.
FOR PECL, USE AZ1000ELT21 OR SIMILAR
Figure 12. CLK Drive Using an LVDS or PECL to CMOS Converter
ETC1-1T
CLK
5pF-30pF
DIFFERENTIAL
CLOCK
INPUT
LTC2298
LTC2297
LTC2296
229876 F13
0.1µF
FERRITE
BEAD
VCM
Figure 13. LVDS or PECL CLK Drive Using a Transformer
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.
229876fa
20
LTC2298/LTC2297/LTC2296
Maximum and Minimum Conversion Rates
The maximum conversion rate for the LTC2298/LTC2297/
LTC2296 is 65Msps (LTC2298), 40Msps (LTC2297), and
25Msps (LTC2296). 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 (LTC2298), 11.8ns
(LTC2297), and 18.9ns (LTC2296) 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 MODE pin controls both Channel A and
Channel B—the duty cycle stabilizer is either on of off for
both channels.
The lower limit of the LTC2298/LTC2297/LTC2296 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 LTC2298/LTC2297/
LTC2296 is 1Msps.
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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