LTC2245
14-Bit, 10Msps
Low Power 3V ADC
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FEATURES
DESCRIPTIO
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The LTC®2245 is a 14-bit 10Msps, low power 3V A/D
converter designed for digitizing high frequency, wide
dynamic range signals. The LTC2245 is perfect for demanding imaging and communications applications with
AC performance that includes 74.4dB SNR and 90dB
SFDR for signals well beyond the Nyquist frequency.
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Sample Rate: 10Msps
Single 3V Supply (2.7V to 3.4V)
Low Power: 60mW
74.4dB 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
Spectral Analysis
Portable Instrumentation
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TYPICAL APPLICATIO
Typical INL, 2V Range
2.0
REFL
FLEXIBLE
REFERENCE
1.5
OVDD
+
ANALOG
INPUT
INPUT
S/H
–
14-BIT
PIPELINED
ADC CORE
CORRECTION
LOGIC
D13
•
•
•
D0
OUTPUT
DRIVERS
OGND
1.0
INL ERROR (LSB)
REFH
0.5
0
–0.5
–1.0
–1.5
CLOCK/DUTY
CYCLE
CONTROL
–2.0
0
2245 TA01
4096
8192
CODE
12288
16384
2245 G01
CLK
2245fa
1
LTC2245
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ABSOLUTE
AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
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
LTC2245C ............................................... 0°C to 70°C
LTC2245I .............................................–40°C to 85°C
Storage Temperature Range ..................–65°C to 125°C
D11
D12
D13
OF
MODE
SENSE
VCM
VDD
TOP VIEW
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
D1
D0
OE
CLK
SHDN
9 10 11 12 13 14 15 16
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
QFN PART MARKING
2245*
ORDER PART NUMBER
LTC2245CUH
LTC2245IUH
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
TYP
MAX
UNITS
Bits
Integral
Linearity Error
Differential Analog Input
(Note 5)
●
–4
±1
4
LSB
Differential
Linearity Error
Differential Analog Input
●
–1
±0.5
1
LSB
Offset Error
(Note 6)
●
–12
±2
12
mV
Gain Error
External Reference
●
–2.5
±0.5
2.5
%FS
±10
µV/°C
Internal Reference
±30
ppm/°C
External Reference
±5
ppm/°C
SENSE = 1V
1
LSBRMS
Offset Drift
Full-Scale Drift
Transition Noise
2245fa
2
LTC2245
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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 – 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
1.5
1.5
V
1.9
2
V
V
–1
1
µA
–3
3
µA
–3
3
µA
0
ns
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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
MIN
TYP
SNR
Signal-to-Noise Ratio
5MHz Input
70MHz Input
●
72.3
74.4
73.2
dB
dB
SFDR
Spurious Free Dynamic Range
2nd or 3rd Harmonic
5MHz Input
70MHz Input
●
76
90
85
dB
dB
SFDR
Spurious Free Dynamic Range
4th Harmonic or Higher
5MHz Input
70MHz Input
●
84
95
95
dB
dB
S/(N+D)
Signal-to-Noise Plus Distortion Ratio
5MHz Input
70MHz Input
●
71.7
74.4
73.1
dB
dB
IMD
Intermodulation Distortion
fIN1 = 4.3MHz, fIN2 = 4.6MHz
90
dB
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I TER AL REFERE CE CHARACTERISTICS
MAX
UNITS
(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
2245fa
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LTC2245
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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)
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
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
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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)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VDD
Analog Supply Voltage
(Note 9)
●
2.7
3
3.4
V
OVDD
Output Supply Voltage
(Note 9)
IVDD
Supply Current
●
●
0.5
3
3.6
V
20
23
mA
PDISS
Power Dissipation
●
60
69
mW
PSHDN
Shutdown Power
SHDN = H, OE = H, No CLK
2
mW
PNAP
Nap Mode Power
SHDN = H, OE = L, No CLK
15
mW
2245fa
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LTC2245
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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
TYP
MAX
UNITS
fs
Sampling Frequency
(Note 9)
●
1
tL
CLK Low Time
Duty Cycle Stabilizer Off
Duty Cycle Stabilizer On
(Note 7)
●
●
40
5
tH
CLK High Time
Duty Cycle Stabilizer Off
Duty Cycle Stabilizer On
(Note 7)
●
●
40
5
tAP
Sample-and-Hold Aperture Delay
tD
CLK to DATA delay
CL = 5pF (Note 7)
●
2.7
5.4
ns
Data Access Time After OE↓
CL = 5pF (Note 7)
●
4.3
10
ns
BUS Relinquish Time
(Note 7)
●
3.3
8.5
ns
10
MHz
50
50
500
500
ns
ns
50
50
500
500
ns
ns
0
1.4
Pipeline
Latency
ns
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 = 10MHz, input range = 2VP-P with differential
drive, unless otherwise noted.
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 = 10MHz, input range = 1VP-P with
differential drive.
Note 9: Recommended operating conditions.
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TYPICAL PERFOR A CE CHARACTERISTICS
Typical INL, 2V Range
2.0
1.5
1.0
0
0.8
–10
–20
0.6
1.0
–30
0.5
0
–0.5
–1.0
0.4
AMPLITUDE (dB)
DNL ERROR (LSB)
INL ERROR (LSB)
8192 Point FFT, fIN = 5.1MHz,
–1dB, 2V Range
Typical DNL, 2V Range
0.2
0
–0.2
–0.4
–110
–1.0
0
4096
8192
CODE
12288
16384
2245 G01
–80
–100
–0.8
–2.0
–60
–70
–90
–0.6
–1.5
–40
–50
0
4096
8192
CODE
12288
16384
2245 G02
–120
0
1
3
2
FREQUENCY (MHz)
4
5
2245 G03
2245fa
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LTC2245
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TYPICAL PERFOR A CE CHARACTERISTICS
8192 Point 2-Tone FFT,
fIN = 4.3MHz and 4.6MHz,
–1dB, 2V Range
0
0
–10
–10
–20
–20
–30
–30
–60
–70
–80
–60
–70
–100
–100
–110
–110
–120
–120
3
2
FREQUENCY (MHz)
4
15000
13373
10000
5
6919
5000
3227
43
0
1
0
3
2
FREQUENCY (MHz)
4
853
278
8179 8180 8181 8182 8183 8184 8185 8186
CODE
5
2245 G06
2245 G05
2245 G04
SNR vs Input Frequency, –1dB,
2V Range
SFDR vs Input Frequency, –1dB,
2V Range
75
SNR and SFDR vs Sample Rate,
2V Range, fIN = 5MHz, –1dB
100
100
74
SFDR
95
SNR AND SFDR (dBFS)
73
90
SFDR (dBFS)
72
SNR (dBFS)
18803
–80
–90
1
20000
–50
–90
0
22016
COUNT
–40
–50
Grounded Input Histogram
25000
–40
AMPLITUDE (dB)
AMPLITUDE (dB)
8192 Point FFT, fIN = 70.1MHz,
–1dB, 2V Range
71
70
69
68
85
80
75
90
80
SNR
70
67
70
66
60
65
65
0
10
40
30
20
50
60
INPUT FREQUENCY (MHz)
70
0
10
40
60
30
50
20
INPUT FREQUENCY (MHz)
SNR vs Input Level, fIN = 5MHz,
2V Range
120
dBFS
110
10
12
4
6
8
SAMPLE RATE (Msps)
14
2225 G09
dBFS
100
60
SFDR (dBc AND dBFS)
SNR (dBc AND dBFS)
2
SFDR vs Input Level, fIN = 5MHz,
2V Range
70
50
40
dBc
30
20
90
80
dBc
70
60
50
40
100dBc SFDR
REFERENCE LINE
30
20
10
0
–70 –60
0
2245 G08
2245 G07
80
70
10
–50
–40 –30 –20
INPUT LEVEL (dBFS)
–10
0
2245 G10
0
–80
–40
–20
–60
INPUT LEVEL (dBFS)
0
2245 G11
2245fa
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LTC2245
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TYPICAL PERFOR A CE CHARACTERISTICS
IOVDD vs Sample Rate, 5MHz Sine
Wave Input, –1dB, OVDD = 1.8V
IVDD vs Sample Rate,
5MHz Sine Wave Input, –1dB
25
1.0
0.9
0.8
2V RANGE
0.7
IOVDD (mA)
IVDD (mA)
20
1V RANGE
15
0.6
0.5
0.4
0.3
0.2
0.1
0
10
0
2
4
6
8
10
SAMPLE RATE (Msps)
12
14
2245 G12
0
2
8
6
4
10
SAMPLE RATE (Msps)
12
14
2245 G13
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PI FU CTIO S
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.
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.
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.
OVDD (Pin 21): Positive Supply for the Output Drivers.
Bypass to ground with 0.1µF ceramic chip capacitor.
VDD (Pins 7, 32): 3V Supply. Bypass to GND with 0.1µF
ceramic chip capacitors.
OF (Pin 28): Over/Under Flow Output. High when an over
or under flow has occurred.
GND (Pin 8): ADC Power Ground.
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.
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
OGND (Pin 20): Output Driver Ground.
2245fa
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LTC2245
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PI FU CTIO S
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.
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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
DIFF
REF
AMP
REFH
0.1µF
CLOCK/DUTY
CYCLE
CONTROL
CONTROL
LOGIC
CLK
SHDN
OUTPUT
DRIVERS
•
•
•
D0
2245 F01
REFL
OGND
MODE
OE
2.2µF
1µF
1µF
Figure 1. Functional Block Diagram
2245fa
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LTC2245
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TI I G DIAGRA
tAP
N+4
N+2
N
ANALOG
INPUT
N+3
tH
N+5
N+1
tL
CLK
tD
N–4
N–5
D0-D13, OF
N–3
N–2
N–1
N
2245 TD01
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APPLICATIO S I FOR ATIO
DYNAMIC PERFORMANCE
Intermodulation Distortion
Signal-to-Noise Plus Distortion Ratio
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.
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.
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.
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.
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.
2245fa
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LTC2245
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APPLICATIO S I FOR ATIO
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)
CONVERTER OPERATION
As shown in Figure 1, the LTC2245 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 LTC2245 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
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 LTC2245
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.
LTC2245
VDD
CSAMPLE
4pF
15Ω
AIN+
CPARASITIC
1pF
VDD
AIN–
CSAMPLE
4pF
15Ω
CPARASITIC
1pF
VDD
CLK
2245 F02
Figure 2. Equivalent Input Circuit
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
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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 VCM or a low noise reference voltage between
0.5V and 1.5V.
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 LTC2245 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 sampleand-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 LTC2245 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
AIN+
LTC2245
0.1µF
12pF
25Ω
T1 = MA/COM ETC1-1T 25Ω
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
AIN–
2245 F03
Figure 3. Single-Ended to Differential Conversion
Using a Transformer
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VCM
HIGH SPEED
DIFFERENTIAL
25Ω
AMPLIFIER
ANALOG
INPUT
+
2.2µF
VCM
1.5V
AIN+
4Ω
1.5V BANDGAP
REFERENCE
2.2µF
LTC2245
0.5V
1V
+
CM
–
LTC2245
12pF
–
25Ω
RANGE
DETECT
AND
CONTROL
AIN–
2245 F04
Figure 4. Differential Drive with an Amplifier
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
VCM
1k
0.1µF
ANALOG
INPUT
1k
2.2µF
25Ω
AIN
2.2µF
0.1µF
+
LTC2245
DIFF AMP
1µF
REFL
12pF
25Ω
INTERNAL ADC
LOW REFERENCE
AIN–
0.1µF
2245 F06
2245 F05
Figure 6. Equivalent Reference Circuit
Figure 5. Single-Ended Drive
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.
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.
Reference Operation
Figure 6 shows the LTC2245 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.
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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 6.
Other voltage ranges in-between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 7. 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.
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.
Driving the Clock Input
The CLK input can be driven directly with a CMOS or TTL
level signal. A differential clock can also be used along with
a low-jitter CMOS converter before the CLK pin (see
Figure 8).
1.5V
The noise performance of the LTC2245 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.
Maximum and Minimum Conversion Rates
The maximum conversion rate for the LTC2245 is 10Msps.
For the ADC to operate properly, the CLK signal should
have a 50% (±10%) duty cycle. Each half cycle must have
at least 40ns 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 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 LTC2245 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 LTC2245 is 1Msps.
CLEAN
SUPPLY
VCM
4.7µF
2.2µF
12k
0.75V
SENSE
FERRITE
BEAD
LTC2245
0.1µF
12k
1µF
CLK
2245 F07
100Ω
LTC2245
Figure 7. 1.5V Range ADC
2245 F08
IF LVDS USE FIN1002 OR FIN1018.
FOR PECL, USE AZ1000ELT21 OR SIMILAR
Figure 8. CLK Drive Using an LVDS or PECL to CMOS Converter
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DIGITAL OUTPUTS
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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