LTC2338-18
18-Bit, 1Msps, ±10.24V
True Bipolar, Fully Differential
Input ADC with 100dB SNR
DESCRIPTION
FEATURES
1Msps Throughput Rate
nn ±4LSB INL (Max)
nn Guaranteed 18-Bit No Missing Codes
nn Fully Differential Inputs
nn True Bipolar Input Ranges ±6.25V, ±10.24V, ±12.5V
nn 100dB SNR (Typ) at f = 2kHz
IN
nn –115dB THD (Typ) at f = 2kHz
IN
nn Guaranteed Operation to 125°C
nn Single 5V Supply
nn Low Drift (20ppm/°C Max) 2.048V Internal Reference
nn Onboard Single-Shot Capable Reference Buffer
nn No Pipeline Delay, No Cycle Latency
nn 1.8V to 5V I/O Voltages
nn SPI-Compatible Serial I/O with Daisy-Chain Mode
nn Internal Conversion Clock
nn Power Dissipation 50mW (Typ)
nn 16-Lead MSOP Package
The LTC®2338-18 is a low noise, high speed 18-bit successive approximation register (SAR) ADC with fully differential
inputs. Operating from a single 5V supply, the LTC2338-18
has a ±10.24V true bipolar input range, making it ideal for
high voltage applications which require a wide dynamic
range. The LTC2338-18 achieves ±4LSB INL maximum,
no missing codes at 18-bits with 100dB SNR.
nn
The LTC2338-18 has an onboard single-shot capable
reference buffer and low drift (20ppm/°C max) 2.048V
temperature compensated reference. The LTC2338-18
also has a high speed SPI-compatible serial interface that
supports 1.8V, 2.5V, 3.3V and 5V logic while also featuring a daisy-chain mode. The fast 1Msps throughput with
no cycle latency makes the LTC2338-18 ideally suited
for a wide variety of high speed applications. An internal
oscillator sets the conversion time, easing external timing
considerations. The LTC2338-18 dissipates only 50mW
and automatically naps between conversions, leading to
reduced power dissipation that scales with the sampling
rate. A sleep mode is also provided to reduce the power
consumption of the LTC2338-18 to 300μW for further
power savings during inactive periods.
APPLICATIONS
Programmable Logic Controllers
Industrial Process Control
nn High Speed Data Acquisition
nn Portable or Compact Instrumentation
nn ATE
nn
nn
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
SoftSpan is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents, including 7705765, 7961132.
TYPICAL APPLICATION
5V
10µF
32k Point FFT fS = 1Msps, fIN = 2kHz
1.8V TO 5V
2.2µF
0
SNR = 100.3dB
THD = –117dB
SINAD = 100.2dB
SFDR = –119dB
–20
0.1µF
–10.24V
+10.24V
–10.24V
+
VDD
VDDLBYP OVDD
IN+
LTC2338-18
–
IN–
REF
REFBUF
47µF
REFIN
100nF
GND
CHAIN
RDL/SDI
SDO
SCK
BUSY
CNV
SAMPLE CLOCK
233818 TA01
AMPLITUDE (dBFS)
–40
+10.24V
–60
–80
–100
–120
–140
–160
–180
0
100
200
300
FREQUENCY (kHz)
400
500
233818 TA01
233818fa
For more information www.linear.com/LTC2338-18
1
LTC2338-18
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Notes 1, 2)
TOP VIEW
Supply Voltage (VDD)...................................................6V
Supply Voltage (OVDD).................................................6V
Supply Bypass Voltage (VDDLBYP)............................3.2V
Analog Input Voltage
IN+, IN–...............................................–16.5V to 16.5V
REFBUF....................................................................6V
REFIN ...................................................................2.8V
Digital Input Voltage
(Note 3)............................ (GND –0.3V) to (OVDD + 0.3V)
Digital Output Voltage
(Note 3)............................ (GND –0.3V) to (OVDD + 0.3V)
Power Dissipation............................................... 500mW
Operating Temperature Range
LTC2338C................................................. 0°C to 70°C
LTC2338I..............................................–40°C to 85°C
LTC2338H........................................... –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
ORDER INFORMATION
VDDLBYP
VDD
GND
IN+
IN–
GND
REFBUF
REFIN
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
GND
OVDD
SDO
SCK
RDL/SDI
BUSY
CHAIN
CNV
MS PACKAGE
16-LEAD PLASTIC MSOP
TJMAX = 150°C, θJA = 110°C/W
http://www.linear.com/product/LTC2338-18#orderinfo
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC2338CMS-18#PBF
LTC2338CMS-18#TRPBF
233818
16-Lead Plastic MSOP
0°C to 70°C
LTC2338IMS-18#PBF
LTC2338IMS-18#TRPBF
233818
16-Lead Plastic MSOP
–40°C to 85°C
LTC2338HMS-18#PBF
LTC2338HMS-18#TRPBF 233818
16-Lead Plastic MSOP
–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 (IN+)
(Note 5)
l
–2.5 • VREFBUF – 0.25
2.5 • VREFBUF + 0.25
V
–
Absolute Input Range (IN–)
(Note 5)
l
–2.5 • VREFBUF – 0.25
2.5 • VREFBUF + 0.25
V
VIN+ – VIN–
Input Differential Voltage Range
VIN = VIN+ – VIN–
l
–5 • VREFBUF
5 • VREFBUF
V
VCM
Common Mode Input Range
(Note 11)
l
–0.5
0.5
V
l
–7.8
4.8
mA
VIN
MIN
TYP
0
MAX
UNITS
IIN
Analog Input Current
CIN
Analog Input Capacitance
5
pF
RIN
Analog Input Resistance
2.083
kΩ
CMRR
Input Common Mode Rejection Ratio
67
dB
fIN = 500kHz
233818fa
2
For more information www.linear.com/LTC2338-18
LTC2338-18
CONVERTER 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
MIN
TYP
UNITS
l
18
Bits
No Missing Codes
l
18
Bits
l
–4
±1
l
–1
l
–15
Transition Noise
INL
Integral Linearity Error
DNL
Differential Linearity Error
BZE
Bipolar Zero-Scale Error
0.8
(Note 6)
(Note 7)
Bipolar Zero-Scale Error Drift
FSE
MAX
Resolution
Bipolar Full-Scale Error
LSBRMS
4
LSB
±0.1
1
LSB
0
15
LSB
0.01
LSB/°C
VREFBUF = 4.096V (REFBUF Overdriven) (Notes 7, 9)
l
–100
100
LSB
REFIN = 2.048V (Note 7)
l
–150
150
LSB
Bipolar Full-Scale Error Drift
±0.5
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
SINAD
Signal-to-(Noise + Distortion) Ratio
±6.25V Range, fIN = 2kHz, REFIN = 1.25V
l
93
97
dB
±10.24V Range, fIN = 2kHz, REFIN = 2.048V
l
95
100
dB
±12.5V Range, fIN = 2kHz, REFBUF = 5V
l
96
101
dB
±6.25V Range, fIN = 2kHz, REFIN = 1.25V
l
93.5
97
dB
±10.24V Range, fIN = 2kHz, REFIN = 2.048V
l
96
100
dB
±12.5V Range, fIN = 2kHz, REFBUF = 5V
l
98
102
dB
±6.25V Range, fIN = 2kHz, REFIN = 1.25V
l
–111
–102
dB
±10.24V Range, fIN = 2kHz, REFIN = 2.048V
l
–115
–102
dB
–112
–100
SNR
THD
SFDR
Signal-to-Noise Ratio
Total Harmonic Distortion
Spurious Free Dynamic Range
MIN
TYP
MAX
UNITS
±12.5V Range, fIN = 2kHz, REFBUF = 5V
l
±6.25V Range, fIN = 2kHz, REFIN = 1.25V
l
102
113
dB
±10.24V Range, fIN = 2kHz, REFIN = 2.048V
l
102
117
dB
±12.5V Range, fIN = 2kHz, REFBUF = 5V
l
100
114
–3dB Input Linear Bandwidth
dB
dB
7
MHz
Aperture Delay
500
ps
Aperture Jitter
4
ps
500
ns
Transient Response
Full-Scale Step
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
VREFIN
Internal Reference Output Voltage
VREFIN Temperature Coefficient
CONDITIONS
(Note 14)
MIN
TYP
MAX
UNITS
2.043
2.048
2.053
V
2
20
l
REFIN Output Impedance
15
VREFIN Line Regulation
VDD = 4.75V to 5.25V
REFIN Input Voltage Range
(REFIN Overdriven) (Note 5)
kΩ
0.08
1.25
ppm/°C
mV/V
2.4
V
233818fa
For more information www.linear.com/LTC2338-18
3
LTC2338-18
REFERENCE BUFFER 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
MIN
TYP
MAX
UNITS
VREFBUF
Reference Buffer Output Voltage
VREFIN = 2.048V
l
4.091
4.096
4.101
V
REFBUF Input Voltage Range
(REFBUF Overdriven) (Notes 5, 9)
l
2.5
5
V
REFBUF Output Impedance
VREFIN = 0V
REFBUF Load Current
VREFBUF = 5V (REFBUF Overdriven) (Notes 9, 10)
VREFBUF = 5V, Nap Mode (REFBUF Overdriven) (Note 9)
IREFBUF
13
kΩ
1.05
0.39
l
1.2
mA
mA
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
CONDITIONS
MIN
VIH
High Level Input Voltage
l
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
TYP
MAX
UNITS
0.8 • OVDD
V
–10
l
0.2 • OVDD
V
10
μA
5
pF
OVDD – 0.2
V
VOL
Low Level Output Voltage
IO = 500µA
l
IOZ
Hi-Z Output Leakage Current
VOUT = 0V to OVDD
l
ISOURCE
Output Source Current
VOUT = 0V
–10
mA
ISINK
Output Sink Current
VOUT = OVDD
10
mA
–10
0.2
V
10
µA
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
CONDITIONS
OVDD
Supply Voltage
IVDD
IOVDD
INAP
ISLEEP
Supply Current
Supply Current
Nap Mode Current
Sleep Mode Current
1Msps Sample Rate (IN+ = IN– = 0V)
1Msps Sample Rate (CL = 20pF)
Conversion Done (IVDD + IOVDD)
Sleep Mode (IVDD + IOVDD)
PD
Power Dissipation
Nap Mode
Sleep Mode
1Msps Sample Rate (IN+ = IN– = 0V)
Conversion Done (IVDD + IOVDD)
Sleep Mode (IVDD + IOVDD)
MIN
TYP
MAX
UNITS
l
4.75
5
5.25
V
l
1.71
5.25
V
11.2
l
l
10
0.4
3.9
60
4.6
225
mA
mA
mA
μA
l
l
l
50
19.5
0.3
56
23
1.1
mW
mW
mW
l
233818fa
4
For more information www.linear.com/LTC2338-18
LTC2338-18
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
fSMPL
Maximum Sampling Frequency
CONDITIONS
tCONV
Conversion Time
tACQ
Acquisition Time
MIN
TYP
l
tACQ = tCYC – tCONV – tBUSYLH (Note 11)
MAX
UNITS
1
Msps
l
460
l
460
ns
1
µs
20
ns
tCYC
Time Between Conversions
l
tCNVH
CNV High Time
l
527
ns
tBUSYLH
CNV↑ to BUSY Delay
CL = 20pF
l
tCNVL
Minimum Low Time for CNV
(Note 12)
l
20
ns
tQUIET
SCK Quiet Time from CNV↑
(Note 11)
l
20
ns
tSCK
SCK Period
(Notes 12, 13)
l
10
ns
13
ns
tSCKH
SCK High Time
l
4
ns
tSCKL
SCK Low Time
l
4
ns
tSSDISCK
SDI Setup Time From SCK↑
(Note 12)
l
4
ns
tHSDISCK
SDI Hold Time From SCK↑
(Note 12)
l
1
ns
tSCKCH
SCK Period in Chain Mode
tSCKCH = tSSDISCK + tDSDO (Note 12)
l
13.5
ns
tDSDO
SDO Data Remains Valid Delay from SCK↑
CL = 20pF, OVDD = 5.25V
CL = 20pF, OVDD = 2.5V
CL = 20pF, OVDD = 1.71V
l
l
l
tHSDO
SDO Data Remains Valid Delay from SCK↑
CL = 20pF (Note 11)
l
7.5
8
9.5
1
ns
ns
ns
ns
tDSDOBUSYL
SDO Data Valid Delay from BUSY↓
CL = 20pF (Note 11)
l
5
ns
tEN
Bus Enable Time After RDL↓
(Note 12)
l
16
ns
tDIS
Bus Relinquish Time After RDL↑
(Note 12)
l
tWAKE
REFBUF Wakeup Time
CREFBUF = 47μF, CREFIN = 100nF
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, ±10.24V Range, REFIN = 2.048V,
fSMPL = 1MHz.
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.
13
200
ns
ms
Note 7: Bipolar zero error is the offset voltage measured from –0.5LSB
when the output code flickers between 00 0000 0000 0000 0000 and 11
1111 1111 1111 1111. Full-scale bipolar error is the worst-case of –FS
or +FS 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 ±20.48V input
with REFIN = 2.048V.
Note 9: When REFBUF is overdriven, the internal reference buffer must be
turned off by setting REFIN = 0V.
Note 10: fSMPL = 1MHz, 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, OVDD = 2.5V
and OVDD = 5.25V.
Note 13: tSCK of 10ns maximum allows a shift clock frequency up to
100MHz for rising edge capture.
Note 14: Temperature coefficient is calculated by dividing the maximum
change in output voltage by the specified temperature range.
0.8 • OVDD
tWIDTH
0.2 • OVDD
tDELAY
tDELAY
0.8 • OVDD
0.8 • OVDD
0.2 • OVDD
0.2 • OVDD
50%
50%
233818 F01
Figure 1. Voltage Levels for Timing Specifications
233818fa
For more information www.linear.com/LTC2338-18
5
LTC2338-18
T
TYPICAL
PERFORMANCE CHARACTERISTICS A = 25°C, VDD = 5V, OVDD = 2.5V, REFIN = 2.048V,
fSMPL = 1Msps, unless otherwise noted.
Integral Nonlinearity
vs Output Code
Differential Nonlinearity
vs Output Code
5000
2.5
0.4
4500
0.3
4000
0.2
3500
0.1
3000
1.5
DNL ERROR (LSB)
1.0
0.5
0
–0.5
–1.0
COUNTS
0.5
2.0
INL ERROR (LSB)
DC Histogram
3.0
0.0
–0.1
1500
1000
–2.5
–0.4
500
–3.0
–131072
–0.5
–131072
–65536
0
65536
OUTPUT CODE
131072
–65536
0
65536
32k Point FFT fS = 1Msps,
fIN = 2kHz
110
–70
–60
–80
–100
–120
–140
SINAD
90
80
70
0
100
200
300
FREQUENCY (kHz)
400
60
500
0
25
50
2
3
4
233818 G03
–90
–100
–110
–120
–130
–150
75 100 125 150 175 200
FREQUENCY (kHz)
SNR, SINAD vs Input Level,
fIN = 2kHz
102.0
SNR, SINAD vs Temperature,
fIN = 2kHz
–105
SNR, SINAD (dBFS)
101.0
SNR
100.5
100.5
100.0
SNR
SINAD
99.5
99.0
–30
–20
–10
INPUT LEVEL (dB)
0
233818 G07
75 100 125 150 175 200
FREQUENCY (kHz)
98.0
–55 –35 –15
THD, Harmonics vs Temperature,
fIN = 2kHz
–115
–120
–125
THD
3RD
2ND
–130
98.5
100.0
–40
50
–110
THD, HARMONICS (dBFS)
101.5
SINAD
25
233818 G06
101.5
101.0
0
233818 G05
233818 G04
102.0
0
1
CODE
–140
–160
–180
–1
THD
2ND
3RD
–80
SNR
100
SNR, SINAD (dBFS)
–40
–2
THD, Harmonics
vs Input Frequency
SNR, SINAD vs Input Frequency
SNR = 100.3dB
THD = –117dB
SINAD = 100.2dB
SFDR = –119dB
–20
–3
233818 G02
THD, HARMONICS (dBFS)
0
0
–4
131072
OUTPUT CODE
233818 G01
AMPLITUDE (dBFS)
2000
–0.3
–2.0
MAGNITUDE (dBFS)
2500
–0.2
–1.5
σ = 0.8
5 25 45 65 85 105 125
TEMPERATURE (°C)
233818 G08
–135
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
233818 G09
233818fa
6
For more information www.linear.com/LTC2338-18
LTC2338-18
T
TYPICAL
PERFORMANCE CHARACTERISTICS A = 25°C, VDD = 5V, OVDD = 2.5V, REFIN = 2.048V,
fSMPL = 1Msps, unless otherwise noted.
Full-Scale Error vs Temperature
Offset Error vs Temperature
20
5
1.5
15
4
MAX INL
0.5
MAX DNL
0
MIN DNL
–0.5
–1.0
MIN INL
3
10
OFFSET ERROR (LSB)
1.0
FULL-SCALE ERROR (LSB)
INL, DNL ERROR (LSB)
INL/DNL vs Temperature
2.0
5
0
–5
–10
–15
–2.0
–55 –35 –15
–20
–55 –35 –15
–5
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
Internal Reference Output vs
Temperature
120
2.0484
100
8
80
INTERNAL REFERENCE OUTPUT (V)
10
CURRENT (µA)
CURRENT (mA)
VDD
60
40
20
OVDD
0
–55 –35 –15
0
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
5 25 45 65 85 105 125
TEMPERATURE (°C)
233818 G13
2.0482
2.0481
2.0480
2.0479
2.0478
2.0477
2.0476
–55 –35 –15
CMRR vs Input Frequency
Supply Current vs Sampling Rate
12
30
75
10
25
SUPPLY CURRENT (mA)
80
70
15
65
60
10
5 25 45 65 85 105 125
TEMPERATURE (°C)
233818 G15
35
CMRR (dB)
NUMBER OF PARTS
2.0483
233818 G14
Internal Reference Output
Temperature Coefficient
Distribution
20
5 25 45 65 85 105 125
TEMPERATURE (°C)
233818 G12
Sleep Current vs Temperature
12
2
–2
233818 G11
Supply Current vs Temperature
4
0
–1
–4
233818 G10
6
1
–3
–1.5
5 25 45 65 85 105 125
TEMPERATURE (°C)
2
VDD
8
6
4
5
55
2
0
50
0
OVDD
–10 –8 –6 –4 –2 0 2 4
DRIFT (ppm/°C)
6
8 10
233818 G16
0
100
200
300
FREQUENCY (kHz)
400
500
233818 G17
0 100 200 300 400 500 600 700 800 900 1000
SAMPLING FREQUENCY (kHz)
233818 G18
233818fa
For more information www.linear.com/LTC2338-18
7
LTC2338-18
PIN FUNCTIONS
VDDLBYP (Pin 1): 2.5V Supply Bypass Pin. The voltage
on this pin is generated via an onboard regulator off of
VDD. This pin must be bypassed with a 2.2μF ceramic
capacitor to GND.
VDD (Pin 2): 5V Power Supply. The range of VDD is 4.75V to
5.25V. Bypass VDD to GND with a 10µF ceramic capacitor.
GND (Pins 3, 6 and 16): Ground.
IN+, IN– (Pins 4, 5): Positive and Negative Differential
Analog Inputs. Typical input range ±10.24V.
REFBUF (Pin 7): Reference Buffer Output. An onboard
buffer nominally outputs 4.096V to this pin. This pin is
referred to GND and should be decoupled closely to the pin
with a 47μF ceramic capacitor. The internal buffer driving
this pin may be disabled by grounding its input at REFIN.
Once the buffer is disabled, an external reference may
overdrive this pin in the range of 2.5V to 5V. A resistive
load greater than 500kΩ can be placed on the reference
buffer output.
REFIN (Pin 8): Reference Output/Reference Buffer Input.
An onboard bandgap reference nominally outputs 2.048V
at this pin. Bypass this pin with a 100nF ceramic capacitor
to GND to limit the reference output noise. If more accuracy is desired, this pin may be overdriven by an external
reference in the range of 1.25V to 2.4V.
CNV (Pin 9): Convert Input. A rising edge on this input
powers up the part and initiates a new conversion. Logic
levels are determined by OVDD.
CHAIN (Pin 10): Chain Mode Selector Pin. When low,
the LTC2338-18 operates in normal mode and the
RDL/SDI input pin functions to enable or disable SDO.
When high, the LTC2338-18 operates in chain mode and the
RDL/SDI pin functions as SDI, the daisy-chain serial data
input. Logic levels are determined by OVDD.
BUSY (Pin 11): BUSY Indicator. Goes high at the start of
a new conversion and returns low when the conversion
has finished. Logic levels are determined by OVDD.
RDL/SDI (Pin 12): When CHAIN is low, the part is in normal mode and the pin is treated as a bus enabling input.
When CHAIN is high, the part is in chain mode and the
pin is treated as a serial data input pin where data from
another ADC in the daisy chain is input. Logic levels are
determined by OVDD.
SCK (Pin 13): Serial Data Clock Input. When SDO is enabled,
the conversion result or daisy-chain data from another
ADC is shifted out on the rising edges of this clock MSB
first. Logic levels are determined by OVDD.
SDO (Pin 14): Serial Data Output. The conversion result or
daisy-chain data is output on this pin on each rising edge
of SCK MSB first. The output data is in 2’s complement
format. Logic levels are determined by OVDD.
OVDD (Pin 15): I/O Interface Digital Power. The range of
OVDD is 1.71V to 5.25V. This supply is nominally set to
the same supply as the host interface (1.8V, 2.5V, 3.3V,
or 5V). Bypass OVDD to GND with a 0.1μF capacitor.
233818fa
8
For more information www.linear.com/LTC2338-18
LTC2338-18
FUNCTIONAL BLOCK DIAGRAM
VDD = 5V
VDDLBYP = 2.5V
REFIN = 1.25V
TO 2.4V
REFBUF = 2.5V
TO 5V
OVDD = 1.8V
TO 5V
LDO
15k
2.048V
REFERENCE
2× REFERENCE
BUFFER
IN+
IN–
4R
4R
R
0.63× BUFFER
R
+
18-BIT SAMPLING ADC
–
SPI
PORT
CONTROL LOGIC
CHAIN
SDO
RDL/SDI
SCK
CNV
BUSY
GND
233818 BD01
233818fa
For more information www.linear.com/LTC2338-18
9
LTC2338-18
TIMING DIAGRAM
Conversion Timing Using the Serial Interface
CHAIN, RDL/SDI = 0
CNV
BUSY
CONVERT
NAP AND ACQUIRE
SCK
SDO
D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
233818 TD01
233818fa
10
For more information www.linear.com/LTC2338-18
LTC2338-18
APPLICATIONS INFORMATION
TRANSFER FUNCTION
The LTC2338-18 is a low noise, high speed 18-bit successive approximation register (SAR) ADC with fully differential
inputs. Operating from a single 5V supply, the LTC2338-18
has a ±10.24V true bipolar input range, making it ideal for
high voltage applications which require a wide dynamic
range. The LTC2338-18 achieves ±4LSB INL maximum,
no missing codes at 18-bits and 100dB SNR.
The LTC2338-18 digitizes the full-scale voltage of ±5 •
REFBUF into 218 levels, resulting in an LSB size of 156µV
with REFBUF = 4.096V. The ideal transfer function is shown
in Figure 2. The output data is in 2’s complement format.
The LTC2338-18 has an onboard single-shot capable
reference buffer and low drift (20ppm/°C max) 2.048V
temperature-compensated reference. The LTC2338-18
also has a high speed SPI-compatible serial interface that
supports 1.8V, 2.5V, 3.3V and 5V logic while also featuring a daisy-chain mode. The fast 1Msps throughput with
no cycle latency makes the LTC2338-18 ideally suited
for a wide variety of high speed applications. An internal
oscillator sets the conversion time, easing external timing
considerations. The LTC2338-18 dissipates only 50mW
and automatically naps between conversions, leading to
reduced power dissipation that scales with the sampling
rate. A sleep mode is also provided to reduce the power
consumption of the LTC2338-18 to 300μW for further
power savings during inactive periods.
CONVERTER OPERATION
The LTC2338-18 operates in two phases. During the
acquisition phase, the charge redistribution capacitor
D/A converter (CDAC) is connected to the outputs of
the resistor divider networks that pins IN+ and IN– drive
to sample an attenuated and level-shifted version of the
differential analog input voltage as shown in Figure 3. A
rising edge on the CNV pin initiates a conversion. During the conversion phase, the 18-bit CDAC is sequenced
through a successive approximation algorithm, effectively
comparing the sampled input with binary-weighted fractions of the reference voltage (e.g. VREFBUF/2, VREFBUF/4
… VREFBUF/262144) using the differential comparator. At
the end of conversion, the CDAC output approximates the
sampled analog input. The ADC control logic then prepares
the 18-bit digital output code for serial transfer.
OUTPUT CODE (TWO’S COMPLEMENT)
OVERVIEW
011...111
BIPOLAR
ZERO
011...110
000...001
000...000
111...111
111...110
100...001
FSR = +FS – –FS
1LSB = FSR/262144
100...000
–FSR/2
–1 0V 1
FSR/2 – 1LSB
LSB
LSB
INPUT VOLTAGE (V)
233818 F02
Figure 2. LTC2338-18 Transfer Function
ANALOG INPUT
The analog inputs of the LTC2338-18 are fully differential
to maximize the signal swing that can be digitized. The
analog inputs can be modeled by the equivalent circuit
shown in Figure 3. The back-to-back diodes at the inputs
form clamps that provide ESD protection. Each input
drives a resistor divider network that has a total impedance of 2kΩ. The resistor divider network attenuates and
level shifts the ±2.5 • REFBUF true bipolar signal swing
of each input to the 0-REFBUF input signal swing of the
ADC core. In the acquisition phase, 45pF (CIN) from the
sampling CDAC in series with approximately 50Ω (RON)
from the on-resistance of the sampling switch is connected to the output of the resistor divider network. Any
unwanted signal that is common to both inputs will be
reduced by the common mode rejection of the ADC core
and resistor divider network. The inputs of the ADC core
draw a current spike while charging the CIN capacitors
during acquisition.
233818fa
For more information www.linear.com/LTC2338-18
11
LTC2338-18
APPLICATIONS INFORMATION
0.63 • VREFBUF
IN+
1.6k
400Ω
RON
50Ω
SINGLE-ENDED
INPUT SIGNAL
CIN
45pF
IN–
400Ω
RON
50Ω
LTC2338-18
IN–
6600pF
0.63 • VREFBUF
1.6k
IN+
500Ω
BIAS
VOLTAGE
CIN
45pF
BW = 48kHz
SINGLE-ENDEDTO-DIFFERENTIAL
DRIVER
233818 F04
Figure 4. Input Signal Chain
233818 F03
Figure 3. The Equivalent Circuit for the Differential
Analog Input of the LTC2338-18
INPUT DRIVE CIRCUITS
A low impedance source can directly drive the high impedance inputs of the LTC2338-18 without gain error. A
high impedance source should be buffered to minimize
settling 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 entering acquisition.
For best performance, a buffer amplifier should be used to
drive the analog inputs of the LTC2338-18. The amplifier
provides low output impedance to minimize gain error
and allows 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 Filtering
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 4 is sufficient for many applications.
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 7MHz. 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-to-Differential Conversion
For single-ended input signals, a single-ended-to-differential conversion circuit must be used to produce a differential
signal at the inputs of the LTC2338-18. The LT®1469 high
speed dual operational amplifier is recommended for performing single-ended-to-differential conversions as shown
in Figure 5a. 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.
Figure 5b shows the resulting FFT when the LT1469 is
used to drive the LTC2338-18 in this configuration.
Fully Differential Inputs
To achieve the full distortion performance of the
LTC2338‑18, a low distortion fully differential signal source
driven through the LT1469 configured as two unity gain
buffers as shown in Figure 6 can be used to get the full
data sheet THD specification of –115dB.
LT1469
3
±10.24V
2
1
+
–
OUT1
±10.24V
5
6
4.99k
+
–
7
OUT2
±10.24V
4.99k
233818 F05a
Figure 5a. LT1469 Converting a ±10.24V Single-Ended
Signal to a ±20.48V Differential Input Signal
233818fa
12
For more information www.linear.com/LTC2338-18
LTC2338-18
APPLICATIONS INFORMATION
0
REFIN
REFBUF
(OVERDRIVE)
BIPOLAR INPUT RANGE
–60
0V
2.5V (Min)
±6.25V
–80
0V
5V (Max)
±12.5V
–40
AMPLITUDE (dBFS)
Table 3. External Reference Unbuffered
SNR = 100.1dB
THD = –110dB
SINAD = 99.7dB
SFDR = –111dB
–20
–100
–120
Internal Reference with Internal Buffer
–140
The LTC2338-18 has an on-chip, low noise, low drift
(20ppm/°C max), temperature compensated bandgap
reference that is factory trimmed to 2.048V. It is internally
connected to a reference buffer as shown in Figure 7a and
is available at REFIN (Pin 8). REFIN should be bypassed to
GND with a 100nF ceramic capacitor to minimize noise. The
reference buffer gains the REFIN voltage by 2 to 4.096V at
REFBUF (Pin 7). So the input range is ±10.24V, as shown
in Table 1. Bypass REFBUF to GND with at least a 47μF
ceramic capacitor (X7R, 10V, 1210 size) to compensate
the reference buffer and minimize noise.
–160
–180
0
100
200
300
FREQUENCY (kHz)
400
500
233818 F05b
Figure 5b. 128k Point FFT Plot with fIN = 2kHz
for Circuit Shown in Figure 5a
LT1469
±10.24V
3
2
5
±10.24V
6
+
–
1
+
–
7
±10.24V
233818 F06
15k
REFIN
±10.24V
BANDGAP
REFERENCE
100nF
Figure 6. LT1469 Buffering a Fully Differential Signal Source
REFBUF
ADC REFERENCE
REFERENCE
BUFFER
6.5k
There are three ways of providing the ADC reference. The
first is to use both the internal reference and reference
buffer. The second is to externally overdrive the internal
reference and use the internal reference buffer. The third
is to disable the internal reference buffer and overdrive
the REFBUF pin from an external source. The following
tables give examples of these cases and the resulting
bipolar input ranges.
Table 1. Internal Reference with Internal Buffer
REFIN
REFBUF
BIPOLAR INPUT RANGE
2.048V
4.096V
±10.24V
Table 2. External Reference with Internal Buffer
REFIN
(OVERDRIVE)
REFBUF
BIPOLAR INPUT RANGE
1.25V (Min)
2.5V
±6.25V
2.048V
4.096V
±10.24V
2.4V (Max)
4.8V
±12V
47µF
6.5k
GND
LTC2338-18
233818 F07a
Figure 7a.LTC2338-18 Internal Reference Circuit
External Reference with Internal Buffer
If more accuracy and/or lower drift is desired, REFIN
can be easily overdriven by an external reference since
a 15k resistor is in series with the reference as shown
in Figure 7b. REFIN can be overdriven in the range from
1.25V to 2.4V. The resulting voltage at REFBUF will be
2 • REFIN. So the input range is ±5 • REFIN, as shown
in Table 2. Linear Technology offers a portfolio of high
performance references designed to meet the needs of
many applications. With its small size, low power, and high
accuracy, the LTC6655-2.048 is well suited for use with
the LTC2338-18 when overdriving the internal reference.
233818fa
For more information www.linear.com/LTC2338-18
13
LTC2338-18
APPLICATIONS INFORMATION
15k
REFIN
15k
REFIN
BANDGAP
REFERENCE
BANDGAP
REFERENCE
2.7µF
REFBUF
LTC6655-2.048
REFBUF
REFERENCE
BUFFER
REFERENCE
BUFFER
47µF
6.5k
6.5k
LTC6655-5
47µF
6.5k
GND
6.5k
LTC2338-18
GND
LTC2338-18
233818 F07b
233818 F07c
Figure7b. Using the LTC6655-2.048 as an External Reference
Figure 7c. Overdriving REFBUF Using the LTC6655-5
The LTC6655-2.048 offers 0.025% (max) initial accuracy
and 2ppm/°C (max) temperature coefficient for high precision applications. The LTC6655-2.048 is fully specified
over the H-grade temperature range and complements
the extended temperature range of the LTC2338-18 up to
125°C. Bypassing the LTC6655-2.048 with a 2.7μF to 100μF
ceramic capacitor close to the REFIN pin is recommended.
external reference must provide all of this charge with a
DC current equivalent to IREFBUF = QCONV/tCYC. Thus, the
DC current draw of REFBUF depends on the sampling rate
and output code. In applications where a burst of samples
is taken after idling for long periods, as shown in Figure 8,
IREFBUF quickly goes from approximately 390µA to a maximum of 1.2mA for REFBUF = 5V at 1Msps. 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 REFBUF will affect the accuracy of the output
code. If an external reference is used to overdrive REFBUF,
the fast settling LTC6655-5 reference is recommended.
External Reference Unbuffered
The internal reference buffer can also be overdriven from
2.5V to 5V with an external reference at REFBUF as shown
in Figure 7c. So the input ranges are ±6.25V to ±12.5V,
respectively, as shown in Table 3. To do so, REFIN must
be grounded to disable the reference buffer. A 13k resistor loads the REFBUF pin when the reference buffer is
disabled. To maximize the input signal swing and corresponding SNR, the LTC6655-5 is recommended when
overdriving REFBUF. The LTC6655-5 offers the same small
size, accuracy, drift and extended temperature range as
the LTC6655-2.048. By using a 5V reference, an SNR of
102dB can be achieved. Bypassing the LTC6655-5 with
a 47μF ceramic capacitor (X5R, 0805 size) close to the
REFBUF pin is recommended.
The REFBUF pin of the LTC2338-18 draws a charge (QCONV)
from the external bypass capacitor during each conversion
cycle. If the internal reference buffer is overdriven, the
Internal Reference Buffer Transient Response
For optimum transient performance, the internal reference
buffer should be used. The internal reference buffer uses a
proprietary design that results in an output voltage change
at REFBUF of less than 1LSB when responding to a sudden
burst of conversions. This makes the internal reference
buffer of the LTC2338-18 truly single-shot capable since the
first sample taken after idling will yield the same result as
a sample taken after the transient response of the internal
reference buffer has settled. Figure 9 shows the transient
responses of the LTC2338-18 with the internal reference
buffer and with the internal reference buffer overdriven by
the LTC6655-5, both with a bypass capacitance of 47μF.
CNV
IDLE
PERIOD
IDLE
PERIOD
233818 F08
Figure 8. CNV Waveform Showing Burst Sampling
233818fa
14
For more information www.linear.com/LTC2338-18
LTC2338-18
APPLICATIONS INFORMATION
0
INTERNAL REFERENCE BUFFER
0
–40
–2
–4
SNR = 100.3dB
THD = –117dB
SINAD = 100.2dB
SFDR = –119dB
–20
AMPLITUDE (dBFS)
DEVIATION FROM FINAL VALUE (LSB)
2
EXTERNAL SOURCE ON REFBUF
–60
–80
–100
–120
–140
–6
–160
–8
–180
0 100 200 300 400 500 600 700 800 900 1000
TIME (µs)
0
100
200
300
FREQUENCY (kHz)
233818 F09
400
500
233818 F10
Figure 9. Transient Response of the LTC2338-18
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 LTC2338-18 provides guaranteed tested
limits for both AC distortion and noise measurements.
Signal-to-Noise and Distortion Ratio (SINAD)
Figure 10. 32k Point FFT of the LTC2338-18
The out-of-band harmonics alias into the frequency band
between DC and half the sampling frequency (fSMPL/2).
THD is expressed as:
THD= 20log
V22 + V32 + V42 +…+ VN 2
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 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 band limited
to frequencies from above DC and below half the sampling
frequency. Figure 10 shows that the LTC2338-18 achieves
a typical SINAD of 100dB at a 1MHz sampling rate with
a 2kHz input.
POWER CONSIDERATIONS
Signal-to-Noise Ratio (SNR)
The LTC2338-18 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 LTC2338‑18
has a power-on reset (POR) circuit that will reset the
LTC2338-18 at initial power-up or whenever the power
supply voltage drops below 2V. Once the supply voltage
reenters the nominal supply voltage range, the POR will
re-initialize the ADC. No conversions should be initiated
until 100ms after a POR event to ensure the re-initialization
period has ended. Any conversions initiated before this
time will produce invalid results.
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 10 shows
that the LTC2338-18 achieves a typical SNR of 100dB at
a 1MHz sampling rate with a 2kHz input.
Total Harmonic Distortion (THD)
Total Harmonic Distortion (THD) is the ratio of the RMS sum
of all harmonics of the input signal to the fundamental itself.
The LTC2338-18 provides two power supply pins: the 5V
power supply (VDD), and the digital input/output interface
power supply (OVDD). The flexible OVDD supply allows the
LTC2338-18 to communicate with any digital logic operating between 1.8V and 5V, including 2.5V and 3.3V systems.
Power Supply Sequencing
233818fa
For more information www.linear.com/LTC2338-18
15
LTC2338-18
APPLICATIONS INFORMATION
12
CNV Timing
10
The LTC2338-18 conversion is controlled by CNV. A rising edge on CNV will start a conversion and power up
the LTC2338-18. Once a conversion has been initiated,
it cannot be restarted until the conversion is complete.
For optimum performance, CNV should be driven by a
clean low jitter signal. Converter status is indicated by the
BUSY output which remains high while the conversion is
in progress. To ensure that no errors occur in the digitized
results, any additional transitions on CNV should occur
within 40ns from the start of the conversion or after the
conversion has been completed. Once the conversion has
completed, the LTC2338-18 powers down and begins
acquiring the input signal.
Internal Conversion Clock
The LTC2338-18 has an internal clock that is trimmed to
achieve a maximum conversion time of 527ns. With a minimum acquisition time of 460ns, throughput performance
of 1Msps is guaranteed without any external adjustments.
Auto Nap Mode
The LTC2338-18 automatically enters nap mode after a
conversion has been completed and completely powers
up once a new conversion is initiated on the rising edge of
CNV. During nap mode, only the ADC core powers down
and all other circuits remain active. During nap, data from
the last conversion can be clocked out. The auto nap mode
feature will reduce the power dissipation of the LTC2338-18
as the sampling frequency is reduced. Since full power is
consumed only during a conversion, the ADC core of the
LTC2338-18 remains powered down for a larger fraction of
the conversion cycle (tCYC) at lower sample rates, thereby
reducing the average power dissipation which scales with
the sampling rate as shown in Figure 11.
Sleep Mode
The auto nap mode feature provides limited power savings
since only the ADC core powers down. To obtain greater power
savings, the LTC2338-18 provides a sleep mode. During sleep
mode, the entire part is powered down except for a small
standby current resulting in a power dissipation of 300μW.
SUPPLY CURRENT (mA)
TIMING AND CONTROL
VDD
8
6
4
2
OVDD
0
0 100 200 300 400 500 600 700 800 900 1000
SAMPLING FREQUENCY (kHz)
233818 F11
Figure 11. Power Supply Current of the LTC2338-18
Versus Sampling Rate.
To enter sleep mode, toggle CNV twice with no intervening
rising edge on SCK. The part will enter sleep mode on the
falling edge of BUSY from the last conversion initiated. Once
in sleep mode, a rising edge on SCK will wake the part up.
Upon emerging from sleep mode, wait tWAKE seconds before
initiating a conversion to allow the reference and reference
buffer to wake up and charge the bypass capacitors at REFIN
and REFBUF. (Refer to the Timing Diagrams section for more
detailed timing information about sleep mode.)
DIGITAL INTERFACE
The LTC2338-18 has a serial digital interface. The flexible
OVDD supply allows the LTC2338-18 to communicate with
any digital logic operating between 1.8V and 5V, including
2.5V and 3.3V systems.
The serial output data is clocked out on the SDO pin when
an external clock is applied to the SCK pin if SDO is enabled.
Clocking out the data after the conversion will yield the
best performance. With a shift clock frequency of at least
40MHz, a 1Msps throughput is still achieved. The serial
output data changes state on the rising edge of SCK and
can be captured on the falling edge or next rising edge of
SCK. D17 remains valid till the first rising edge of SCK.
The serial interface on the LTC2338-18 is simple and
straightforward to use. The following sections describe the
operation of the LTC2338-18. Several modes are provided
depending on whether a single or multiple ADCs share the
SPI bus or are daisy-chained.
233818fa
16
For more information www.linear.com/LTC2338-18
LTC2338-18
APPLICATIONS INFORMATION
Normal Mode, Single Device
shows a single LTC2338-18 operated in normal mode
with CHAIN and RDL/SDI tied to ground. With RDL/SDI
grounded, SDO is enabled and the MSB(D17) of the new
conversion data is available at the falling edge of BUSY.
This is the simplest way to operate the LTC2338-18.
When CHAIN = 0, the LTC2338-18 operates in normal
mode. In normal mode, RDL/SDI enables or disables the
serial data output pin SDO. If RDL/SDI is high, SDO is in
high impedance. If RDL/SDI is low, SDO is driven. Figure 12
CONVERT
DIGITAL HOST
CNV
CHAIN
LTC2338-18
RDL/SDI
BUSY
IRQ
SDO
DATA IN
SCK
CLK
233818 F13a
NAP AND ACQUIRE
CONVERT
NAP AND ACQUIRE
CHAIN = 0
RDL/SDI = 0
CONVERT
tCYC
tCNVH
tCNVL
CNV
tACQ = tCYC – tCONV – tBUSYLH
tCONV
BUSY
tACQ
tSCK
tBUSYLH
tSCKH
1
SCK
2
3
tHSDO
tDSDOBUSYL
SDO
tQUIET
16
17
18
tSCKL
tDSDO
D17
D16
D15
D1
D0
233818 F12
Figure 12. Using a Single LTC2338-18 in Normal Mode
233818fa
For more information www.linear.com/LTC2338-18
17
LTC2338-18
APPLICATIONS INFORMATION
Normal Mode, Multiple Devices
be used to allow only one LTC2338-18 to drive SDO at a
time in order to avoid bus conflicts. As shown in Figure 13,
the RDL/SDI inputs idle high and are individually brought
low to read data out of each device between conversions.
When RDL/SDI is brought low, the MSB of the selected
device is output onto SDO.
Figure 13 shows multiple LTC2338-18 devices operating
in normal mode (CHAIN = 0) sharing CNV, SCK and SDO.
By sharing CNV, SCK and SDO, the number of required
signals to operate multiple ADCs in parallel is reduced.
Since SDO is shared, the RDL/SDI input of each ADC must
RDLB
RDLA
CONVERT
CNV
CHAIN
CNV
CHAIN
LTC2338-18
SDO
B
BUSY
LTC2338-18
A
IRQ
DIGITAL HOST
SDO
RDL/SDI
RDL/SDI
SCK
SCK
DATA IN
CLK
233818 F13a
NAP AND
ACQUIRE
CONVERT
CONVERT
NAP AND ACQUIRE
CHAIN = 0
tCNVL
CNV
tCONV
BUSY
tBUSYLH
RDL/SDIA
RDL/SDIB
tSCK
SCK
1
2
tSCKH
3
16
17
18
tHSDO
SDO
Hi-Z
D17A
D16A
D15A
19
20
21
34
35
36
tSCKL
tDSDO
tEN
tQUIET
tDIS
D1A
D0A
Hi-Z
D17B
D16B
D15B
D1B
D0B
Hi-Z
233818 F13
Figure 13. Normal Mode with Multiple Devices Sharing CNV, SCK, and SDO
233818fa
18
For more information www.linear.com/LTC2338-18
LTC2338-18
APPLICATIONS INFORMATION
Chain Mode, Multiple Devices
may limit the number of lines needed to interface to a large
number of converters. Figure 14 shows an example with
two daisy-chained devices. The MSB of converter A will
appear at SDO of converter B after 18 SCK cycles. The
MSB of converter A is clocked in at the SDI/RDL pin of
converter B on the rising edge of the first SCK.
When CHAIN = OVDD, the LTC2338-18 operates in
chain mode. In chain mode, SDO is always enabled and
RDL/SDI serves as the serial data input pin (SDI) where
daisy-chain data output from another ADC can be input.
This is useful for applications where hardware constraints
CONVERT
OVDD
OVDD
CNV
CHAIN
RDL/SDI
CNV
CHAIN
LTC2338-18
RDL/SDI
SDO
A
DIGITAL HOST
LTC2338-18
IRQ
BUSY
B
DATA IN
SDO
SCK
SCK
CLK
233818 F14a
NAP AND
ACQUIRE
CONVERT
CONVERT
NAP AND ACQUIRE
CHAIN = OVDD
RDL/SDIA = 0
tCYC
tCNVL
CNV
BUSY
tCONV
tBUSYLH
tSCKCH
SCK
1
2
3
16
17
tSSDISCK
18
19
20
34
35
36
tSCKL
tHSDO
tHSDISCK
SDOA = RDL/SDIB
tQUIET
tSCKH
tDSDO
D17A
D16A
D15A
D1A
D0A
D17B
D16B
D15B
D1B
D0B
tDSDOBUSYL
SDOB
D17A
D16A
D1A
D0A
233818 F14
Figure 14. Chain Mode Timing Diagram
233818fa
For more information www.linear.com/LTC2338-18
19
LTC2338-18
APPLICATIONS INFORMATION
Sleep Mode
To enter sleep mode, toggle CNV twice with no intervening
rising edge on SCK as shown in Figure 15. The part will
enter sleep mode on the falling edge of BUSY from the
CHAIN = DON’T CARE
RDL/SDI = DON’T CARE
NAP AND
ACQUIRE
CONVERT
last conversion initiated. Once in sleep mode, a rising edge
on SCK will wake the part up. Upon emerging from sleep
mode, wait tWAKE seconds before initiating a conversion
to allow the reference and reference buffer to wake up
and charge the bypass capacitors at REFIN and REFBUF.
CONVERT
SLEEP
NAP AND
ACQUIRE
CONVERT
tWAKE
tCNVH
CNV
tCONV
BUSY
tCONV
tBUSYLH
SCK
CHAIN = DON’T CARE
RDL/SDI = DON’T CARE
CONVERT
SLEEP
NAP AND
ACQUIRE
CONVERT
tWAKE
tCNVH
CNV
tCONV
BUSY
tBUSYLH
SCK
233818 F15
Figure 15. Sleep Mode Timing Diagram
233818fa
20
For more information www.linear.com/LTC2338-18
LTC2338-18
BOARD LAYOUT
To obtain the best performance from the LTC2338-18 a
printed circuit board (PCB) is recommended. Layout for the
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 alongside
analog signals or underneath the ADC.
Recommended Layout
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 DC1908, the
evaluation kit for the LTC2338-18.
Partial Top Silkscreen
Partial Layer 1 Component Side
233818fa
For more information www.linear.com/LTC2338-18
21
LTC2338-18
BOARD LAYOUT
Partial Layer 2 Ground Plane
Partial Layer 3 Power Plane
Partial Layer 4 Bottom Layer
233818fa
22
For more information www.linear.com/LTC2338-18
R5
49.9
1206
BNC
R14 0 OHM
BNC
J6
BNC
C47
OPT
0603
C59
1.0uF
50V
R39 0 OHM
OPT
C61
C44
1.0uF
25V
C57 0.1uF 25V
V+
2
C60
0.1uF
25V
R6
1K
U8
4
5
6
R41
4.99K
2
3
-
+
U10A
C42
OPT
0603
V+
1
R3
OPT
C58
7
C49 100pF
R40 4.99K
U10B
LT1469CS8
V-
4
C2
0.1uF
OPT
R31
CLK
R32 0 OHM
OPT
R38
R35 0 OHM
0 OHM
R9
33 OHM
U2
NC7SVU04P5X
LT1469CS8
R33 0 OHM
2
5
3
C1
0.1uF
NC7SZ04P5X
R36
0 OHM
R34 OPT
C18
OPT
0603
C5
0.1uF
BYPASS CAPS FOR U10
C55 1.0uF 50V
J4
C43
0.1uF
25V
AIN+ / - 10.24V
V-
AIN+
+ / - 10.24V
CLK IN
100MHz MAX
3.3VPP
J1
R2
1K
8
5
3
+3.3V
OPT
R16
C19
OPT
0805
OPT
R15
C40
OPT
1206
C39
OPT
1206
SDO
SDO
5
4
*
+5V
4
3
2
1
TP1
IN-
IN+
OUTS
OUTF
GND
C10
0.1uF
C7
0.1uF
E3
VREF
4
6
5
8
10
12
14
7
9
11
13
DC590
2
3
J3
C17
0.1uF
RDL/SDI
BUSY
SDO
SCK
CNV
12
11
14
13
9
R1
CNV
CNV
TP3
SCK
SCK
TP2
B
BUSY
RDL
RDL
TP5
BUSY
SDO
SCK
2
3
A
U7
R4
2K
R17
1K
R13
A2
A1
A0
SCL
SDA
WP
4
3
2
1
6
5
7
+3.3V
2
33
31
DB2
DB3
DB15
DB14
DB13
DB12
2
PROG
WP
R10
4.99K
4
2
1
6
5
3
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
R12
4.99K
1
3
5
7
9
11
13
15
17
19
21
23
25
27
JP1
EEPROM
R11
4.99K
CLKOUT
0.1uF
DB11
C16 DB10
DB9
DB8
DB7
DB6
DB5
29
35
DB1
DB4
37
39
DB0
DB17
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
TP7
DB16
CNVST_33
TP6
C4 0.1uF
DB16
P1
CON-EDGE40-100
DB17
NC7SVU04P5X
U4
C3 0.1uF
33 OHM
DC590_DETECT
2
0.1uF
NC7SZ04P5X
U9
4
5
3
7
8
+3.3V
+3.3V
C15
Q
PR
VCC
D
2
Q
C14
0.1uF
4
1
CP
R8
33 OHM
CLR
GND
1
24LC024-I /ST
6
4
U3
NL17SZ74
OE
GND
VCC
5
C13 +3.3V
0.1uF
EXT
JP2
REF
INT
+3.3V
U6
NC7SZ66P5X
CNV
C56
0.1uF
2
C46
2.2uF
10V
33 OHM
C48 TP4
10uF
6.3V
C12
0.1uF
10V
1210
C20
47uF
R37
OPT
9V-10V
(NC) 1
R7
1K
5
6
7
8
VREF
GND
GND
LTC6655BHMS8-5
GND
VIN
SHDN
U1
LTC233X- CMS
C9
10uF
6.3V
C6
10uF
6.3V
+3.3V
C11
10uF
+8.6V
U15
2
VDD
1
3
+3.3V
1
15
OVDD
GND
GND
GND
3
6
16
7
8
VDDLBYP
REFBUF
REFIN
CHAIN
10
5
3
8
VCC
VSS
4
5
3
1
For more information www.linear.com/LTC2338-18
3
4
+
-
+3.3V
LTC2338-18
BOARD LAYOUT
Partial Schematic of Demoboard
233818fa
23
LTC2338-18
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/product/LTC2338-18#packaging for the most recent package drawings.
MS Package
16-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1669 Rev Ø)
0.889 ±0.127
(.035 ±.005)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
0.50
(.0197)
BSC
0.305 ±0.038
(.0120 ±.0015)
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
DETAIL “A”
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
4.90 ±0.152
(.193 ±.006)
0° – 6° TYP
0.280 ±0.076
(.011 ±.003)
REF
16151413121110 9
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
DETAIL “A”
0.18
(.007)
SEATING
PLANE
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
1234567 8
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MS16) 1107 REV Ø
233818fa
24
For more information www.linear.com/LTC2338-18
LTC2338-18
REVISION HISTORY
REV
DATE
DESCRIPTION
A
08/16
Updated graphs G01, G02, and G03
PAGE NUMBER
6
233818fa
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 representation that the interconnection
of itsinformation
circuits as described
herein will not infringe on existing patent rights.
For more
www.linear.com/LTC2338-18
25
LTC2338-18
TYPICAL APPLICATION
LT1469 Configured to Convert a ±10.24V Single-Ended Signal to a ±20.48V Differential Signal
15V
8
10.24V
0V
–10.24V
2
3
4.99k
6
5
–
+
1
5V
IN+
7
4
VDD
LTC2338-18
LT1469
–
+
4.99k
10.24V
0V
–10.24V
10.24V
0V
–10.24V
IN–
REFBUF
–15V
REFIN
47µF
100nF
233818 TA02
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
ADCs
LTC2379-18/LTC2378-18/ 18-Bit, 1.6Msps/1Msps/500ksps/250ksps
LTC2377-18/LTC2376-18 Serial, Low Power ADC
2.5V Supply, Differential Input, 101.2dB SNR, ±5V Input Range, DGC,
Pin-Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2380-16/LTC2378-16/ 16-Bit, 2Msps/1Msps/500ksps/250ksps
LTC2377-16/LTC2376-16 Serial, Low Power ADC
2.5V Supply, Differential Input, 96.2dB SNR, ±5V Input Range, DGC,
Pin-Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2369-18/LTC2368-18/ 18-Bit, 1.6Msps/1Msps/500ksps/250ksps
LTC2367-18/LTC2364-18 Serial, Low Power ADC
2.5V Supply, Pseudo-Differential Unipolar Input, 96.5dB SNR, 0V to 5V Input
Range, Pin-Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2370-16/LTC2368-16/ 16-Bit, 2Msps/1Msps/500ksps/250ksps
LTC2367-16/LTC2364-16 Serial, Low Power ADC
2.5V Supply, Pseudo-Differential Unipolar Input, 94dB SNR, 0V to 5V Input
Range, Pin-Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2389-18/LTC2389-16 18-Bit/16-Bit, 2.5Msps Parallel/Serial ADC
5V Supply, Pin-Configurable Input Range, 99.8dB/96dB SNR, Parallel or Serial
I/O 7mm × 7mm LQFP-48 and QFN-48 Packages
LTC1609
16-Bit, 200ksps Serial ADC
±10V, Configurable Unipolar/Bipolar Input, Single 5V Supply, SSOP-28 and
SO-20 Packages
LTC1606/LTC1605
16-Bit, 250ksps/100ksps Parallel ADCs
±10V Input, 5V Supply, 75mW/55mW, SSOP-28 and SO-28 Packages
LTC1859/LTC1858/
LTC1857
16-/14-/12-Bit, 8-Channel 100ksps Serial
ADCs
±10V, SoftSpan™, Single-Ended or Differential Inputs, Single 5V Supply,
SSOP-28 Package
LTC2756/LTC2757
18-Bit, Single Serial/Parallel IOUT SoftSpan
DAC
±1LSB INL/DNL, Software-Selectable Ranges, SSOP-28/7mm × 7mm LQFP-48
Package
LTC2641
16-Bit/14-Bit/12-Bit Single Serial VOUT DAC
±1LSB INL /DNL, MSOP-8 Package, 0V to 5V Output
LTC2630
12-Bit/10-Bit/8-Bit Single VOUT DACs
Internal Reference, ±1LSB INL (12 Bits), SC70 6-Pin Package
LTC6655
Precision Low Drift Low Noise Buffered
Reference
5V/2.5V/2.048V/1.2V, 2ppm/°C, 0.25ppm Peak-to-Peak Noise, MSOP-8 Package
LTC6652
Precision Low Drift Low Noise Buffered
Reference
5V/2.5V/2.048V/1.2V, 5ppm/°C, 2.1ppm Peak-to-Peak Noise, MSOP-8 Package
DACs
References
Amplifiers
LT1468/LT1469
Single/Dual 90MHz, 22V/μs, 16-Bit Accurate Low Input Offset: 75μV/125µV
Op Amp
233818fa
26 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC2338-18
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LTC2338-18
LT 0816 REV A • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2013
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