LTC2321-14
Dual, 14-Bit + Sign, 2Msps
Differential Input ADC with Wide
Input Common Mode Range
Description
Features
2Msps Throughput Rate
nn ±1LSB INL (Typ)
nn Guaranteed 14-Bit, No Missing Codes
nn 8V
P-P Differential Inputs with Wide Input Common
Mode Range
nn 80dB SNR (Typ) at f = 500kHz
IN
nn –90dB THD (Typ) at f = 500kHz
IN
nn No-Cycle Latency
nn Guaranteed Operation to 125°C
nn Single 3.3V or 5V Supply
nn Low Drift (20ppm/°C Max) 2.048V or 4.096V Internal
Reference
nn 1.8V to 2.5V I/O Voltages
nn CMOS or LVDS SPI-Compatible Serial I/O
nn Power Dissipation 33mW/Ch (Typ)
nn Small 28-Lead (4mm × 5mm) QFN Package
The LTC®2321-14 is a low noise, high speed dual 14-bit +
sign successive approximation register (SAR) ADC with
differential inputs and wide input common mode range.
Operating from a single 3.3V or 5V supply, the LTC232114 has an 8VP-P differential input range, making it ideal
for applications which require a wide dynamic range with
high common mode rejection. The LTC2321-14 achieves
±1LSB INL typical, no missing codes at 14 bits and 80dB
SNR.
nn
The LTC2321-14 has an onboard low drift (20ppm/°C max)
2.048V or 4.096V temperature-compensated reference.
The LTC2321-14 also has a high speed SPI-compatible
serial interface that supports CMOS or LVDS. The fast
2Msps per channel throughput with no-cycle latency
makes the LTC2321-14 ideally suited for a wide variety of
high speed applications. The LTC2321-14 dissipates only
33mW per channel and offers nap and sleep modes to
reduce the power consumption to 5μW for further power
savings during inactive periods.
Applications
High Speed Data Acquisition Systems
Communications
nn Remote Data Acquisition
nn Imaging
nn Optical Networking
nn Automotive
nn Multiphase Motor Control
nn
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Analog
Devices, Inc. All other trademarks are the property of their respective owners.
nn
Typical Application
0
DIFFERENTIAL INPUTS
NO CONFIGURATION REQUIRED
10µF
0V
DIFFERENTIAL
VDD
25Ω
AIN1+
REFOUT1
VBYP1
LTC2321-14
REFOUT2
0V
220pF
VBYP2
BIPOLAR
0V
UNIPOLAR
0V
25Ω
AIN1–
AIN2+
AIN2–
VDD CMOS/LVDS
REFINT
GND
SDO1
SDO2
CLKOUT
SCK
CNV
OGND OVDD
10µF
AMPLITUDE (dBFS)
IN+, IN –
INSTRUMENTATION
32k Point FFT fS = 2Msps, fIN = 500kHz
3.3V OR 5V
1µF
10µF
1µF
TO CONTROL
LOGIC
(FPGA, CPLD,
DSP, ETC.)
1.8V TO 2.5V
1µF
SNR = 80.2dB
THD = –92.5dB
–20 SINAD = 79.9dB
SFDR = 98.6dB
–40
–60
–80
–100
–120
–140
0
232114 TA01a
0.2
0.4
0.6
FREQUENCY (MHz)
0.8
1.0
232114 TA01b
232114fc
For more information www.linear.com/LTC2321-14
1
LTC2321-14
Absolute Maximum Ratings
Pin Configuration
(Notes 1, 2)
Order Information
OGND
VBYP2
CMOS/LVDS
REFOUT2
REFRTN2
REFINT
TOP VIEW
28 27 26 25 24 23
VDD 1
22 SCK –
AIN2+ 2
21 SCK+
20 SDO2 –
AIN2 – 3
29
GND
GND 4
GND 5
19 SDO2+
18 CLKOUT –
17 CLKOUT+
AIN1 – 6
AIN1+ 7
16 SDO1 –
VDD 8
15 SDO1+
OVDD
VBYP1
REFOUT1
REFRTN1
CNV
9 10 11 12 13 14
GND
Supply Voltage (VDD)...................................................6V
Supply Voltage (OVDD).................................................3V
Supply Bypass Voltage (VBYP1, VBYP2)........................3V
Analog Input Voltage
AIN+, AIN – (Note 3).................... –0.3V to (VDD + 0.3V)
REFOUT1,2.............................. .–0.3V to (VDD + 0.3V)
CNV (Note 15)........................... –0.3V to (VDD + 0.3V)
Digital Input Voltage
(Note 3)........................... (GND – 0.3V) to (OVDD + 0.3V)
Digital Output Voltage..... (GND – 0.3V) to (OVDD + 0.3V)
Power Dissipation................................................200mW
Operating Temperature Range
LTC2321C................................................. 0°C to 70°C
LTC2321I..............................................–40°C to 85°C
LTC2321H........................................... –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFN
TJMAX = 125°C, θJA = 43°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
http://www.linear.com/product/LTC2321-14#orderinfo
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC2321CUFD-14#PBF
LTC2321CUFD-14#TRPBF
23214
28-Lead (4mm × 5mm) Plastic QFN
0°C to 70°C
LTC2321IUFD-14#PBF
LTC2321IUFD-14#TRPBF
23214
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 85°C
LTC2321HUFD-14#PBF
LTC2321HUFD-14#TRPBF
23214
28-Lead (4mm × 5mm) Plastic QFN
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C (Note 4).
SYMBOL
PARAMETER
CONDITIONS
VIN+
Absolute Input Range (AIN1+, AIN2+)
(Note 5)
l
VIN–
Absolute Input Range (AIN1–, AIN2–)
(Note 5)
l
0
VDD
V
VIN+ – VIN–
Input Differential Voltage Range
VIN = VIN+ – VIN–
l
–REFOUT1,2
REFOUT1,2
V
VCM
Common Mode Input Range
VIN = (VIN+ + VIN–)/2
l
0
VDD
V
IIN
Analog Input DC Leakage Current
l
–1
1
µA
CIN
Analog Input Capacitance
CMRR
Input Common Mode Rejection Ratio
fIN = 500kHz
85
dB
IREFOUT
External Reference Current
REFINT = 0V, REFOUT = 4.096V
310
µA
2
MIN
TYP
0
10
MAX
UNITS
VDD
V
pF
232114fc
For more information www.linear.com/LTC2321-14
LTC2321-14
Converter Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C (Notes 4, 16).
SYMBOL PARAMETER
CONDITIONS
MIN
MAX
UNITS
Resolution
14
Bits
No Missing Codes
l
14
Bits
l
–4
±1
4
LSB
l
–0.99
±0.4
0.99
LSB
l
–4
0
4
Transition Noise
INL
Integral Linearity Error
DNL
Differential Linearity Error
BZE
Bipolar Zero-Scale Error
0.5
(Note 6)
(Note 7)
Bipolar Zero-Scale Error Drift
FSE
TYP
l
LSBRMS
0.006
Bipolar Full-Scale Error
VREFOUT1,2 = 4.096V (REFINT Grounded) (Note 7)
Bipolar Full-Scale Error Drift
VREFOUT1,2 = 4.096V (REFINT Grounded)
l
–25
±3
LSB
LSB/°C
25
15
LSB
ppm/°C
Dynamic Accuracy
The l denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C and AIN = –1dBFS (Notes 4, 8).
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
80
dB
80
dB
80
dB
80.7
dB
SINAD
Signal-to-(Noise + Distortion) Ratio fIN = 500kHz, VREFOUT1,2 = 4.096V, Internal Reference
fIN = 500kHz, VREFOUT1,2 = 5V, External Reference
l
72.5
SNR
Signal-to-Noise Ratio
fIN = 500kHz, VREFOUT1,2 = 4.096V, Internal Reference
l
73
fIN = 500kHz, VREFOUT1,2 = 5V, External Reference
THD
Total Harmonic Distortion
fIN = 500kHz, VREFOUT1,2 = 4.096V, Internal Reference
–90
l
fIN = 500kHz, VREFOUT1,2 = 5V, External Reference
SFDR
Spurious Free Dynamic Range
fIN = 500kHz, VREFOUT1,2 = 4.096V, Internal Reference
l
80
fIN = 500kHz, VREFOUT1,2 = 5V, External Reference
MAX
UNITS
–80
dB
–84
dB
88
dB
88
dB
–3dB Input Linear Bandwidth
10
MHz
Aperture Delay
500
ps
Aperture Delay Matching
500
ps
Aperture Jitter
Transient Response
Full-Scale Step
1
psRMS
3
ns
Internal Reference Characteristics
The l denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C (Note 4).
SYMBOL
PARAMETER
CONDITIONS
VREFOUT1,2
Internal Reference Output Voltage
4.75V < VDD < 5.25V
3.13V < VDD < 3.47V
l
l
VREFOUT1,2 Temperature Coefficient
(Note 14)
l
REFOUT1,2 Output Impedance
VREFOUT1,2 Line Regulation
VDD = 4.75V to 5.25V
MIN
TYP
MAX
UNITS
4.088
2.044
4.096
2.048
4.106
2.053
V
3
20
ppm/°C
0.25
Ω
0.3
mV/V
232114fc
For more information www.linear.com/LTC2321-14
3
LTC2321-14
Digital Inputs And Digital Outputs
The l denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C (Note 4).
SYMBOL
PARAMETER
VIH
High Level Input Voltage
CONDITIONS
l
MIN
VIL
Low Level Input Voltage
l
IIN
Digital Input Current
CIN
Digital Input Capacitance
VIN = 0V to OVDD
VOH
High Level Output Voltage
IO = -500µA
l
VOL
Low Level Output Voltage
IO = 500µA
l
IOZ
Hi-Z Output Leakage Current
VOUT = 0V to OVDD
l
l
TYP
MAX
UNITS
0.8 • OVDD
V
–10
0.2 • OVDD
V
10
μA
5
pF
OVDD – 0.2
V
–10
ISOURCE
Output Source Current
VOUT = 0V
ISINK
Output Sink Current
VOUT = OVDD
VID
LVDS Differential Input Voltage
100Ω Differential Termination OVDD = 2.5V
l
1
0.2
V
10
µA
–10
mA
10
mA
240
600
mV
VIS
LVDS Common Mode Input Voltage
100Ω Differential Termination OVDD = 2.5V
l
1.45
V
VOD
LVDS Differential Output Voltage
100Ω Differential Load, LVDS Mode
OVDD = 2.5V
l
100
150
300
mV
VOS
LVDS Common Mode Output Voltage
100Ω Differential Load, LVDS Mode
OVDD = 2.5V
l
0.85
1.2
1.4
V
VOD_LP
Low Power LVDS Differential Output
Voltage
100Ω Differential Load, LVDS Mode
OVDD = 2.5V
l
75
100
250
mV
VOS_LP
Low Power LVDS Common Mode
Output Voltage
100Ω Differential Load, LVDS Mode
OVDD = 2.5V
l
0.9
1.2
1.4
V
Power Requirements
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C (Note 4).
SYMBOL PARAMETER
VDD
Supply Voltage
OVDD
Supply Voltage
CONDITIONS
MIN
5V Operation
3.3V Operation
IVDD
Supply Current
2Msps Sample Rate (IN+ = IN– = 0V)
IOVDD
Supply Current
2Msps Sample Rate (CL = 5pF)
2Msps Sample Rate (RL = 100Ω)
INAP
Nap Mode Current
Conversion Done (IVDD)
ISLEEP
Sleep Mode Current
Sleep Mode (IVDD + IOVDD)
Sleep Mode (IVDD + IOVDD)
CMOS Mode
LVDS Mode
PD_3.3V
Power Dissipation
PD_5V
4
MAX
UNITS
l
l
4.75
3.13
TYP
5.25
3.47
V
V
l
1.71
2.63
V
l
11.8
15
mA
l
l
1.8
7.1
2
11
mA
mA
l
2.55
5
mA
l
l
1
1
5
5
μA
μA
VDD = 3.3V 2Msps Sample Rate (IN+ = IN– = 0V) CMOS Mode
VDD = 3.3V 2Msps Sample Rate (IN+ = IN– = 0V) LVDS Mode
l
l
37
52
58
86
mW
mW
Nap Mode
VDD = 3.3V Conversion Done (IVDD + IOVDD)
VDD = 3.3V Conversion Done (IVDD + IOVDD)
CMOS Mode
LVDS Mode
l
l
7.8
26
13
41
mW
mW
Sleep Mode
VDD = 3.3V Sleep Mode (IVDD + IOVDD)
VDD = 3.3V Sleep Mode (IVDD + IOVDD)
CMOS Mode
LVDS Mode
l
l
5
5
16.5
16.5
μW
μW
Power Dissipation
VDD = 5V 2Msps Sample Rate (IN+ = IN– = 0V) CMOS Mode
VDD = 5V 2Msps Sample Rate (IN+ = IN– = 0V) LVDS Mode
l
l
66
77
80
102.5
mW
mW
Nap Mode
VDD = 5V Conversion Done (IVDD + IOVDD)
VDD = 5V Conversion Done (IVDD + IOVDD)
CMOS Mode
LVDS Mode
l
l
13
31
25
40
mW
mW
Sleep Mode
VDD = 5V Sleep Mode (IVDD + IOVDD)
VDD = 5V Sleep Mode (IVDD + IOVDD)
CMOS Mode
LVDS Mode
l
l
5
5
25
25
μW
μW
CMOS Mode
LVDS Mode
232114fc
For more information www.linear.com/LTC2321-14
LTC2321-14
ADC Timing Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C (Note 4).
SYMBOL
PARAMETER
CONDITIONS
fSMPL
Maximum Sampling Frequency
tCYC
Time Between Conversions
tCONV
Conversion Time
MIN
TYP
l
(Note 11) tCYC = tCNVH + tCONV + tREADOUT
MAX
UNITS
2
Msps
l
500
l
220
1000000
ns
ns
tCNVH
CNV High Time
l
25
ns
tDSCKHCNVH
SCK Delay Time to CNV↑
(Note 11)
l
0
ns
tSCK
SCK Period
(Notes 12, 13)
l
15.6
ns
tSCKH
SCK High Time
l
7
ns
tSCKL
SCK Low Time
l
7
ns
2.8
tDSCKCLKOUT
SCK to CLKOUT Delay
(Note 12)
l
tDCLKOUTSDOV
SDO Data Valid Delay from
CLKOUT↓
CL = 5pF (Note 12)
l
2
ns
ns
tHSDO
SDO Data Remains Valid Delay
from CLKOUT↓
CL = 5pF (Note 11)
l
2
ns
tDCNVSDOV
SDO Data Valid Delay from CNV↓ CL = 5pF (Note 11)
l
tDCNVSDOZ
Bus Relinquish Time After CNV↑ (Note 11)
l
tWAKE
REFOUT1,2 Wakeup Time
2.5
CREFOUT1,2 = 10μF
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.
Note 3: When these pin voltages are taken below ground, or above VDD
or OVDD, they will be clamped by internal diodes. This product can handle
input currents up to 100mA below ground, or above VDD or OVDD, without
latch-up.
Note 4: VDD = 5V, OVDD = 2.5V, REFOUT1,2 = 4.096V, fSMPL = 2MHz.
Note 5: Recommended operating conditions.
Note 6: 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 7: Bipolar zero error is the offset voltage measured from –0.5LSB
when the output code flickers between 0000 0000 0000 000 and 1111
1111 1111 111. Full-scale bipolar error is the worst-case of –FS or +FS
10
3
ns
3
ns
ms
untrimmed deviation from ideal first and last code transitions and includes
the effect of offset error.
Note 8: All specifications in dB are referred to a full-scale ±4.096V input
with REFOUT = 4.096V.
Note 9: When REFOUT1,2 is overdriven, the internal reference buffer must
be turned off by setting REFINT = 0V.
Note 10: fSMPL = 2MHz, IREFBUF varies proportionally with sample rate.
Note 11: Guaranteed by design, not subject to test.
Note 12: Parameter tested and guaranteed at OVDD = 1.71V and
OVDD = 2.5V.
Note 13: tSCK of 15.6ns maximum allows a shift clock frequency up to
64MHz for rising edge capture.
Note 14: Temperature coefficient is calculated by dividing the maximum
change in output voltage by the specified temperature range.
Note 15: CNV is driven from a low jitter digital source, typically at OVDD
logic levels. This input pin has a TTL style input that will draw a small
amount of current.
Note 16: 1LSB = 2 • REFOUT1, 2/214
0.8 • OVDD
tWIDTH
0.2 • OVDD
tDELAY
tDELAY
0.8 • OVDD
0.8 • OVDD
0.2 • OVDD
0.2 • OVDD
50%
50%
232114 F01
Figure 1. Voltage Levels for Timing Specifications
232114fc
For more information www.linear.com/LTC2321-14
5
LTC2321-14
Typical
Performance Characteristics A = 25°C, VDD = 5V, OVDD = 2.5V, REFOUT1,2 =
T
4.096V, fSMPL = 2Msps, unless otherwise noted. (Note 16)
Integral Nonlinearity
vs Output Code
Differential Nonlinearity
vs Output Code
DC Histogram
1.0
3
35000
30000
2
0.5
0
–1
25000
COUNTS
DNL ERROR (LSB)
INL ERROR (LSB)
1
0
20000
15000
10000
–0.5
–2
5000
–8192
8192
0
OUTPUT CODE
–100
0.4
0.8
0.6
FREQUENCY (MHz)
SNR
SINAD
79.5
79.0
0.5
FREQUENCY (MHz)
1
–100
–105
–110
THD
–95
H2
–100
–105
1.9
2.1
2.3
2.5
2.7
2.9
INPUT COMMON MODE (V)
3.1
3.3
232114 G07
0.5
FREQUENCY (MHz)
76
74
72
1
232114 G06
F1 = 100kHz
F2 = 500kHz
IMD = –97dBc
–20
SINAD
78
68
0.5
0
32k Point FFT, IMD, fS = 2Msps,
VIN+ = 100kHz, VIN– = 500kHz
SNR
70
H3
3
232114 G03
H3
232114 G05
AMPLITUDE (dBFS)
SNR, SINAD (dBFS)
–90
2
H2
0
80
–85
1
THD
–95
82
–80
0
CODE
–90
SNR and SINAD vs VREF at 500kHz
–75
–110
1.7
0
232114 G04
THD, Harmonics vs Input
Common Mode (500kHz)
–1
–85
80.0
78.0
1.0
–2
THD, Harmonics
vs Input Frequency
78.5
0.2
–3
232114 G02
80.5
–80
0
0
16384
81.0
–120
THD, HARMONICS (dBFS)
8192
SNR, SINAD vs Input Frequency
(100kHz to 1MHz)
–60
6
0
OUTPUT CODE
32k Point FFT, fS = 2Msps,
fIN = 500kHz
SNR = 80.2dB
THD = –92.5dB
–20 SINAD = 79.9dB
SFDR = 98.6dB
–40
–140
–8192
232114 G01
SNR, SINAD LEVEL (dBFS)
AMPLITUDE (dBFS)
0
–1.0
–16384
16384
THD, HARMONICS (dBFS)
–3
–16384
–40
–60
–80
–100
–120
1
1.5
2
2.5
3
VREF (V)
3.5
4
4.5
5
232114 G08
–140
0
0.2
0.4
0.6
FREQUENCY (MHz)
0.8
1.0
232114 G09
232114fc
For more information www.linear.com/LTC2321-14
LTC2321-14
Typical
Performance Characteristics A = 25°C, VDD = 5V, OVDD = 2.5V, REFOUT1,2 =
T
4.096V, fSMPL = 2Msps, unless otherwise noted. (Note 16)
Crosstalk vs Input Frequency
CMRR (dB)
–132
–89
20000
–92
15000
OUTPUT CODE (CH1, CH2)
–130
CROSSTALK (dBc)
Output Match with Simultaneous
Input Steps at CH1 and CH2
CMRR vs Input Frequency
–95
–98
–134
–101
–136
0
0.2
0.4
0.6
FREQUENCY (MHz)
0.8
–104
1.0
0
0.5
FREQUENCY (MHz)
232114 G10
Offset Error vs Temperature
1.0
0
–5000
0
CH1
–0.2
–0.4
0
25
50
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.5
–50
75
100 125 150
TEMPERATURE (°C)
–25
0
25
50
75
TEMPERATURE (°C)
232114 G13
Reference Current vs Temperature,
VREF = 4.096V
300
400
500
232114 G12
REFOUT1,2 Output vs Temperature
–0.4
–0.6
–50 –25
200
TIME (ns)
REFOUT (ppm, NORMALIZED TO 20°C)
GAIN ERROR (14-BIT LSB)
0.2
100
200
0.4
CH2
0
232114 G11
0.5
0.4
LSB
5000
Gain Error vs Temperature
0.6
0.350
10000
100
125
100
0
4.096V
–100
–200
2.048V
–300
–400
–500
–50
232114 G14
Supply Current
vs Sample Frequency
8
12.0
0
50
100
TEMPERATURE (°C)
150
232114 G15
OVDD Current vs SCK Frequency,
CLOAD = 10pF
0.345
0.340
11.0
OVDD CURRENT (mA)
SUPPLY CURRENT (mA)
REFERENCE CURRENT (mA)
11.5
10.5
10.0
9.5
9.0
6
4
2
8.5
0.335
–40 –20
0
20
40
60
TEMPERATURE (°C)
80
100 120
232114 G16
8.0
0
0.5
1.0
1.5
SAMPLE RATE (Msps)
2.0
232114 G17
0
0 10 20 30 40 50 60 70 80 90 100 110
SCK FREQUENCY (MHz)
232114 G18
232114fc
For more information www.linear.com/LTC2321-14
7
LTC2321-14
Pin Functions
VDD (Pins 1, 8): Power Supply. Bypass VDD to GND with
a 10µF ceramic and a 0.1µF ceramic close to the part. The
VDD pins should be shorted together and driven from the
same supply.
AIN2+, AIN2– (Pins 2, 3): Analog Differential Input Pins.
Full-scale range (AIN2+ – AIN2–) is ±REFOUT2 voltage.
These pins can be driven from VDD to GND.
GND (Pins 4, 5, 10, 29): Ground. These pins and exposed
pad (Pin 29) must be tied directly to a solid ground plane.
AIN1–, AIN1+ (Pins 6, 7): Analog Differential Input Pins.
Full-scale range (AIN1+ – AIN1–) is ±REFOUT1 voltage.
These pins can be driven from VDD to GND.
CNV (Pin 9): Conversion Start Input. A falling edge on
CNV puts the internal sample-and-hold into the hold mode
and starts a conversion cycle. CNV must be driven by a
low jitter clock as shown in the Typical Application circuit
on the back page. The CNV pin is unaffected by the CMOS/
LVDS pin.
REFRTN1 (Pin 11): Reference Buffer 1 Output Return.
Bypass REFRTN1 to REFOUT1. Do not tie the REFRTN1
pin to the ground plane.
REFOUT1 (Pin 12): Reference Buffer 1 Output. An onboard
buffer nominally outputs 4.096V to this pin. This pin is
referred to REFRTN1 and should be decoupled closely to
the pin (no vias) with a 0.1µF (X7R, 0402 size) capacitor
and a 10μF (X5R, 0805 size) ceramic capacitor in parallel. The internal buffer driving this pin may be disabled
by grounding the REFINT pin. If the buffer is disabled,
an external reference may drive this pin in the range of
1.25V to 5V.
VBYP1 (Pin 13): Bypass this internally supplied pin to
ground with a 1µF ceramic capacitor. The nominal output
voltage on this pin is 1.6V.
OVDD (Pin 14): I/O Interface Digital Power. The range of
OVDD is 1.71V to 2.5V. This supply is nominally set to
the same supply as the host interface (CMOS: 1.8V or
2.5V, LVDS: 2.5V). Bypass OVDD to OGND with a 0.1μF
capacitor.
SDO1+,
SDO1–
(Pins 15, 16): Channel 1 Serial Data
Output. The conversion result is shifted MSB first on each
falling edge of SCK. In CMOS mode, the result is output
8
on SDO1+. The logic level is determined by OVDD. Do
not connect SDO1–. In LVDS mode, the result is output
differentially on SDO1+ and SDO1–. These pins must be
differentially terminated by an external 100Ω resistor at
the receiver (FPGA).
CLKOUT+, CLKOUT– (Pins 17, 18): Serial Data Clock
Output. CLKOUT provides a skew-matched clock to latch
the SDO output at the receiver. In CMOS mode, the skewmatched clock is output on CLKOUT+. The logic level is
determined by OVDD. Do not connect CLKOUT–. For low
throughput applications using SCK to latch the SDO output, CLKOUT+ can be disabled by tying CLKOUT– to OVDD.
In LVDS mode, the skew-matched clock is output differentially on CLKOUT+ and CLKOUT–. These pins must be
differentially terminated by an external 100Ω resistor at
the receiver (FPGA).
SDO2+, SDO2– (Pins 19, 20): Channel 2 Serial Data
Output. The conversion result is shifted MSB first on each
falling edge of SCK. In CMOS mode, the result is output
on SDO2+. The logic level is determined by OVDD. Do
not connect SDO2–. In LVDS mode, the result is output
differentially on SDO2+ and SDO2–. These pins must be
differentially terminated by an external 100Ω resistor at
the receiver (FPGA).
SCK+, SCK– (Pins 21, 22): Serial Data Clock Input. The
falling edge of this clock shifts the conversion result MSB
first onto the SDO pins. In CMOS mode, drive SCK+ with
a single-ended clock. The logic level is determined by
OVDD. Do not connect SCK–. In LVDS mode, drive SCK+
and SCK–. with a differential clock. These pins must be
differentially terminated by an external 100Ω resistor at
the receiver (ADC).
OGND (Pin 23): I/O Ground. This ground must be tied to
the ground plane at a single point. OVDD is bypassed to
this pin.
VBYP2 (Pin 24): Bypass this internally supplied pin to
ground with a 1µF ceramic capacitor. The nominal output
voltage on this pin is 1.6V
CMOS/LVDS (Pin 25): I/O Mode Select. Ground this pin
to enable CMOS mode, tie to OVDD to enable LVDS mode.
Float this pin to enable low power LVDS mode.
For more information www.linear.com/LTC2321-14
232114fc
LTC2321-14
Pin Functions
REFOUT2 (Pin 26): Reference Buffer 2 Output. An onboard
buffer nominally outputs 4.096V to this pin. This pin is
referred to REFRTN2 and should be decoupled closely to
the pin (no vias) with a 0.1µF (X7R, 0402 size) capacitor
and a 10μF (X5R, 0805 size) ceramic capacitor in parallel. The internal buffer driving this pin may be disabled
by grounding the REFINT pin. If the buffer is disabled,
an external reference may drive this pin in the range of
1.25V to VDD.
REFRTN2 (Pin 27): Reference Buffer 2 Output Return.
Bypass REFRTN2 to REFOUT2. Do not tie the REFRTN2
pin to the ground plane.
REFINT (Pin 28): Reference Buffer Output Enable. Tie to
VDD when using the internal reference. Tie to ground to
disable the internal REFOUT1 and REFOUT2 buffers for
use with external voltage references. This pin has a 500k
internal pull-up to VDD.
Exposed Pad (Pin 29): Ground. Solder this pad to ground.
Functional Block Diagram
VDD
1,8
7
6
AIN1+
AIN1–
VBYP1 13
LDO
+
14-BIT + SIGN
SAR ADC
S/H
–
28 REFINT
REFOUT1
12
LVDS/CMOS
TRI-STATE
SERIAL OUTPUT
1.2V REF
26
9
G
CNV
3
AIN2+
AIN2–
VDD
1,8
15
16
OVDD 14
TIMING CONTROL
LOGIC
OUTPUT
CLOCK DRIVER
LVDS/CMOS
RECEIVERS
2
SDO1–
GND
4, 5, 10, 29
G
REFOUT2
SDO1+
+
14-BIT + SIGN
SAR ADC
S/H
–
LVDS/CMOS
TRI-STATE
SERIAL OUTPUT
LDO
CLKOUT+
CLKOUT–
SCK+
SCK –
SDO2+
SDO2 –
17
18
21
22
19
20
VBYP2 24
232114 BD
232114fc
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9
LTC2321-14
Timing Diagram
ACQUISITION
CONVERSION
READOUT
CNV
1
SCK
HI-Z
B14
SDO
1
CLKOUT
2
3
B13
2
4
B12
3
5
B11
4
6
13
B10
5
14
B2
6
SERIAL DATA BITS B[14:0] CORRESPOND TO CURRENT CONVERSION
13
15
B1
14
16
B0
15
0
HI-Z
16
232114 TD
Applications Information
OVERVIEW
CONVERTER OPERATION
The LTC2321-14 is a low noise, high speed 14-bit + sign
dual successive approximation register (SAR) ADC with
differential inputs and wide input common mode range.
The flexible analog inputs support fully differential,
pseudo-differential bipolar and pseudo-differential unipolar drive without requiring any hardware configuration.
The MSB of the 14-bit + sign two’s complement output
indicates the sign of the differential analog input voltage.
The LTC2321-14 operates in two phases. During the
acquisition phase, the sample capacitor is connected
to the analog input pins AIN+ and AIN – to sample the
differential analog input voltage, as shown in Figure
3. A falling edge on the CNV pin initiates a conversion. During the conversion phase, the 15-bit CDAC is
sequenced through a successive approximation algorithm, effectively comparing the sampled input with
binary-weighted fractions of the reference voltage
(e.g., VREFOUT/2, VREFOUT/4 … VREFOUT/16384) using
the differential comparator. At the end of conversion, the
CDAC output approximates the sampled analog input. The
ADC control logic then prepares the 15-bit digital output
code for serial transfer . The data is clocked out on each
falling edge of the SCK+ input clock.
The ADC’s transfer function provides 15-bits of resolution across the full-scale span of 2 • REFOUT as shown in
Figure 2. If the analog input spans less than this full-scale,
such as in the case of pseudo-differential drive, the ADC
provides 14-bits of resolution across this reduced span,
with the additional benefit of digitizing over- and underrange conditions, as shown in Table 1. This unique feature
is particularly useful in control loop applications.
10
232114fc
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LTC2321-14
Applications Information
The LTC2321-14 digitizes the full-scale voltage of 2 •
REFOUT into 215 levels, resulting in a 15-bit resolution
size of 250µV with REFBUF = 4.096V. The ideal transfer
function is shown in Figure 2. The output data is in 2’s
complement format. When driven by fully differential
inputs, the transfer function spans 215 codes. When
driven by pseudo-differential inputs, the transfer function
spans 214 codes.
OUTPUT CODE (TWO’S COMPLEMENT)
TRANSFER FUNCTION
011 1111 1111 1111
011 1111 1111 1110
000 0000 0000 0001
000 0000 0000 0000
111 1111 1111 1111
1LSB = 2 • REFOUT
32768
100 0000 0000 0001
100 0000 0000 0000
–REFOUT/2
Analog Input
–1 0 1
LSB
LSB
REFOUT/2
–1LSB
INPUT VOLTAGE (V)
The differential inputs of the LTC2321-14 provide great
flexibility to convert a wide variety of analog signals with
no configuration required. The LTC2321-14 digitizes the
difference voltage between the AIN+ and AIN – pins while
supporting a wide common mode input range. The analog
input signals can have an arbitrary relationship to each
other, provided that they remain between VDD and GND.
The LTC2321-14 can also digitize more limited classes
of analog input signals such as pseudo-differential unipolar/bipolar and fully differential with no configuration
required.
The analog inputs of the LTC2321-14 can be modeled
by the equivalent circuit shown in Figure 3. The backto-back diodes at the inputs form clamps that provide
ESD protection. In the acquisition phase, 10pF (CIN) from
the sampling capacitor in series with approximately 15Ω
(RON) from the on-resistance of the sampling switch is
connected to the input. Any unwanted signal that is common to both inputs will be reduced by the common mode
rejection of the ADC sampler. The inputs of the ADC core
draw a small current spike while charging the CIN capacitors during acquisition.
232114 F02
Figure 2. LTC2321-14 Transfer Function
VDD
RON
15Ω
AIN1+
CIN
10pF
BIAS
VOLTAGE
VDD
AIN1–
RON
15Ω
CIN
10pF
232114 F03
Figure 3. The Equivalent Circuit for the Differential
Analog Input of the LTC2321-14
Table 1. Code Ranges for the Analog Input Operational Modes
MODE
SPAN (VIN+ – VIN–)
MIN CODE
MAX CODE
Fully Differential
–REFOUT to +REFOUT
100 0000 0000 0000
011 1111 1111 1111
Pseudo-Differential Bipolar
–-REFOUT/2 to +REFOUT/2
110 0000 0000 0000
001 1111 1111 1111
Pseudo-Differential Unipolar
0 to REFOUT
000 0000 0000 0000
011 1111 1111 1111
232114fc
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11
LTC2321-14
Applications Information
VREF
LT1819
+
–
0V
VREF
0V
LTC2321-14
25Ω
AIN1+
REFOUT1
10µF
VREF
VBYP1
220pF
10k
VREF /2
10k
1µF
+
–
VREF /2
25Ω
AIN1–
1µF
TO CONTROL
LOGIC
(FPGA, CPLD,
DSP, ETC.)
SDO1
CLKOUT
SCK
ONLY CHANNEL 1 SHOWN FOR CLARITY
232114 F04
Figure 4. Pseudo-Differential Bipolar Application Circuit
Single-Ended Signals
Single-ended signals can be directly digitized by the
LTC2321-14. These signals should be sensed pseudodifferentially for improved common mode rejection. By
connecting the reference signal (e.g., ground sense) of
the main analog signal to the other AIN pin, any noise or
disturbance common to the two signals will be rejected
by the high CMRR of the ADC. The LTC2321-14 flexibility handles both pseudo-differential unipolar and bipolar
signals, with no configuration required. The wide common mode input range relaxes the accuracy requirements of any signal conditioning circuits prior to the
analog inputs.
Pseudo-Differential Bipolar Input Range
The pseudo-differential bipolar configuration represents
driving one of the analog inputs at a fixed voltage, typically VREF /2, and applying a signal to the other AIN pin. In
this case the analog input swings symmetrically around
the fixed input yielding bipolar two’s complement output codes with an ADC span of half of full-scale. This
12
configuration is illustrated in Figure 4, and the corresponding transfer function in Figure 5. The fixed analog
input pin need not be set at VREF /2, but at some point
within the VDD rails allowing the alternate input to swing
symmetrically around this voltage. If the input signal (AIN+
– AIN –) swings beyond ±REFOUT/2, valid codes will be
generated by the ADC and must be clamped by the user,
if necessary.
ADC CODE
(2’s COMPLEMENT)
16383
8191
–VREF
–VREF /2
–8192
–16384
0
VREF /2
VREF
AIN
(AIN+ – AIN–)
DOTTED REGIONS AVAILABLE
BUT UNUSED
232114 F05
Figure 5. Pseudo-Differential Bipolar Transfer Function
232114fc
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LTC2321-14
Applications Information
Pseudo-Differential Unipolar Input Range
complement output codes with an ADC span of half of fullscale. This configuration is illustrated in Figure 6, and the
corresponding transfer function in Figure 7. If the input
signal (AIN+ – AIN –) swings negative, valid codes will be
generated by the ADC and must be clamped by the user,
if necessary.
The pseudo-differential unipolar configuration represents
driving one of the analog inputs at ground and applying a
signal to the other AIN pin. In this case, the analog input
swings between ground and VREF yielding unipolar two’s
VREF
0V
LT1818
VREF
+
–
0V
LTC2321-14
25Ω
AIN1+
VBYP1
220pF
25Ω
REFOUT1
AIN1–
SDO1
CLKOUT
SCK
10µF
1µF
TO CONTROL
LOGIC
(FPGA, CPLD,
DSP, ETC.)
232114 F06
Figure 6. Pseudo-Differential Unipolar Application Circuit
ADC CODE
(2’s COMPLEMENT)
16383
8191
–VREF
–VREF /2
–8192
–16384
0
VREF /2
VREF
AIN
(AIN+ – AIN–)
DOTTED REGIONS AVAILABLE
BUT UNUSED
232114 F07
Figure 7. Pseudo-Differential Unipolar Transfer Function
232114fc
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13
LTC2321-14
Applications Information
Single-Ended-to-Differential Conversion
While single-ended signals can be directly digitized as
previously discussed, single-ended to differential conversion circuits may also be used when higher dynamic range
is desired. By producing a differential signal at the inputs
of the LTC2321-14, the signal swing presented to the ADC
is maximized, thus increasing the achievable SNR.
The LT®1819 high speed dual operational amplifier is
recommended for performing single-ended-to-differential conversions, as shown in Figure 8. In this case, the
first amplifier is configured as a unity-gain buffer and the
single-ended input signal directly drives the high impedance input of this amplifier.
Fully-Differential Inputs
To achieve the full distortion performance of the LTC2321-14,
a low distortion fully-differential signal source driven
through the LT1819 configured as two unity-gain buffers, as shown in Figure 9, can be used. This circuit
achieves the full data sheet THD specification of –85dB
at input frequencies of 500kHz and less. Data sheet typical
VREF
0V
200Ω
VREF /2
+
–
VREF
+
–
VREF
0V
0V
VREF
0V
0V
LT1819
+
–
VREF
+
–
VREF
0V
0V
232114 F09
232114 F08
Figure 8. Single-Ended to Differential Driver
14
The fully-differential configuration yields an analog input
span (AIN+ – AIN –) of ±REFOUT1,2. In this configuration,
the input signal is driven on each AIN pin, typically at
equal spans but opposite polarity. This yields a high common mode rejection on the input signals. The common
mode voltage of the analog input can be anywhere within
the VDD input range, but will be limited by the peak swing
of the full-range input signal. For example, if the internal
reference is used with VDD = 5VDC, the full-range input
span will be ±4.096V. Half of the input span is typically
driven on each AIN pin, yielding a signal span for each
AIN pin of 4.096VP-P. This leaves ~0.9V of common mode
variation tolerance. When using external references, it is
possible to increase common mode tolerance by compressing the ADC full-range codes into a tighter range.
For example, using an external 2.048V reference with VDD
= 5V the total span would be ±2.048V and each AIN span
would be limited to 2.048VP-P allowing a common mode
VREF
LT1819
200Ω
performance curves taken at higher frequencies used a
harmonic rejection filter between the ADC and the signal
source to eliminate the op amp as the dominant source
of distortion.
Figure 9. LT1819 Buffering a Fully-Differential Signal Source
232114fc
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LTC2321-14
Applications Information
range of ~3V. Compressing the input span would incur a
SNR penalty of approximately 2dB. Input span compression may be useful if single-supply analog input drivers
are used which cannot swing rail-to-rail. The fully-differential configuration is illustrated in Figure 10, with the
corresponding transfer function illustrated in Figure 11.
time during acquisition and to optimize the distortion performance of the ADC. Minimizing settling time is important even for DC inputs, because the ADC inputs draw a
current spike when during acquisition.
For best performance, a buffer amplifier should be used to
drive the analog inputs of the LTC2321-14. The amplifier
provides low output impedance to minimize gain error
and allow for fast settling of the analog signal during the
acquisition phase. It also provides isolation between the
signal source and the ADC inputs, which draw a small
current spike during acquisition.
INPUT DRIVE CIRCUITS
A low impedance source can directly drive the high impedance inputs of the LTC2321-14 without gain error. A high
impedance source should be buffered to minimize settling
VREF
0V
VREF
LT1819
+
–
LTC2321-14
25Ω
0V
AIN1+
REFOUT1
VBYP1
220pF
VREF
0V
VREF
+
–
25Ω
0V
AIN1–
SDO1
CLKOUT
SCK
ONLY CHANNEL 1 SHOWN FOR CLARITY
10µF
1µF
TO CONTROL
LOGIC
(FPGA, CPLD,
DSP, ETC.)
232114 F10
Figure 10. Fully-Differential Application Circuit
ADC CODE
(2’s COMPLEMENT)
16383
8192
–VREF
–VREF /2
0
VREF /2
VREF
AIN
(AINn + – AINn –)
–8192
–16384
232114 F11
Figure 11. Fully-Differential Transfer Function
232114fc
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15
LTC2321-14
Applications Information
Input Filtering
ADC REFERENCE
The noise and distortion of the buffer amplifier and signal
source must be considered since they add to the ADC
noise and distortion. Noisy input signals should be filtered
prior to the buffer amplifier input with a low bandwidth
filter to minimize noise. The simple 1-pole RC lowpass filter shown in Figure 12 is sufficient for many applications.
Internal Reference
The input resistor divider network, sampling switch onresistance (RON) and the sample capacitor (CIN) form a
second lowpass filter that limits the input bandwidth to
the ADC core to 110MHz. A buffer amplifier with a low
noise density must be selected to minimize the degradation of the SNR over this bandwidth.
High quality capacitors and resistors should be used in
the RC filters since these components can add distortion. NPO and silver mica type dielectric capacitors have
excellent linearity. Carbon surface mount resistors can
generate distortion from self heating and from damage
that may occur during soldering. Metal film surface mount
resistors are much less susceptible to both problems.
SINGLE-ENDED
INPUT SIGNAL
50Ω
3.3nF
BW = 1MHz
The LTC2321-14 has an on-chip, low noise, low drift
(20ppm/°C max), temperature compensated bandgap
reference. It is internally buffered and is available at
REFOUT1,2 (Pins 12, 26). The reference buffer gains
the internal reference voltage to 4.096V for supply voltages VDD = 5V and to 2.048V for VDD = 3.3V. Bypass
REFOUT1,2 to REFRTN1,2 with the parallel combination of
a 0.1µF (X7R, 0402 size) capacitor and a 10μF (X5R, 0805
size) ceramic capacitor to compensate the reference buffer and minimize noise (Figure 13a). The 0.1µF capacitor
should be as close as possible to the LTC2321-14 package to minimize wiring inductance. Tie the REFINT pin to
VDD to enable the internal reference buffer.
Table 2. REFOUT1,2 Sources and Ranges vs VDD
VDD
REFINT
PIN
REFOUT1,2 PIN
DIFFERENTIAL
SPAN
5V
5V
Internal 4.096V
±4.096V
5V
0V
External (1.25V to 5V)
±1.25V to ±5V
3.3V
3.3V
Internal 2.048V
±2.048V
3.3V
0V
External (1.25V to 3.3V)
±1.25V to ±3.3V
IN+
LTC2321
IN–
SINGLE-ENDED
TO DIFFERENTIAL
DRIVER
232114 F12
Figure 12. Input Signal Chain
16
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LTC2321-14
Applications Information
External Reference
The internal reference buffer can also be overdriven from
1.25V to 5V with an external reference at REFOUT1,2
as shown in Figure 13 (b and c). To do so, REFINT must
be grounded to disable the reference buffer. A 55k internal resistance loads the REFOUT1,2 pins when the reference buffer is disabled. To maximize the input signal
swing and corresponding SNR, the LTC6655-5 is recommended when overdriving REFOUT1,2. The LTC6655-5
offers the same small size, accuracy, drift and extended
temperature range as the LTC6655-4.096. By using a 5V
reference, a higher SNR can be achieved. We recommend
bypassing the LTC6655-5 with a parallel combination of
a 0.1µF (X7R, 0402 size) ceramic capacitor and a 10μF
ceramic capacitor (X5R, 0805 size) close to each of the
REFOUT1,2 and REFRTN1,2 pins.
Internal Reference Buffer Transient Response
The REFOUT1,2 pins of the LTC2321-14 draw charge
(QCONV) from the external bypass capacitors during each
conversion cycle. If the internal reference buffer is overdriven, the external reference must provide all of this charge
with a DC current equivalent to IREF = QCONV/tCYC.
Thus, the DC current draw of REFOUT1,2 depends
on the sampling rate and output code. In applications
where a burst of samples is taken after idling for long
REFINT
VDD
REFINT
REFOUT1
3.3V TO 5V
0.1µF
10µF
REFOUT1
LTC2321-14
0.1µF
5V TO 13.2V
REFRTN1
0.1µF
REFRTN2
0.1µF
10µF
LTC6655-4.096
VIN
VOUT_F
SHDN VOUT_S
10µF
LTC2321-14
REFRTN1
0.1µF
REFRTN2
10µF
REFOUT2
GND
REFOUT2
GND
232114 F13b
232114 F13a
(13a) LTC2321-14 Internal Reference Circuit
(13b) LTC2321-14 with a Shared External Reference Circuit
5V TO 13.2V
0.1µF
REFINT
LTC6655-4.096
VIN
VOUT_F
SHDN VOUT_S
REFOUT1
0.1µF
10µF
REFRTN1
0.1µF
0.1µF
LTC6655-2.048
VIN
VOUT_F
SHDN VOUT_S
LTC2321-14
REFRTN2
10µF
REFOUT2
GND
232114 F13c
(13c) LTC2321-14 with Different External Reference Voltages
Figure 13.
232114fc
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17
LTC2321-14
Applications Information
periods, as shown in Figure 14, IREFBUF quickly goes
from approximately ~75µA to a maximum of 500µA for
REFOUT1,2 = 5V at 2Msps. This step in DC current draw
triggers a transient response in the external reference that
must be considered since any deviation in the voltage at
REFOUT1,2 will affect the accuracy of the output code. If
an external reference is used to overdrive REFOUT1,2 the
fast settling LTC6655 reference is recommended.
DYNAMIC PERFORMANCE
Fast Fourier transform (FFT) techniques are used to test
the ADC’s frequency response, distortion and noise at the
rated throughput. By applying a low distortion sine wave
and analyzing the digital output using an FFT algorithm,
the ADC’s spectral content can be examined for frequencies outside the fundamental. The LTC2321-14 provides
guaranteed tested limits for both AC distortion and noise
measurements.
CNV
Signal-to-Noise and Distortion Ratio (SINAD)
IDLE
PERIOD
232114 F14
Figure 14. CNV Waveform Showing Burst Sampling
OUTPUT CODE (CH1, CH2)
20000
15000
10000
Signal-to-Noise Ratio (SNR)
5000
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. Figure 16 shows
that the LTC2321-14 achieves a typical SNR of 80dB at a
2MHz sampling rate with a 500kHz input.
0
–5000
The signal-to-noise and distortion ratio (SINAD) is the
ratio between the RMS amplitude of the fundamental input
frequency and the RMS amplitude of all other frequency
components at the A/D output. The output is bandlimited
to frequencies from above DC and below half the sampling
frequency. Figure 16 shows that the LTC2321-14 achieves
a typical SINAD of 80dB at a 2MHz sampling rate with a
500kHz input.
0
100
200
300
400
500
TIME (ns)
232114 F15
Figure 15. Transient Response of the LTC2321-14
AMPLITUDE (dBFS)
0
SNR = 80.2dB
THD = –92.5dB
–20 SINAD = 79.9dB
SFDR = 98.6dB
–40
–60
–80
–100
–120
–140
0
0.2
0.4
0.6
FREQUENCY (MHz)
0.8
1.0
232114 F16
Figure 16. 32k Point FFT of the LTC2321-14
18
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LTC2321-14
Applications Information
Total Harmonic Distortion (THD)
POWER CONSIDERATIONS
Total harmonic distortion (THD) 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 (fSMPL /2). THD is expressed as:
The LTC2321-14 requires two power supplies: the 5V
power supply (VDD), and the digital input/output interface
power supply (OVDD). The flexible OVDD supply allows
the LTC2321-14 to communicate with any digital logic
operating between 1.8V and 2.5V. When using LVDS I/O,
the OVDD supply must be set to 2.5V.
THD=20log
V22 + V32 + V42 +…+ VN2
Power Supply Sequencing
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 LTC2321-14 does not have any specific power supply sequencing requirements. Care should be taken to
adhere to the maximum voltage relationships described
in the Absolute Maximum Ratings section. The LTC232114 has a power-on-reset (POR) circuit that will reset the
LTC2321-14 at initial power-up or whenever the power
supply voltage drops below 2V. Once the supply voltage
re-enters the nominal supply voltage range, the POR will
reinitialize the ADC. No conversions should be initiated
until 10ms after a POR event to ensure the reinitialization
period has ended. Any conversions initiated before this
time will produce invalid results.
12.0
SUPPLY CURRENT (mA)
11.5
11.0
10.5
10.0
9.5
9.0
8.5
8.0
0
0.5
1.0
1.5
SAMPLE RATE (Msps)
2.0
232114 F17
Figure 17. Power Supply Current of the LTC2321-14 Versus Sampling Rate
232114fc
For more information www.linear.com/LTC2321-14
19
LTC2321-14
Applications Information
TIMING AND CONTROL
to capture the SDO output eases timing requirements at
the receiver. For low throughput applications, CLKOUT+
can be disabled by tying CLKOUT– to OVDD.
CNV Timing
A rising edge on CNV initiates the acquisition phase and
puts the internal sample-and-hold into the sample mode.
A falling edge on CNV puts the internal sample-and-hold
into the hold mode and starts a conversion cycle. The
CNV pulse must be at least 25ns wide for proper operation. CNV must be driven by a fast low jitter signal with a
fall time from OVDD to below 100mV of less than 1ns. To
achieve this fast falling edge, the distance from the CNV
source to the CNV pin should be minimized. The trace
for this pulse should be kept as narrow as possible and
routed away from adjacent traces or planes to minimize
capacitance. The drive strength of the gate driving the
CNV line must be sufficient to yield a fast falling edge at
the ADC pin to below 100mV. We recommend the Typical
Application circuit on the back page, which uses a high
speed flip-flop to generate the CNV pulse to the ADC,
eliminating the effect of jitter from the FPGA. If jitter from
the FPGA is not a concern, the flip-flop can be eliminated
and replaced with an inverter such as the NC7SZ04P5X.
SCK Serial Data Clock Input
The falling edge of this clock shifts the conversion result
MSB first onto the SDO pins. A 64MHz external clock must
be applied at the SCK pin to achieve 2Msps throughput.
CLKOUT Serial Data Clock Output
The CLKOUT output provides a skew-matched clock to
latch the SDO output at the receiver. The timing skew
of the CLKOUT and SDO outputs are matched. For high
throughput applications, using CLKOUT instead of SCK
CNV
1
Nap/Sleep Modes
Nap mode is a method to save power without sacrificing
power-up delays for subsequent conversions. Sleep mode
has substantial power savings, but a power-up delay is
incurred to allow the reference and power systems to
become valid. To enter nap mode on the LTC2321-14,
the SCK signal must be held high or low and a series
of two CNV pulses must be applied. This is the case for
both CMOS and LVDS modes. The second rising edge of
CNV initiates the nap state. The nap state will persist until
either a single rising edge of SCK is applied, or further
CNV pulses are applied. The SCK rising edge will put the
LTC2321-14 back into the operational (full-power) state.
When in nap mode, two additional pulses will put the
LTC2321-14 in sleep mode. When configured for CMOS
I/O operation, a single rising edge of SCK can return the
LTC2321-14 into operational mode. A 10ms delay is necessary after exiting sleep mode to allow the reference buffer to recharge the external filter capacitor. In LVDS mode,
exit sleep mode by supplying a fifth CNV pulse. The fifth
pulse will return the LTC2321-14 to operational mode,
and further SCK pulses will keep the part from re-entering
nap and sleep modes. The fifth SCK pulse also works in
CMOS mode as a method to exit sleep. In the absence of
SCK pulses, repetitive CNV pulses will cycle the LTC232114 between operational, nap and sleep modes indefinitely.
Refer to the timing diagrams in Figure 18, Figure 19, Figure 20
and Figure 21 for more detailed timing information about
sleep and nap modes.
2
NAP MODE
SCK
SDO1
SDO2
FULL POWER MODE
HOLD STATIC HIGH OR LOW
WAKE ON 1ST SCK EDGE
Z
Z
232114 F18
Figure 18. CMOS and LVDS Mode NAP and WAKE Using SCK
20
232114fc
For more information www.linear.com/LTC2321-14
LTC2321-14
Applications Information
REFOUT1
REFOUT2
REFOUT
RECOVERY
4.096V
4.096V
tWAKE
CNV
1
2
3
4
NAP MODE
SCK
SLEEP MODE
FULL POWER MODE
HOLD STATIC HIGH OR LOW
WAKE ON 1ST SCK EDGE
SDO1
SDO2
Z
Z
Z
Z
232114 F19
Figure 19. CMOS Mode SLEEP and WAKE Using SCK
REFOUT1
REFOUT2
REFOUT
RECOVERY
4.096V
4.096V
tWAKE
CNV
1
2
3
4
NAP MODE
SCK
SDO1
SDO2
WAKE ON 5TH
CSB EDGE
5
SLEEP MODE
FULL POWER MODE
HOLD STATIC HIGH OR LOW
Z
Z
Z
Z
Z
232114 F20
Figure 20. LVDS and CMOS Mode SLEEP and WAKE Using CNV
232114fc
For more information www.linear.com/LTC2321-14
21
LTC2321-14
Applications Information
DIGITAL INTERFACE
skew of the CLKOUT and SDO outputs are matched. For
high throughput applications, using CLKOUT instead of
SCK to capture the SDO output eases timing requirements
at the receiver.
The LTC2321-14 features a serial digital interface that
is simple and straight forward to use. The flexible OVDD
supply allows the LTC2321-14 to communicate with any
digital logic operating between 1.8V and 2.5V. A 64MHz
external clock must be applied at the SCK pin to achieve
2Msps throughput.
In CMOS mode, use the SDO1+, SDO2+ and CLKOUT+
pins as outputs. Use the SCK+ pin as an input. Do not
connect the SDO1–, SDO2–, SCK– and CLKOUT– pins,
as they each have internal pull-down circuitry to OGND.
In addition to a standard CMOS SPI interface, the LTC2321-14
provides an optional LVDS SPI interface to support low
noise digital design. The CMOS/LVDS pin is used to select
the digital interface mode.
In LVDS mode, use the SDO1+/SDO1–, SDO2+/SDO2– and
CLKOUT+/CLKOUT– pins as differential outputs. These
pins must be differentially terminated by an external 100Ω
resistor at the receiver (FPGA). The SCK+/SCK– pins are
differential inputs and must be terminated differentially by
an external 100Ω resistor at the receiver (ADC).
The falling edge of SCK outputs the conversion result MSB
first on the SDO pins. CLKOUT provides a skew-matched
clock to latch the SDO output at the receiver. The timing
tDSCKHCNVH
CNV
tSCKL
1
SCK
3
4
5
tSCK
6
13
14
15
16
tDCNVSDOV
HI-Z
B14
SDO
B13
B12
B11
B10
tDCLKOUTSDOV
1
CLKOUT
tCNVH
2
tSCKH
2
3
4
B2
B1
B0
0
HI-Z
tHSDO
5
tCONV
6
tREADOUT
13
14
15
16
tDCNVSDOZ
tCYC
SERIAL DATA BITS B[15:0] CORRESPOND TO CURRENT CONVERSION
tDSCKCLKOUT IS THE DELAY FROM SCK↓ TO CLKOUT↓
232114 F21
Figure 21. LTC2321-14 Timing Diagram
22
232114fc
For more information www.linear.com/LTC2321-14
LTC2321-14
Applications Information
BOARD LAYOUT
Recommended Layout
To obtain the best performance from the LTC2321-14,
a printed circuit board is recommended. Layout for the
printed circuit board (PCB) should ensure the digital and
analog signal lines are separated as much as possible.
In particular, care should be taken not to run any digital
clocks or signals adjacent to analog signals or underneath
the ADC.
The following is an example of a recommended PCB layout. A single solid ground plane is used. Bypass capacitors to the supplies are placed as close as possible to the
supply pins. Low impedance common returns for these
bypass capacitors are essential to the low noise operation of the ADC. The analog input traces are screened by
ground. For more details and information, refer to the
DC1996A, the evaluation kit for the LTC2321-14.
LTC2321-14
2.5V
FPGA OR DSP
OVDD
SDO1+
SDO1–
100Ω
+
–
100Ω
+
–
CLKOUT+
CLKOUT –
SCK+
2.5V
+
–
100Ω
CMOS/LVDS
SCK–
SDO2+
100Ω
SDO2–
+
–
CNV
232114 F22
Figure 22. LTC2321 Using the LVDS Interface
232114fc
For more information www.linear.com/LTC2321-14
23
LTC2321-14
Applications Information
24
Figure 23. Layer 1, Top Layer
Figure 24. Layer 2, Ground Plane
Figure 25. Layer 3, Power Plane
Figure 26. Layer 4, Bottom Layer
232114fc
For more information www.linear.com/LTC2321-14
LTC2321-14
Typical Application
Low Jitter Clock Timing with RF Sine Generator Using Clock Squaring/Level-Shifting Circuit and Retiming Flip-Flop
VCC
0.1µF
50Ω
1k
NC7SZ04P5X
MASTER_CLOCK
VCC
1k
D
PRE
NL17SZ74USG Q
CLR
CONV
CONV ENABLE
LTC2321-14
CONTROL
LOGIC
(FPGA, CPLD,
DSP, ETC.)
CNV
SCK
CLKOUT
GND
CMOS/LVDS
SDO1
SDO2
10Ω
10Ω
10Ω
NC7SZ04P5X (× 3)
232114 TA02
232114fc
For more information www.linear.com/LTC2321-14
25
LTC2321-14
Package Description
Please refer to http://www.linear.com/product/LTC2321-14#packaging for the most recent package drawings.
UFD Package
UFD Package
28-Lead Plastic
QFN (4mm × 5mm)
28-Lead
QFN
(4mm × 5mm)
(ReferencePlastic
LTC DWG
# 05-08-1712
Rev C)
(Reference LTC DWG # 05-08-1712 Rev C)
0.70 ±0.05
4.50 ±0.05
3.10 ±0.05
2.50 REF
2.65 ±0.05
3.65 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
3.50 REF
4.10 ±0.05
5.50 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ±0.10
(2 SIDES)
0.75 ±0.05
R = 0.05
TYP
PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER
2.50 REF
R = 0.115
TYP
27
28
0.40 ±0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
5.00 ±0.10
(2 SIDES)
3.50 REF
3.65 ±0.10
2.65 ±0.10
(UFD28) QFN 0816 REV C
0.200 REF
0.00 – 0.05
0.25 ±0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGHD-3).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
26
232114fc
For more information www.linear.com/LTC2321-14
LTC2321-14
Revision History
REV
DATE
DESCRIPTION
A
11/14
Updated Timing Characteristics and Figure 21
5, 22
B
05/17
Changed the CNV pin description in the Pin Functions section, and the CNV Timing section in the Applications
Information section.
8, 20
C
09/17
PAGE NUMBER
Changed Fairchild components on the Typical Application.
25
Changed CNV minimum pulse width to 25ns.
20
232114fc
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representaFor more
www.linear.com/LTC2321-14
tion that the interconnection
of itsinformation
circuits as described
herein will not infringe on existing patent rights.
27
LTC2321-14
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
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Precision Low Drift, Low Noise Buffered Reference
5V/4.096V/3.3V/3V/2.5V/2.048V/1.25V, 5ppm/°C, 0.25ppm
Peak-to-Peak Noise, MSOP-8 Package
LTC6652
Precision Low Drift, Low Noise Buffered Reference
5V/4.096V/3.3V/3V/2.5V/2.048V/1.25V, 5ppm/°C, 2.1ppm
Peak-to-Peak Noise, MSOP-8 Package
LT1818/LT1819
400MHz, 2500V/µs, 9mA Single/Dual Operational
Amplifiers
–85dBc Distortion at 5MHz, 6nV/√Hz Input Noise Voltage,
9mA Supply Current, Unity-Gain Stable
LT1806
325MHz, Single, Rail-to-Rail Input and Output,
Low Distortion, Low Noise Precision Op Amps
–80dBc Distortion at 5MHz, 3.5nV/√Hz Input Noise Voltage,
9mA Supply Current, Unity-Gain Stable
LT6200
165MHz, Rail-to-Rail Input and Output,
0.95nV/√Hz Low Noise, Op Amp Family
Low Noise, Low Distortion, Unity-Gain Stable
Amplifiers
28
232114fc
LT 0917 REV C • PRINTED IN USA
For more information www.linear.com/LTC2321-14
www.linear.com/LTC2321-14
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