LTC2308
Low Noise, 500ksps,
8-Channel, 12-Bit ADC
FEATURES
DESCRIPTION
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The LTC®2308 is a low noise, 500ksps, 8-channel, 12-bit
ADC with an SPI/MICROWIRE compatible serial interface.
This ADC includes an internal reference and a fully differential sample-and-hold circuit to reduce common mode
noise. The internal conversion clock allows the external
serial output data clock (SCK) to operate at any frequency
up to 40MHz.
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12-Bit Resolution
500ksps Sampling Rate
Low Noise: SINAD = 73.3dB
Guaranteed No Missing Codes
Single 5V Supply
Auto-Shutdown Scales Supply Current with Sample
Rate
Low Power: 17.5mW at 500ksps
0.9mW Nap Mode
35μW Sleep Mode
Internal Reference
Internal 8-Channel Multiplexer
Internal Conversion Clock
SPI/MICROWIRETM Compatible Serial Interface
Unipolar or Bipolar Input Ranges (Software Selectable)
Separate Output Supply OVDD (2.7V to 5.25V)
24-Pin 4mm × 4mm QFN Package
The LTC2308 operates from a single 5V supply and draws
just 3.5mA at a sample rate of 500ksps. The auto-shutdown
feature reduces the supply current to 200μA at a sample
rate of 1ksps.
The LTC2308 is packaged in a small 24-pin 4mm × 4mm
QFN. The internal 2.5V reference and 8-channel multiplexer
further reduce PCB board space requirements.
The low power consumption and small size make the
LTC2308 ideal for battery operated and portable applications, while the 4-wire SPI compatible serial interface
makes this ADC a good match for isolated or remote data
acquisition systems.
APPLICATIONS
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High Speed Data Acquisition
Industrial Process Control
Motor Control
Accelerometer Measurements
Battery Operated Instruments
Isolated and/or Remote Data Acquisition
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
5V
10μF
AVDD
CH0
8192 Point FFT, fIN = 1kHz
0.1μF
10μF
OVDD
DVDD
LTC2308
CH1
CH2
CH3
CH0-CH7
CH4
ANALOG INPUTS
0V TO 4.096V UNIPOLAR
CH5
±2.048V BIPOLAR
CH6
2.7V TO 5.25 V
0.1μF
SDI
ANALOG
INPUT
MUX
+
–
12-BIT
500ksps
ADC
SERIAL
PORT
SERIAL DATA LINK TO
ASIC, PLD, MPU, DSP
OR SHIFT REGISTER
SDO
SCK
CONVST
VREF
CH7
INTERNAL
2.5V REF
COM
2.2μF
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
–140
fSMPL = 500kHz
SINAD = 73.6dB
THD = –89.5dB
MAGNITUDE (dB)
0.1μF
0
REFCOMP
GND
0.1μF
10μF
50
150
100
FREQUENCY (kHz)
200
250
2308 TA01b
2308 TA01
2308fb
1
LTC2308
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Notes 1, 2)
OVDD
GND
DVDD
CH0
CH2
CH1
TOP VIEW
Supply Voltage (AVDD, DVDD, OVDD) ...........................6V
Analog Input Voltage (Note 3)
CH0-CH7, COM, REF,
REFCOMP ...................(GND – 0.3V) to (AVDD + 0.3V)
Digital Input Voltage
(Note 3).......................... (GND – 0.3V) to (DVDD + 0.3V)
Digital Output Voltage .... (GND – 0.3V) to (OVDD + 0.3V)
Power Dissipation ...............................................500mW
Operating Temperature Range
LTC2308C ................................................ 0°C to 70°C
LTC2308I.............................................. –40°C to 85°C
Storage Temperature Range................... –65°C to 150°C
24 23 22 21 20 19
CH3 1
18 GND
CH4 2
17 SD0
CH5 3
16 SCK
25
CH6 4
15 SDI
AVDD
9 10 11 12
GND
8
GND
7
GND
13 AVDD
VREF
14 CONVST
COM 6
REFCOMP
CH7 5
UF PACKAGE
24-LEAD (4mm s 4mm) PLASTIC QFN
TJMAX = 150°C, θJA = 37°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC2308CUF#PBF
LTC2308CUF#TRPBF
2308
24-Lead (4mm × 4mm) Plastic QFN
0°C to 70°C
LTC2308IUF#PBF
LTC2308IUF#TRPBF
2308
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
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/
CONVERTER AND MULTIPLEXER CHARACTERISTICS
The l denotes the specifications
which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 4, 5)
PARAMETER
CONDITIONS
MIN
l
Resolution (No Missing Codes)
TYP
MAX
12
UNITS
Bits
l
±0.3
±1
LSB
Differential Linearity Error
l
±0.25
±1
LSB
Bipolar Zero Error
l
±1
±6
Integral Linearity Error
(Note 6)
(Note 7)
Bipolar Zero Error Drift
0.002
Bipolar Zero Error Match
Unipolar Zero Error
(Note 7)
l
±0.3
±3
LSB
l
±0.5
±3
LSB
Unipolar Zero Error Drift
Unipolar Zero Error Match
LSB
LSB/°C
0.002
l
±0.3
LSB/°C
±2
LSB
2308fb
2
LTC2308
CONVERTER AND MULTIPLEXER CHARACTERISTICS
The l denotes the specifications
which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Notes 4, 5)
PARAMETER
CONDITIONS
Bipolar Full-Scale Error
External Reference (Note 8)
Bipolar Full-Scale Error Drift
External Reference
MIN
l
External Reference (Note 8)
Unipolar Full-Scale Error Drift
External Reference
MAX
±1
±9
0.05
Bipolar Full-Scale Error Match
Unipolar Full-Scale Error
TYP
LSB
LSB/°C
l
±0.5
±3
LSB
l
±1.5
±8
LSB
0.05
l
Unipolar Full-Scale Error Match
UNITS
LSB/°C
±0.4
±3
LSB
ANALOG INPUT
The l denotes the specifications which apply over the full operating temperature range, otherwise
specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
VIN
+
Absolute Input Range (CH0 to CH7)
VIN–
Absolute Input Range (CH0 to CH7,
COM)
VIN+ – VIN–
Input Differential Voltage Range
IIN
Analog Input Leakage Current
CIN
Analog Input Capacitance
CMRR
Input Common Mode Rejection Ratio
CONDITIONS
MIN
TYP
MAX
UNITS
(Note 9)
l
–0.05
REFCOMP
V
Unipolar (Note 9)
Bipolar (Note 9)
l
l
–0.05
–0.05
0.25 • REFCOMP
0.75 • REFCOMP
V
V
VIN = VIN+ – VIN– (Unipolar)
VIN = VIN+ – VIN– (Bipolar)
l
l
0 to REFCOMP
±REFCOMP/2
l
V
V
±1
Sample Mode
Hold Mode
μA
55
5
pF
pF
70
dB
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, 10)
SYMBOL
PARAMETER
CONDITIONS
SINAD
Signal-to-(Noise + Distortion) Ratio
fIN = 1kHz
MIN
TYP
l
71
73.3
71
73.4
SNR
Signal-to-Noise Ratio
fIN = 1kHz
l
THD
Total Harmonic Distortion
fIN = 1kHz, First 5 Harmonics
l
SFDR
Spurious Free Dynamic Range
fIN = 1kHz
l
Channel-to-Channel Isolation
Full Linear Bandwidth
–90
UNITS
dB
dB
–78
dB
90
dB
fIN = 1kHz
–109
dB
(Note 11)
700
kHz
–3dB Input Linear Bandwidth
25
MHz
Aperture Delay
13
ns
240
ns
Transient Reponse
Full-Scale Step
80
MAX
2308fb
3
LTC2308
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)
PARAMETER
CONDITIONS
VREF Output Voltage
IOUT = 0
VREF Output Tempco
IOUT = 0
VREF Output Impedance
–0.1mA ≤ IOUT ≤ 0.1mA
VREFCOMP Output Voltage
IOUT = 0
VREF Line Regulation
AVDD = 4.75V to 5.25V
l
MIN
TYP
MAX
UNITS
2.47
2.50
2.53
V
±25
ppm/°C
8
kΩ
4.096
V
0.8
mV/V
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
TYP
MAX
UNITS
VIH
High Level Input Voltage
DVDD = 5.25V
l
VIL
Low Level Input Voltage
DVDD = 4.75V
l
0.8
V
IIN
High Level Input Current
VIN = VDD
l
±10
μA
CIN
Digital Input Capacitance
VOH
High Level Output Voltage
VOL
Low Level Input Voltage
OVDD = 4.75V, IOUT = –10μA
OVDD = 4.75V, IOUT = –200μA
l
OVDD = 4.75V, IOUT = 160μA
OVDD = 4.75V, IOUT = 1.6mA
l
l
2.4
V
5
pF
4.74
V
V
4
0.05
0.4
V
V
±10
μA
IOZ
Hi-Z Output Leakage
VOUT = 0V to OVDD, CONVST High
COZ
Hi-Z Output Capacitance
CONVST High
15
ISOURCE
Output Source Current
VOUT = 0V
–10
mA
ISINK
Output Sink Current
VOUT = OVDD
10
mA
pF
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
AVDD
DVDD
CONDITIONS
MIN
TYP
MAX
UNITS
Analog Supply Voltage
4.75
5
5.25
V
Digital Supply Voltage
4.75
5
5.25
V
OVDD
Output Driver Supply Voltage
2.7
5.25
V
IDD
Supply Current
Nap Mode
Sleep Mode
4.2
400
20
mA
μA
μA
PD
Power Dissipation
Nap Mode
Sleep Mode
CL = 25pF
CONVST = 5V, Conversion Done
CONVST = 5V, Conversion Done
l
l
l
3.5
180
7
17.5
0.9
35
mW
mW
μW
2308fb
4
LTC2308
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
MAX
UNITS
fSMPL(MAX)
Maximum Sampling Frequency
CONDITIONS
l
MIN
500
kHz
fSCK
Shift Clock Frequency
l
40
MHz
tWHCONV
CONVST High Time
tHD
Hold Time SDI After SCK↑
tSUDI
Setup Time SDI Valid Before SCK↑
l
0
ns
tWHCLK
SCK High Time
fSCK = fSCK(MAX)
l
10
ns
tWLCLK
SCK Low Time
fSCK = fSCK(MAX)
l
10
ns
tWLCONVST
CONVST Low Time During Data Transfer
(Note 9)
l
410
ns
tHCONVST
Hold Time CONVST Low After Last SCK↓
(Note 9)
l
20
ns
(Note 9)
TYP
l
20
ns
l
2.5
ns
l
tCONV
Conversion Time
tACQ
Acquisition Time
7th SCK↑ to CONVST↑ (Note 9)
tREFWAKE
REFCOMP Wakeup Time (Note 12)
CREFCOMP = 10μF, CREF = 2.2μF
tdDO
SDO Data Valid After SCK↓
CL = 25pF (Note 9)
l
thDO
SDO Hold Time After SCK↓
CL = 25pF
l
l
1.3
1.6
240
μs
ns
200
10.8
ms
12.5
4
ns
ns
ten
SDO Valid After CONVST↓
CL = 25pF
l
11
15
ns
tdis
Bus Relinquish Time
CL = 25pF
l
11
15
ns
tr
SDO Rise Time
CL = 25pF
4
ns
tf
SDO Fall Time
CL = 25pF
4
ns
tCYC
Total Cycle Time
2
μs
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All voltage values are with respect to ground with AVDD, DVDD and
OVDD wired together (unless otherwise noted).
Note 3: When these pin voltages are taken below ground or above VDD,
they will be clamped by internal diodes. These products can handle input
currents greater than 100mA below ground or above VDD without latchup.
Note 4: AVDD = 5V, DVDD = 5V, OVDD = 5V, fSMPL = 500kHz, internal
reference unless otherwise specified.
Note 5: Linearity, offset and full-scale specifications apply for a singleended analog input with respect to COM.
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 and 1111 1111
1111. Unipolar zero error is the offset voltage measured from +0.5LSB
when the output code flickers between 0000 0000 0000 and
0000 0000 0001.
Note 8: 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. Unipolar full-scale error is the deviation of the last code
transition from ideal and includes the effect of offset error.
Note 9: Guaranteed by design, not subject to test.
Note 10: All specifications in dB are referred to a full-scale ±2.048V input
with a 2.5V reference voltage.
Note 11: Full linear bandwidth is defined as the full-scale input frequency
at which the SINAD degrades to 60dB or 10 bits of accuracy.
Note 12: REFCOMP wakeup time is the time required for the REFCOMP pin
to settle within 0.5LSB at 12-bit resolution of its final value after waking up
from SLEEP mode.
2308fb
5
LTC2308
TYPICAL PERFORMANCE CHARACTERISTICS
fSMPL = 500ksps, Internal Reference, unless otherwise noted.
Differential Nonlinearity vs
Output Code
1.00
0.75
0.75
0.50
0.50
0.25
0.25
0
–0.25
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
–140
0
–0.25
–0.50
–0.50
–0.75
–0.75
–1.00
1kHz Sine Wave
8192 Point FFT Plot
0
1024
2048
3072
4096
–1.00
SNR = 73.7dB
SINAD = 73.6dB
THD = –89.5dB
MAGNITUDE (dB)
1.00
DNL (LSB)
INL (LSB)
Integral Nonlinearity vs
Output Code
TA = 25°C, AVDD = DVDD = OVDD = 5V,
0
OUTPUT CODE
1024
2048
3072
0
4096
50
OUTPUT CODE
2308 G01
150
100
FREQUENCY (kHz)
SNR vs Input Frequency
–60
–70
250
2308 G03
2308 G02
Crosstalk vs Frequency for
an Adjacent Pair
200
SINAD vs Input Frequency
80
80
75
75
70
70
–100
–110
SINAD (dB)
–90
SNR (dB)
CROSSTALK (dB)
–80
65
65
60
60
55
55
–120
–130
–140
0.1
50
1
10
100
FREQUENCY (kHz)
50
1
1000
10
100
FREQUENCY (kHz)
3208 G04
1
1000
10
100
FREQUENCY (kHz)
3208 G05
3208 G06
Supply Current vs
Sampling Frequency
THD vs Input Frequency
–60
3.5
–65
3.0
1000
Supply Current vs Temperature
5
SUPPLY CURRENT (mA)
THD (dB)
–75
–80
–85
–90
SUPPLY CURRENT (mA)
4
–70
2.5
2.0
1.5
1.0
3
2
1
0.5
–95
0
–100
1
10
100
FREQUENCY (kHz)
1000
3208 G07
1
10
100
SAMPLING FREQUENCY (ksps)
1000
3208 G08
0
–50
–25
50
25
0
75
TEMPERATURE (oC)
100
125
3208 G09
2308fb
6
LTC2308
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, AVDD = DVDD = OVDD = 5V,
fSMPL = 500ksps, Internal Reference, unless otherwise noted.
Analog Input Leakage Current vs
Temperature
10
1000
8
800
LEAKAGE CURRENT (nA)
SLEEP CURRENT (μA)
Sleep Current vs Temperature
6
4
2
fSMPL = 0ksps
600
CH (ON)
400
CH (OFF)
200
0
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
0
–50
125
–25
50
25
0
75
TEMPERATURE (°C)
3208 G10
Offset vs Temperature
Full-Scale Error vs Temperature
4
FULL-SCALE ERROR (LSB)
OFFSET (LSB)
125
3208 G11
1.5
BIPOLAR
1.0
UNIPOLAR
0.5
EXTERNAL REFERENCE
0
–50
100
–25
75
0
25
50
TEMPERATURE (°C)
100
125
2308 G12
2
BIPOLAR
0
UNIPOLAR
–2
–4
–6
–50
EXTERNAL REFERENCE
–25
50
25
0
75
TEMPERATURE (°C)
100
125
2308 G13
PIN FUNCTIONS
CH3-CH7 (Pins 1, 2, 3, 4, 5): Channel 3 to Channel 7
Analog Inputs. CH3-CH7 can be configured as singleended or differential input channels. See the Analog Input
Multiplexer section.
capacitor. The internal reference may be over driven by an
external 2.5V reference at this pin.
COM (Pin 6): Common Input. This is the reference point
for all single-ended inputs. It must be free of noise and
connected to ground for unipolar conversions and midway
between GND and REFCOMP for bipolar conversions.
REFCOMP (Pin 8): Reference Buffer Output. Bypass to
GND with a 10μF tantalum and 0.1μF ceramic capacitor
in parallel. Nominal output voltage is 4.096V. The internal
reference buffer driving this pin is disabled by grounding
VREF , allowing REFCOMP to be overdriven by an external
source (see Figure 6c).
VREF (Pin 7): 2.5V Reference Output. Bypass to GND with
a minimum 2.2μF tantalum capacitor or low ESR ceramic
GND (Pins 9, 10, 11, 18, 20): Ground. All GND pins must
be connected to a solid ground plane.
2308fb
7
LTC2308
PIN FUNCTIONS
AVDD (Pins 12, 13): 5V Analog Supply. The range of AVDD is
4.75V to 5.25V. Bypass AVDD to GND with a 0.1μF ceramic
and a 10μF tantalum capacitor in parallel.
SDO (Pin 17): Serial Data Out. SDO outputs the data from
the previous conversion. SDO is shifted out serially on the
falling edge of each SCK pulse.
CONVST (Pin 14): Conversion Start. A rising edge at
CONVST begins a conversion. For best performance, ensure
that CONVST returns low within 40ns after the conversion
starts or after the conversion ends.
OVDD (Pin 19): Output Driver Supply. Bypass OVDD to
GND with a 0.1μF ceramic capacitor close to the pin. The
range of OVDD is 2.7V to 5.25V.
SDI (Pin 15): Serial Data Input. The SDI serial bit stream
configures the ADC and is latched on the rising edge of
the first 6 SCK pulses.
SCK (Pin 16): Serial Data Clock. SCK synchronizes the
serial data transfer. The serial data input at SDI is latched
on the rising edge of SCK. The serial data output at SDO
transitions on the falling edge of SCK.
DVDD (Pin 21): 5V Digital Supply. The range of DVDD is
4.75V to 5.25V. Bypass DVDD to GND with a 0.1 μF ceramic
and a 10μF tantalum capacitor in parallel.
CH0-CH2 (Pins 22, 23, 24): Channel 0 to Channel 2
Analog Inputs. CH0-CH2 can be configured as singleended or differential input channels. See the Analog Input
Multiplexer section.
GND (Pin 25): Exposed Pad Ground. Must be soldered
directly to ground plane.
BLOCK DIAGRAM
AVDD
DVDD
OVDD
LTC2308
CH0
CH1
CH2
CH3
CH4
SDI
ANALOG
INPUT
MUX
+
–
12-BIT
500ksps
ADC
SERIAL
PORT
CH5
SDO
SCK
CONVST
CH6
CH7
INTERNAL
2.5V REF
COM
VREF
8k
GAIN = 1.6384x
REFCOMP
2308 BD
GND
2308fb
8
LTC2308
TEST CIRCUIT
Load Circuit for tdis WAVEFORM 1
Load Circuit for tdis WAVEFORM 2, ten
VDD
3k
SDO
SDO
TEST POINT
TEST POINT
CL
3k
CL
2308 TC02
2308 TC01
TIMING DIAGRAM
Voltage Waveforms for SDO Delay Times, tdDO and thDO
tWLCLK (SCK Low Time)
tWHCLK (SCK High Time)
tHD (Hold Time SDI After SCK↑)
tSUDI (Setup Time SDI Stable Before SCK↑)
SCK
VIL
tdDO
tWLCLK
thDO
VOH
tWHCLK
SCK
SDO
VOL
tHD
2308 TD01
SDI
tSUDI
2308 TD03
Voltage Waveforms for tdis
Voltage Waveforms for ten
VIH
CONVST
CONVST
SDO
WAVEFORM 1
(SEE NOTE 1)
90%
SDO
tdis
SDO
WAVEFORM 2
(SEE NOTE 2)
2308 TD04
ten
10%
NOTE 1: WAVEFORM 1 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH
THAT THE OUTPUT IS HIGH UNLESS DISABLED BY THE OUTPUT CONTROL
NOTE 2: WAVEFORM 2 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH
THAT THE OUTPUT IS LOW UNLESS DISABLED BY THE OUTPUT CONTROL
Voltage Waveforms for SDO Rise and Fall Times tr, tf
2308 TD02
VOH
SDO
VOL
tr
tf
2308 TD05
2308fb
9
LTC2308
APPLICATIONS INFORMATION
Overview
The LTC2308 is a low noise, 500ksps, 8-channel, 12-bit
successive approximation register (SAR) A/D converter.
The LTC2308 includes a precision internal reference, a
configurable 8-channel analog input multiplexer (MUX)
and an SPI-compatible serial port for easy data transfers.
The ADC may be configured to accept single-ended or
differential signals and can operate in either unipolar or
bipolar mode. A sleep mode option is also provided to
save power during inactive periods.
Conversions are initiated by a rising edge on the CONVST
input. Once a conversion cycle has begun, it cannot be
restarted. Between conversions, a 6-bit input word (DIN)
at the SDI input configures the MUX and programs various modes of operation. As the DIN bits are shifted in,
data from the previous conversion is shifted out on SDO.
After the 6 bits of the DIN word have been shifted in, the
ADC begins acquiring the analog input in preparation for
the next conversion as the rest of the data is shifted out.
The acquire phase requires a minimum time of 240ns
for the sample-and-hold capacitors to acquire the analog
input signal.
During the conversion, the internal 12-bit capacitive
charge-redistribution DAC output is sequenced through a
successive approximation algorithm by the SAR starting
from the most significant bit (MSB) to the least significant
bit (LSB). The sampled input is successively compared
with binary weighted charges supplied by the capacitive
DAC using a differential comparator. At the end of a conversion, the DAC output balances the analog input. The SAR
contents (a 12-bit data word) that represent the sampled
analog input are loaded into 12 output latches that allow
the data to be shifted out.
Programming the LTC2308
The various modes of operation of the LTC2308 are
programmed by a 6-bit DIN word. The SDI data bits are
loaded on the rising edge of SCK, with the S/D bit loaded
on the first rising edge and the SLP bit on the sixth rising
edge (see Figure 8 in the Timing and Control section). The
input data word is defined as follows:
S/D
O/S
S1
S0
UNI
SLP
S/D = SINGLE-ENDED/DIFFERENTIAL BIT
O/S = ODD/SIGN BIT
S1 = ADDRESS SELECT BIT 1
S0 = ADDRESS SELECT BIT 0
UNI = UNIPOLAR/BIPOLAR BIT
SLP = SLEEP MODE BIT
Analog Input Multiplexer
The analog input MUX is programmed by the S/D, O/S,
S1 and S0 bits of the DIN word. Table 1 lists the MUX
configurations for all combinations of the configuration
bits. Figure 1a shows several possible MUX configurations
and Figure 1b shows how the MUX can be reconfigured
from one conversion to the next.
Table 1. Channel Configuration
S/D O/S S1
S0
0
1
+
–
0
0
0
0
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
0
1
1
0
0
1
1
1
1
0
0
0
1
0
0
1
1
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
–
2
3
+
–
4
5
+
–
6
7
+
–
–
+
COM
+
–
+
–
+
+
–
+
–
+
–
+
–
+
–
+
–
+
–
+
–
2308fb
10
LTC2308
APPLICATIONS INFORMATION
4 Differential
+ (–)
– (+) {
+ (–)
– (+) {
CH0
CH1
+ (–)
– (+) {
+ (–)
– (+) {
CH4
CH5
8 Single-Ended
+
+
+
+
+
+
+
+
CH2
CH3
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
CH6
CH7
Bipolar Mode
COM
COM
REFCOMP/2
COM (–)
+
–{
CH0
CH1
–
+{
+
+
+
+
CH2
CH3
Input Filtering
2308 F01a
Figure 1a. Example MUX Configurations
2nd Conversion
CH2
CH3
–
+
CH4
CH5
+
+
COM
(UNUSED)
2308 F02
mode. In conversion mode, the analog inputs draw only
a small leakage current. If the source impedance of the
driving circuit is low, the ADC inputs can be driven directly.
Otherwise, more acquisition time should be allowed for a
source with higher impedance.
CH4
CH5
CH6
CH7
COM (–)
1st Conversion
+
–
Figure 2. Driving COM in UNIPOLAR and BIPOLAR Modes
Combinations of Differential
and Single-Ended
+
–{
+
–{
Unipolar Mode
{
CH2
CH3
{
CH4
CH5
COM (–)
2308 F01b
Figure 1b. Changing the MUX Assignment “On the Fly”
Driving the Analog Inputs
The analog inputs of the LTC2308 are easy to drive. Each
of the analog inputs can be used as a single-ended input
relative to the COM pin (CH0-COM, CH1-COM, etc.) or in
differential input pairs (CH0 and CH1, CH2 and CH3, CH4
and CH5, CH6 and CH7). Figure 2 shows how to drive COM
for single-ended inputs in unipolar and bipolar modes.
Regardless of the MUX configuration, the “+” and “–”
inputs are sampled at the same instant. Any unwanted
signal that is common to both inputs will be reduced by
the common mode rejection of the sample-and-hold circuit.
The inputs draw only one small current spike while charging the sample-and-hold capacitors during the acquire
The noise and distortion of the input amplifier and other
circuitry must be considered since they will add to the
ADC noise and distortion. Therefore, noisy input circuitry
should be filtered prior to the analog inputs to minimize
noise. A simple 1-pole RC filter is sufficient for many
applications.
The analog inputs of the LTC2308 can be modeled as
a 55pF capacitor (CIN) in series with a 100Ω resistor
(RON) as shown in Figure 3a. CIN gets switched to the
selected input once during each conversion. Large filter
RC time constants will slow the settling of the inputs. It
is important that the overall RC time constants be short
enough to allow the analog inputs to completely settle to
12-bit resolution within the acquisition time (tACQ) if DC
accuracy is important.
When using a filter with a large CFILTER value (e.g. 1μF),
the inputs do not completely settle and the capacitive input
switching currents are averaged into a net DC current
(IDC). In this case, the analog input can be modeled by an
equivalent resistance (REQ = 1/(fSMPL • CIN)) in series with
an ideal voltage source (VREFCOMP/2) as shown in Figure 3b.
The magnitude of the DC current is then approximately
IDC = (VIN – VREFCOMP/2)/REQ, which is roughly proportional to VIN. To prevent large DC drops across the resistor
RFILTER, a filter with a small resistor and large capacitor
should be chosen. When running at the minimum cycle
2308fb
11
LTC2308
APPLICATIONS INFORMATION
INPUT
CH0-CH7
RSOURCE
LTC2308
RON
100Ω
ANALOG
INPUT
50Ω
CH0
LTC2308
VIN
2000pF
CIN
55pF
C1
COM
2308 F03a
REFCOMP
10μF
0.1μF
2308 F04a
Figure 3a. Analog Input Equivalent Circuit
Figure 4a. Optional RC Input Filtering for Single-Ended Input
RFILTER
IDC
INPUT
CH0-CH7
LTC2308
VIN
CFILTER
+
–
1000pF
50Ω
REQ
1/(fSMPL • CIN)
CH0
DIFFERENTIAL
ANALOG
INPUTS
VREFCOMP/2
LTC2308
1000pF
50Ω
CH1
1000pF
2308 F03b
Figure 3b. Analog Input Equivalent Circuit
for Large Filter Capacitances
Figures 4a and 4b show respective examples of input
filtering for single-ended and differential inputs. For the
single-ended case in Figure 4a, a 50Ω source resistor
and a 2000pF capacitor to ground on the input will limit
the input bandwidth to 1.6MHz. High quality capacitors
and resistors should be used in the RC filter 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.
Dynamic Performance
FFT (Fast Fourier Transform) test 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.
0.1μF
2308 F04b
Figure 4b. Optional RC Input Filtering for Differential Inputs
Signal-to-Noise and Distortion Ratio (SINAD)
The signal-to-noise and distortion ratio (SINAD) is the
ratio between the RMS amplitude of the fundamental input
frequency to 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 5 shows a typical SINAD of 73.3dB with
a 500kHz sampling rate and a 1kHz input. A SNR of 73.4dB
can be achieved with the LTC2308.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
–140
MAGNITUDE (dB)
time of 2μs, the input current equals 106μA at VIN = 5V,
which amounts to a full-scale error of 0.5LSBs when using
a filter resistor (RFILTER) of 4.7Ω. Applications requiring
lower sample rates can tolerate a larger filter resistor for
the same amount of full-scale error.
REFCOMP
10μF
0
50
150
100
FREQUENCY (kHz)
200
250
2308 TA01b
Figure 5. 1kHz Sine Wave 8192 Point FFT Plot
2308fb
12
LTC2308
APPLICATIONS INFORMATION
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 out-of-band harmonics alias into the frequency
band between DC and half the sampling frequency(fSMPL/2).
THD is expressed as:
R1
8k
VREF
2.5V
BANDGAP
REFERENCE
2.2μF
REFCOMP
4.096V
REFERENCE
AMP
10μF
R2
THD = 20 log
V22 + V32 + V42... + VN2
0.1μF
R3
GND
LTC2308
V1
2308 F06a
where V1 is the RMS amplitude of the fundamental frequency and V2 through VN are the amplitudes of the second
through Nth harmonics.
Internal Reference
The LTC2308 has an on-chip, temperature compensated
bandgap reference that is factory trimmed to 2.5V (Refer
to Figure 6a). It is internally connected to a reference
amplifier and is available at VREF (Pin 7). VREF should
be bypassed to GND with a 2.2μF tantalum capacitor
to minimize noise. An 8k resistor is in series with the
output so that it can be easily overdriven by an external
reference if more accuracy and/or lower drift are required
as shown in Figure 6b. The reference amplifier gains the
VREF voltage by 1.638 to 4.096V at REFCOMP (Pin 8). To
compensate the reference amplifier, bypass REFCOMP
with a 10μF ceramic or tantalum capacitor in parallel with
a 0.1μF ceramic capacitor for best noise performance. The
internal reference buffer can also be overdriven from 1V
to AVDD with an external reference at REFCOMP as shown
in Figure 6c. To do so VREF must be grounded to disable
the reference buffer. This will result in an input range of
0V to VREFCOMP in unipolar mode and ±0.5 • VREFCOMP in
bipolar mode.
Figure 6a. LTC2308 Reference Circuit
5V
0.1MF
VIN
LT1790A-2.5
VOUT
VREF
2.2μF
LTC2308
REFCOMP
+
10μF
0.1μF
GND
2308 F06b
Figure 6b. Using the LT1790A-2.5 as an External Reference
5V
VREF
VIN
LT1790A-4.096
VOUT
LTC2308
REFCOMP
+
10μF
0.1μF
GND
2308 F06c
Figure 6c. Overdriving REFCOMP Using the LT1790A-4.096
Digital Interface
Internal Conversion Clock
The internal conversion clock is factory trimmed to
achieve a typical conversion time (tCONV) of 1.3μs and a
maximum conversion time of 1.6μs over the full operating temperature range. With a typical acquisition time of
240ns, a throughput sampling rate of 500ksps is tested
and guaranteed.
The LTC2308 communicates via a standard 4-wire SPI
compatible digital interface. The rising edge of CONVST
initiates a conversion. After the conversion is finished, pull
CONVST low to enable the serial output (SDO). The ADC
shifts out the digital data in 2’s complement format when
operating in bipolar mode or in straight binary format when
in unipolar mode, based on the setting of the UNI bit.
2308fb
13
LTC2308
APPLICATIONS INFORMATION
For best performance, ensure that CONVST returns low
within 40ns after the conversion starts (i.e., before the first
bit decision) or after the conversion ends. If CONVST is
low when the conversion ends, the MSB bit will appear at
SDO at the end of the conversion and the ADC will remain
powered up.
Timing and Control
The start of a conversion is triggered by a rising edge at
CONVST. Once initiated, a new conversion cannot be restarted until the current conversion is complete. Figures 8
and 9 show the timing diagrams for two different examples
of CONVST pulses. Example 1 (Figure 8) shows CONVST
staying HIGH after the conversion ends. If CONVST is high
after the tCONV period, the LTC2308 enters NAP or SLEEP
mode, depending on the setting of SLP bit from the DIN
word that was shifted in after the previous conversion.
(see Nap Mode and Sleep Mode for more detail).
When CONVST returns low, the ADC wakes up and the
most significant bit (MSB) of the output data sequence
at SDO becomes valid after the serial data bus is enabled.
All other data bits from SDO transition on the falling edge
of each SCK pulse. Configuration data (DIN) is loaded into
the LTC2308 at SDI, starting with the first SCK rising edge
after CONVST returns low. The S/D bit is loaded on the
first SCK rising edge.
Example 2 (Figure 9) shows CONVST returning low before the conversion ends. In this mode, the ADC and all
internal circuitry remain powered up. When the conversion is complete, the MSB of the output data sequence at
SDO becomes valid after the data bus is enabled. At this
point(tCONV 1.3μs after the rising edge of CONVST), pulsing SCK will shift data out at SDO and load configuration
data (DIN) into the LTC2308 at SDI. The first SCK rising
edge loads the S/D bit into the LTC2308. SDO transitions
on the falling edge of each SCK pulse.
Nap Mode
The ADC enters nap mode when CONVST is held high
after the conversion is complete (tCONV) if the SLP bit is
set to a logic 0. The supply current decreases to 180μA
in nap mode between conversions, thereby reducing the
average power dissipation as the sample rate decreases.
For example, the LTC2308 draws an average of 200μA
with a 1ksps sampling rate. The LTC2308 keeps only the
reference(VREF) and reference buffer(REFCOMP) circuitry
active when in nap mode.
Sleep Mode
The ADC enters sleep mode when CONVST is held high
after the conversion is complete (tCONV) if the SLP bit is
set to a logic 1. The ADC draws only 7μA in sleep mode,
provided that none of the digital inputs are switching. When
CONVST returns low, the LTC2308 is released from the
SLEEP mode and requires 200ms to wake up and charge
the respective 2.2μF and 10μF bypass capacitors on the
VREF and REFCOMP pins.
Board Layout and Bypassing
To obtain the best performance, a printed circuit board with
a solid ground plane is required. Layout for the printed
circuit board should ensure digital and analog signal lines
are separated as much as possible. Care should be taken
not to run any digital signal alongside an analog signal. All
analog inputs should be shielded by GND. VREF, REFCOMP
and AVDD should be bypassed to the ground plane as
close to the pin as possible. Maintaining a low impedance
path for the common return of these bypass capacitors
is essential to the low noise operation of the ADC. These
traces should be as wide as possible. See Figure 7 for a
suggested layout.
Figures 10 and 11 are the transfer characteristics for the
bipolar and unipolar modes. Data is output at SDO in 2’s
complement format for bipolar readings and in straight
binary for unipolar readings.
2308fb
14
LTC2308
APPLICATIONS INFORMATION
Figure 7b. Layer 1 Component Side
Figure 7a. Top Silkscreen
Figure 7c. Layer 2 Ground Plane
Figure 7d. Layer 3 Power Plane
2308fb
15
LTC2308
APPLICATIONS INFORMATION
Figure 7e. Layer Back Solder Side
J1
CH0
R1
OPT
1
2
3
JP1
CH0
JP4
OVDD
SMA
5V
5V
OPEN
C2
10μF
E1
TP1
OPT
J2
HEADER 12x2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
R2 100Ω
E2
CH0
22 CH0
R3 100Ω
E3
CH1
24 CH2
R5 100Ω
E4
CH2
R7 100Ω
E6
CH4
5 CH7
E10
CH6
R11 100Ω
E11
CH7
C7
47pF
C6
47pF
C9
47pF
C8
47pF
C11
47pF
C10
47pF
C13
47pF
C12
47pF
C1
0.1μF
21
19
13
DVDD
0VDD
AVDD AVDD
12
CONV 14
SDO 17
REFCOMP 8
VREF 7
GND GND GND GND GND GND
25
9
10
11
20
JP3
COM
1
OPEN
2
GND
3
18
C15
10μF
R13
4.99k
C16
1μF
E14
DC_BIAS/2
R14
4.99k
CONV_AT_ADC
SDO_AT_ADC
SCK_AT_ADC
SDI 15
LTC2308
6 COM
C14
47pF
R4 301Ω
SCK 16
4 CH6
R12 100Ω
E12
TP2
OPT
5V
C3
0.1μF
2 CH4
R6 100Ω
R10 100Ω
C4
0.1μF
1 CH3
3 CH5
E7
CH5
E26
EXT OVDD
23 CH1
E5
CH3
R8 100Ω
3
2
1
SDI_AT_ADC
R9 49.9Ω
C5
2.2μF
E8
VREF
C38
10μF
E9
REFCOMP
E13
EXTERNAL
BIAS
JP2
DC BIAS
1
EXTERNAL
2
3
REFCOMP
2308 F07F
Figure 7f. Partial Demo Board Schematic
2308fb
16
LTC2308
APPLICATIONS INFORMATION
tWLCONVST
tACQ
CONVST
NAP OR
SLEEP
tCONV
tCYC
1
2
3
4
5
6
7
8
9
10 11 12
SCK
SDI
S/D O/S S1 S0 UNI SLP
MSB
LSB
Hi-Z
SDO
B11 B10 B9
B8
B7
B6
B5
B4
B3
B2
B1
B0
Hi-Z
2308 F08
Figure 8. LTC2308 Timing with a Long CONVST Pulse
tACQ
tHCONVST
tWHCONV
CONVST
tCYC
tCONV
1
2
3
4
5
6
7
8
9
10
11
12
SCK
SDI
S/D O/S S1 S0 UNI SLP
MSB
LSB
Hi-Z
SDO
B11
B10 B9
B8
B7
B6
B5
B4
B3
B2
B1 B0
Hi-Z
2308 F09
Figure 9. LTC2308 Timing with a Short CONVST Pulse
2308fb
17
LTC2308
OUTPUT CODE (TWO’S COMPLEMENT)
APPLICATIONS INFORMATION
011...111
BIPOLAR
ZERO
011...110
000...001
000...000
111...111
111...110
FS = 4.096V
1LSB = FS/2N
1LSB = 1mV
100...001
100...000
–FS/2
–1 0V 1
LSB
LSB
INPUT VOLTAGE (V)
FS/2 – 1LSB
2308 F10
Figure 10. LTC2308 Bipolar Transfer
Characteristics (2’s Complement)
111...111
OUTPUT CODE
111...110
100...001
100...000
011...111 UNIPOLAR
ZERO
011...110
FS = 4.096V
1LSB = FS/2N
1LSB = 1mV
000...001
000...000
0V
FS – 1LSB
INPUT VOLTAGE (V)
2308 F11
Figure 11. LTC2308 Unipolar Transfer
Characteristics (Straight Binary)
2308fb
18
LTC2308
PACKAGE DESCRIPTION
UF Package
24-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 p0.05
4.50 p 0.05
2.45 p 0.05
3.10 p 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 p0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
4.00 p 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
R = 0.115
TYP
0.75 p 0.05
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 s 45o CHAMFER
23 24
PIN 1
TOP MARK
(NOTE 6)
0.40 p 0.10
1
2
2.45 p 0.10
(4-SIDES)
(UF24) QFN 0105
0.200 REF
0.00 – 0.05
0.25 p 0.05
0.50 BSC
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)—TO BE APPROVED
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, IF PRESENT
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
2308fb
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 its circuits as described herein will not infringe on existing patent rights.
19
LTC2308
TYPICAL APPLICATION
Clock Squaring/Level Shifting Circuit Allows Testing with RF Sine Generator,
Convert Re-Timing Flip-Flop Preserves Low Jitter Clock Timing
5V
2.7V TO 5V
10μF
0.1μF
AVDD
CH0
0.1μF
10μF
DVDD
0.1μF
OVDD
LTC2308
CH1
SDI
CH2
CH3
CH4
ANALOG
INPUT
MUX
+
–
12-BIT
500ksps
ADC
SERIAL
PORT
SDO
SCK
VCC
CONVST
CH5
CH6
VREF
CH7
INTERNAL
2.5V REF
COM
CONTROL
LOGIC
(FPGA, CPLD,
DSP, ETC.)
PRE
2.2μF
Q
Q
NL17SZ74
D
CLR
CONVERT ENABLE
REFCOMP
GND
0.1μF
10μF
VCC
RF SIGNAL GENERATOR OR
OTHER LOW JITTER SOURCE
0.1μF
50Ω
MASTER CLOCK
• • • • • •
CONVERT ENABLE
• • • • • •
CONVST
• • • • • •
1k
1k
MASTER
CLOCK
NC7SVU04P5X
• • • • • •
JITTER
• • • • • •
• • • • • •
2308 TA02
DATA TRANSFER
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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Low Input Offset: 75μV/125μV
LTC1609
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LTC1790
Micropower Low Dropout Reference
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LTC1850/LTC1851
10-Bit/12-Bit, 8-Channel, 1.25Msps ADC
Parallel Output, Programmable MUX and Sequencer, 5V Supply
LTC1852/LTC1853
10-Bit/12-Bit, 8-Channel, 400ksps ADC
Parallel Output, Programmable MUX and Sequencer, 3V or 5V Supply
LTC1860/LTC1861
12-Bit, 1-/2-Channel, 250ksps ADC in MSOP
850μA at 250ksps, 2μA at 1ksps, SO-8 and MSOP Packages
LTC1860L/LTC1861L
3V, 12-Bit, 1-/2-Channel, 150ksps ADC
450μA at 150ksps, 10μA at 1ksps, SO-8 and MSOP Packages
LTC1863/LTC1867
12-/16-Bit, 8-Channel, 200ksps ADC
6.5mW, Unipolar or Bipolar, Internal Reference, SSOP-16 Package
LTC1863L/LTC1867L
3V, 12-/16-Bit, 8-Channel, 175ksps ADC
2mW, Unipolar or Bipolar, Internal Reference, SSOP-16 Package
LTC1864/LTC1865
16-Bit, 1-/2-Channel, 250ksps ADC in MSOP
850μA at 250ksps, 2μA at 1ksps, SO-8 and MSOP Packages
LTC1864L/LTC1865L
3V, 16-Bit, 1-/2-Channel, 150ksps ADC in MSOP
450μA at 150ksps, 10μA at 1ksps, SO-8 and MSOP Packages
2308fb
20 Linear Technology Corporation
LT 0908 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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