LTC2400 - 24-Bit µPower No Latency Delta-Sigma ADC in SO-8

40 Pages·2000·0.46 MB·English

by  Linear Technology Corporation

LTC2400
m
24-Bit Power
No Latency DS TM ADC in SO-8
FEATURES DESCRIPTIOU
n 24-Bit ADC in SO-8 Package The LTC®2400 is a 2.7V to 5.5V micropower 24-bit
n 4ppm INL, No Missing Codes converter with an integrated oscillator, 4ppm INL and
n 4ppm Full-Scale Error 0.3ppm RMS noise. It uses delta-sigma technology and
n Single Conversion Settling Time provides single cycle settling time for multiplexed appli-
for Multiplexed Applications cations. Through a single pin the LTC2400 can be config-
n 0.5ppm Offset ured for better than 110dB rejection at 50Hz or 60Hz – 2%,
n 0.3ppm Noise or it can be driven by an external oscillator for a user
n Internal Oscillator—No External Components defined rejection frequency in the range 1Hz to 120Hz.
Required The internal oscillator requires no external frequency
n 110dB Min, 50Hz/60Hz Notch Filter setting components.
n Reference Input Voltage: 0.1V to V
CC The converter accepts any external reference voltage from
n Live Zero—Extended Input Range Accommodates
0.1V to V . With its extended input conversion range of
CC
12.5% Overrange and Underrange
–12.5% V to 112.5% V , the LTC2400 smoothly
REF REF
n Single Supply 2.7V to 5.5V Operation
resolves the offset and overrange problems of preceding
n Low Supply Current (200m A) and Auto Shutdown
sensors or signal conditioning circuits.
APPLICATIOU S The LTC2400 communicates through a flexible 3-wire
digital interface which is compatible with SPI and
n Weight Scales MICROWIRETM protocols.
n Direct Temperature Measurement
, LTC and LT are registered trademarks of Linear Technology Corporation.
n Gas Analyzers No Latency ∆S is a trademark of Linear Technology Corporation.
MICROWIRE is a trademark of National Semiconductor Corporation.
n Strain-Gage Transducers
n Instrumentation
n Data Acquisition
n Industrial Process Control
n 6-Digit DVMs
TYPICAL APPLICATIOU
Total Unadjusted Error vs Output Code
10
2.7V TO 5.5V VCC = 5V
1µF VCC= INTERNAL OSC/50Hz REJECTION 86 TVAR E=F 2=5 5°CV
VCC FO == EINXTTEERRNNAALL OCSLCO/C6K0 HSzO RUERJCEECTION ppm) 4 FO = LOW
LTC2400 OR ( 2
R
REFERENCE R 0
E
0.1VVO TLOT AVGCEC VREF SCK RITY –2
3-WIRE EA –4
INPUTA RNAANLOGGE VIN SDO SPI INTERFACE LIN –6
–0.12VREF TO 1.12VREF GND CS –8
2400 TA01 –10
0 8,338,608 16,777,215
OUTPUT CODE (DECIMAL)
2400 TA02
1
LTC2400
ABSOLUTE WMAXIWMUWM RATINUGS PACKAGE/ORDER INUFORWMATIOUN
(Notes 1, 2)
ORDER PART NUMBER
Supply Voltage (VCC) to GND.......................–0.3V to 7V TOP VIEW
Analog Input Voltage to GND.......–0.3V to (V + 0.3V)
CC VCC 1 8 FO LTC2400CS8
Reference Input Voltage to GND..–0.3V to (V + 0.3V)
CC VREF 2 7 SCK LTC2400IS8
Digital Input Voltage to GND........–0.3V to (V + 0.3V)
CC VIN 3 6 SDO
Digital Output Voltage to GND .....–0.3V to (V + 0.3V)
CC GND 4 5 CS S8 PART MARKING
Operating Temperature Range
S8 PACKAGE
LTC2400C ...............................................0(cid:176) C to 70(cid:176) C 8-LEAD PLASTIC SO 2400
LTC2400I............................................ –40(cid:176) C to 85(cid:176) C TJMAX = 125(cid:176)C, q JA = 130(cid:176)C/W 2400I
Storage Temperature Range................. –65(cid:176) C to 150(cid:176) C
Consult factory for Military grade parts.
Lead Temperature (Soldering, 10 sec)..................300(cid:176) C
COUNVERTER CHARACTERISTICS
The l denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25(cid:176) C. (Notes 3, 4)
A
PARAMETER CONDITIONS MIN TYP MAX UNITS
Resolution (No Missing Codes) 0.1V £ V £ V , (Note 5) l 24 Bits
REF CC
Integral Nonlinearity V = 2.5V (Note 6) l 2 10 ppm of V
REF REF
V = 5V (Note 6) l 4 15 ppm of V
REF REF
Offset Error 2.5V £ V £ V l 0.5 2 ppm of V
REF CC REF
Offset Error Drift 2.5V £ V £ V 0.01 ppm of V /(cid:176) C
REF CC REF
Full-Scale Error 2.5V £ V £ V l 4 10 ppm of V
REF CC REF
Full-Scale Error Drift 2.5V £ V £ V 0.02 ppm of V /(cid:176) C
REF CC REF
Total Unadjusted Error V = 2.5V 5 ppm of V
REF REF
V = 5V 10 ppm of V
REF REF
Output Noise V = 0V (Note 13) 1.5 m V
IN RMS
Normal Mode Rejection 60Hz – 2% (Note 7) l 110 130 dB
Normal Mode Rejection 50Hz – 2% (Note 8) l 110 130 dB
Power Supply Rejection, DC V = 2.5V, V = 0V 100 dB
REF IN
Power Supply Rejection, 60Hz – 2% V = 2.5V, V = 0V, (Notes 7, 15) 110 dB
REF IN
Power Supply Rejection, 50Hz – 2% V = 2.5V, V = 0V, (Notes 8, 15) 110 dB
REF IN
AU ALOG IU PUT AU D REFEREU CE
The l denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25(cid:176) C. (Note 3)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V Input Voltage Range (Note 14) l –0.125 • V 1.125 • V V
IN REF REF
V Reference Voltage Range l 0.1 V V
REF CC
C Input Sampling Capacitance 10 pF
S(IN)
C Reference Sampling Capacitance 15 pF
S(REF)
I Input Leakage Current CS = V l –10 1 10 nA
IN(LEAK) CC
I Reference Leakage Current V = 2.5V, CS = V l –10 1 10 nA
REF(LEAK) REF CC
2
LTC2400
DIGITAL I U PUTS AU D DIGITAL OUTPUTS
The l denotes specifications which apply over the full
operating temperature range, otherwise specifications are at T = 25(cid:176) C. (Note 3)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V High Level Input Voltage 2.7V £ V £ 5.5V l 2.5 V
IH CC
CS, F 2.7V £ V £ 3.3V 2.0 V
O CC
V Low Level Input Voltage 4.5V £ V £ 5.5V l 0.8 V
IL CC
CS, F 2.7V £ V £ 5.5V 0.6 V
O CC
V High Level Input Voltage 2.7V £ V £ 5.5V (Note 9) l 2.5 V
IH CC
SCK 2.7V £ V £ 3.3V (Note 9) 2.0 V
CC
V Low Level Input Voltage 4.5V £ V £ 5.5V (Note 9) l 0.8 V
IL CC
SCK 2.7V £ V £ 5.5V (Note 9) 0.6 V
CC
I Digital Input Current 0V £ V £ V l –10 10 m A
IN IN CC
CS, F
O
I Digital Input Current 0V £ V £ V (Note 9) l –10 10 m A
IN IN CC
SCK
C Digital Input Capacitance 10 pF
IN
CS, F
O
C Digital Input Capacitance (Note 9) 10 pF
IN
SCK
V High Level Output Voltage I = –800m A l V – 0.5V V
OH O CC
SDO
V Low Level Output Voltage I = 1.6mA l 0.4V V
OL O
SDO
V High Level Output Voltage I = –800m A (Note 10) l V – 0.5V V
OH O CC
SCK
V Low Level Output Voltage I = 1.6mA (Note 10) l 0.4V V
OL O
SCK
I High-Z Output Leakage l –10 10 m A
OZ
SDO
POWER REQUIREW E U TS
The l denotes specifications which apply over the full operating temperature range,
otherwise specifications are at T = 25(cid:176) C. (Note 3)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V Supply Voltage l 2.7 5.5 V
CC
I Supply Current
CC
Conversion Mode CS = 0V (Note 12) l 200 300 m A
Sleep Mode CS = V (Note 12) l 20 30 m A
CC
3
LTC2400
TI W I U G CHARACTERISTICS
The l denotes specifications which apply over the full operating temperature
range, otherwise specifications are at T = 25(cid:176) C. (Note 3)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
f External Oscillator Frequency Range l 2.56 307.2 kHz
EOSC
t External Oscillator High Period l 0.5 390 m s
HEO
t External Oscillator Low Period l 0.5 390 m s
LEO
t Conversion Time F = 0V l 130.66 133.33 136 ms
CONV O
F = V l 156.80 160 163.20 ms
O CC
External Oscillator (Note 11) l 20480/f (in kHz) ms
EOSC
f Internal SCK Frequency Internal Oscillator (Note 10) 19.2 kHz
ISCK
External Oscillator (Notes 10, 11) f /8 kHz
EOSC
D Internal SCK Duty Cycle (Note 10) l 45 55 %
ISCK
f External SCK Frequency Range (Note 9) l 2000 kHz
ESCK
t External SCK Low Period (Note 9) l 250 ns
LESCK
t External SCK High Period (Note 9) l 250 ns
HESCK
t Internal SCK 32-Bit Data Output Time Internal Oscillator (Notes 10, 12) l 1.64 1.67 1.70 ms
DOUT_ISCK
External Oscillator (Notes 10, 11) l 256/f (in kHz) ms
EOSC
t External SCK 32-Bit Data Output Time (Note 9) l 32/f (in kHz) ms
DOUT_ESCK ESCK
t CS fl to SDO Low Z l 0 150 ns
1
t2 CS › to SDO High Z l 0 150 ns
t3 CS fl to SCK fl (Note 10) l 0 150 ns
t4 CS fl to SCK › (Note 9) l 50 ns
t SCK fl to SDO Valid l 200 ns
KQMAX
t SDO Hold After SCK fl (Note 5) l 15 ns
KQMIN
t SCK Set-Up Before CS fl l 50 ns
5
t SCK Hold After CS fl l 50 ns
6
Note 1: Absolute Maximum Ratings are those values beyond which the Note 10: The converter is in internal SCK mode of operation such that
life of the device may be impaired. the SCK pin is used as digital output. In this mode of operation the
Note 2: All voltage values are with respect to GND. SCK pin has a total equivalent load capacitance CLOAD = 20pF.
Note 3: VCC = 2.7 to 5.5V unless otherwise specified. Note 11: The external oscillator is connected to the FO pin. The external
oscillator frequency, f , is expressed in kHz.
Note 4: Internal Conversion Clock source with the F pin tied EOSC
O
to GND or to V or to external conversion clock source with Note 12: The converter uses the internal oscillator.
CC
fEOSC = 153600Hz unless otherwise specified. FO = 0V or FO = VCC.
Note 5: Guaranteed by design, not subject to test. Note 13: The output noise includes the contribution of the internal
calibration operations.
Note 6: Integral nonlinearity is defined as the deviation of a code from
a straight line passing through the actual endpoints of the transfer Note 14: For reference voltage values VREF > 2.5V the extended input
curve. The deviation is measured from the center of the quantization of –0.125 • VREF to 1.125 • VREF is limited by the absolute maximum
band. rating of the Analog Input Voltage pin (Pin 3). For 2.5V < VREF £
Note 7: F = 0V (internal oscillator) or f = 153600Hz – 2% 0.267V + 0.89 • VCC the input voltage range is –0.3V to 1.125 • VREF.
O EOSC For 0.267V + 0.89 • V < V £ V the input voltage range is –0.3V
(external oscillator). CC REF CC
to V + 0.3V.
Note 8: F = V (internal oscillator) or f = 128000Hz – 2% CC
O CC EOSC
Note 15: The DC voltage at V = 4.1V, and the AC voltage applied to
(external oscillator). CC
V is 2.8V
Note 9: The converter is in external SCK mode of operation such that CC P-P
the SCK pin is used as digital input. The frequency of the clock signal
driving SCK during the data output is f and is expressed in kHz.
ESCK
4
LTC2400
TYPICAL PERFORW AU CE CHARACTERISTICS
Total Unadjusted Error Negative Input Extended Total
(3V Supply) INL (3V Supply) Unadjusted Error (3V Supply)
10 10 10
VCC = 3V VCC = 3V VCC = 3V
VREF = 3V VREF = 3V VREF = 3V TA = 90°C
5 5 5
m) m) TA = –55°C, –45°C, 25°C, 90°C m) TA = 25°C
p p p
p p p
R ( 0 R ( 0 R ( 0
O O O
ERR TA = –55°C, –45°C, 25°C, 90°C ERR ERR TA = –45°C
–5 –5 –5 TA = –55°C
–10 –10 –10
0 0.5 1.0 1.5 2.0 2.5 3.0 0 0.5 1.0 1.5 2.0 2.5 3.0 0 –0.05 –0.10 –0.15 –0.20 –0.25 –0.30
INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V)
2400 G01 2400 G02 2400 G03
Positive Input Extended Total Total Unadjusted Error
Unadjusted Error (3V Supply) (5V Supply) INL (5V Supply)
10 10 10
VCC = 3V VCC = 5V VCC = 5V
VREF = 3V 8 VREF = 5V VREF = 5V
6
5 5
TA = –55°C 4
m) m) 2 m)
p p p
R (p 0 TA = –45°C R (p 0 R (p 0
ERRO TA = 90°C TA = 25°C ERRO –2 TA = –55°C, –45°C, 25°C, 90°C ERRO TA = –55°C, –45°C, 25°C, 90°C
–4
–5 –5
–6
–8
–10 –10 –10
3.0 3.1 3.2 3.3 0 1 2 3 4 5 0 1 2 3 4 5
INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V)
2400 G04 2400 G05 2400 G06
Negative Input Extended Total Positive Input Extended Total
Unadjusted Error (5V Supply) Unadjusted Error (5V Supply) Offset Error vs Reference Voltage
10 10 6
VCC = 5V VCC = 5V VCC = 5V
VREF = 5V TA = 90°C VREF = 5V 5 TA = 25°C
5 5
TA = –55°C pm) 4
ppm) TA = 25°C ppm) OR (p 3
ERROR ( 0 TA = –45°C ERROR ( 0 TA = 90°C TA = 2T5A° =C –45°C FSET ERR 2
–5 –5 OF 1
TA = –55°C
0
–10 –10 –1
0 –0.05 –0.10 –0.15 –0.20 –0.25 –0.30 5.0 5.1 5.2 5.3 0 1 2 3 4 5
INPUT VOLTAGE (V) INPUT VOLTAGE (V) REFERENCE VOLTAGE
2400 G07 2400 G08 2400 G09
5
LTC2400
TYPICAL PERFORW AU CE CHARACTERISTICS
RMS Noise vs Reference Voltage Offset Error vs V RMS Noise vs V
CC CC
20 5.0 5.0
VCC = 5V VREF = 2.5V VREF = 2.5V
TA = 25°C TA = 25°C TA = 25°C
ppm OF V)REF 1105 ROR (ppm) 2.50 SE (ppm) 2.5
MS NOISE ( 5 OFFSET ER–2.5 RMS NOI
R
0 –5.0 0
0 1 2 3 4 5 2.7 3.2 3.7 4.2 4.7 5.2 2.7 3.2 3.7 4.2 4.7 5.2
REFERENCE VOLTAGE (V) VCC VCC
2400 G10 2400 G11 2400 G12
Noise Histogram RMS Noise vs Code Out Offset Error vs Temperature
1500 1.00 5.0
VCC = 5V VCC = 5V VCC = 5V
VREF = 5V VREF = 5V VREF = 5V
VIN = 0V VIN = –0.3V TO 5.3V VIN = 0V
GS 0.75 TA = 25°C m) 2.5
NUMBER OF READIN1500000 RMS NOISE (ppm)00..2550 OFFSET ERROR (pp–2.50
0 0 –5.0
–1.0 –0.5 0 0.5 1.0 1.5 0 7FFFFF FFFFFF –55 –30 –5 20 45 70 95 120
OUTPUT CODE (ppm) CODE OUT (HEX) TEMPERATURE (°C)
2400 G14 2400 G18 2400 G13
Full-Scale Error
Full-Scale Error vs Temperature vs Reference Voltage Full-Scale Error vs V
CC
5.0 10.0 6
VCC = 5V VCC = 5V
VREF = 5V VIN = VREF
VIN = 5V 5
m) 2.5 m) 7.5 m)
FULL-SCALE ERROR (pp–2.50 FULL-SCALE ERROR (pp 25..50 FULL-SCALE ERROR (pp 243
1 VREF = 2.5V
VIN = 2.5V
TA = 25°C
–5.0 0 0
–55 –30 –5 20 45 70 95 120 0 1 2 3 4 5 2.7 3.2 3.7 4.2 4.7 5.2
TEMPERATURE (°C) REFERENCE VOLTAGE (V) VCC
2400 G15 2400 G16 2400 G17
6
LTC2400
TYPICAL PERFORW AU CE CHARACTERISTICS
Conversion Current vs Temperature Sleep Current vs Temperature PSRR vs Frequency at V
CC
230 30 0
VCC = 4.1V
220 VCC = 5.5V 25 –20 TVAIN = = 2 05V°C
µURRENT (A) 122910000 VCC = 4.1V µURRENT (A) 1250 VCC = 2.7V, 5.5V TION (dB)––6400 FO = 0
LY C 180 VCC= 2.7V LY C EJEC
PP PP 10 R–80
U U 15,360Hz 153,600Hz
S 170 S
5 –100
160
150 0 –120
–55 –30 –5 20 45 70 95 120 –55 –30 –5 20 45 70 95 120 1 100 10k 1M
TEMPERATURE (°C) TEMPERATURE (°C) FREQUENCY AT VCC (Hz)
2400 G19 2400 G20 2400 G23
PSRR vs Frequency at V PSRR vs Frequency at V Rejection vs Frequency at V
CC CC IN
–10 0 0
VCC = 4.1V VCC = 4.1V VCC = 5V
VIN = 0V VIN = 0V VREF = 5V
–30 TA = 25°C –20 TA = 25°C –20 VIN = 2.5V
F0 = 0 FO = 0 FO = 0
N (dB)–50 N (dB)–40 N (dB)–40
REJECTIO––9700 REJECTIO––8600 REJECTIO––8600
–110 –100 –100
–130 –120 –120
0 50 100 150 200 250 15200 15250 15300 15350 15400 15450 15500 1 50 100 150 200 250
FREQUENCY AT VCC (Hz) FREQUENCY AT VCC (Hz) FREQUENCY AT VIN (Hz)
2400 G21 1635 G22 2400 G24
Rejection vs Frequency at V Rejection vs Frequency at V Rejection vs Frequency at V
IN IN IN
–60 0 0
VCC = 5V
–70 –20 VVRINE F= =2 .55VV –20
–80 FO = 0
–40
N (dB) –90 N (dB)–40 N (dB) –60
REJECTIO ––110100 REJECTIO––8600 REJECTIO –80
–100
–120
–100
–130 –120
SAMPLE RATE = 15.36kHz – 2%
–140 –120 –140
–12 –8 –4 0 4 8 12 15100 15200 15300 15400 15500 0 fS/2 fS
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%) FREQUENCY AT VIN (Hz) INPUT FREQUENCY
2400 G25 2400 G26 2400 F26
7
LTC2400
TYPICAL PERFORW AU CE CHARACTERISTICS
INL vs Output Rate Resolution vs Output Rate
24 24
VCC = 5V VCC = 5V
22 VREF = 5V 22 VREF = 5V
TA = 25°C TA = 25°C
20 F0 = EXTERNAL 20 FO = EXTERNAL
*
S)
INL (BITS) 111648 OLUTION (BIT 111648
S
E
R
12 12
10 10 *RESOLUTION = LOG(VREF/RMS NOISE)
LOG (2)
8 8
0 5 10 15 20 25 30 35 40 45 50 55 60 0 5 10 15 20 25 30 35 40 45 50 55 60
OUTPUT RATE (Hz) OUTPUT RATE (Hz)
2400 G27 2400 G28
PIUN FUUNCTIOUNS
V (Pin 1): Positive Supply Voltage. Bypass to GND SDO (Pin 6): Three-State Digital Output. During the data
CC
(Pin␣4) with a 10m F tantalum capacitor in parallel with output period, this pin is used for serial data output. When
0.1m F ceramic capacitor as close to the part as possible. the chip select CS is HIGH (CS = V ), the SDO pin is in a
CC
high impedance state. During the Conversion and Sleep
V (Pin 2): Reference Input. The reference voltage range
REF
periods this pin can be used as a conversion status output.
is 0.1V to V .
CC
The conversion status can be observed by pulling CS LOW.
V (Pin 3): Analog Input. The input voltage range is
IN
SCK (Pin 7): Bidirectional Digital Clock Pin. In Internal
–0.125 • V to 1.125 • V . For V > 2.5V, the input
REF REF REF
Serial Clock Operation mode, SCK is used as digital output
voltage range may be limited by the pin absolute maxi-
for the internal serial interface clock during the data output
mum rating of –0.3V to V + 0.3V.
CC
period. In External Serial Clock Operation mode, SCK is
GND (Pin 4): Ground. Shared pin for analog ground, used as digital input for the external serial interface. A
digital ground, reference ground and signal ground. Should weak internal pull-up is automatically activated in Internal
be connected directly to a ground plane through a mini- Serial Clock Operation mode. The Serial Clock mode is
mum length trace or it should be the single-point-ground determined by the level applied to SCK at power up and the
in a single point grounding system. falling edge of CS.
CS (Pin 5): Active LOW Digital Input. A LOW on this pin F (Pin 8): Frequency Control Pin. Digital input that
O
enables the SDO digital output and wakes up the ADC. controls the ADC’s notch frequencies and conversion
Following each conversion the ADC automatically enters time. When the F pin is connected to V (F = V ), the
O CC O CC
the Sleep mode and remains in this low power state as converter uses its internal oscillator and the digital filter
long as CS is HIGH. A LOW on CS wakes up the ADC. A first null is located at 50Hz. When the F pin is connected
O
LOW-to-HIGH transition on this pin disables the SDO to GND (F = OV), the converter uses its internal oscillator
O
digital output. A LOW-to-HIGH transition on CS during the and the digital filter first null is located at 60Hz. When F
O
Data Output transfer aborts the data transfer and starts a is driven by an external clock signal with a frequency f
EOSC,
new conversion. the converter uses this signal as its clock and the digital
filter first null is located at a frequency f /2560.
EOSC
8
LTC2400
FUUCTIOUAL BLOCK DIAGRAW
INTERNAL
VCC OSCILLATOR
GND AUTOCALIBRATION
AND CONTROL FO
(INT/EXT)
VIN ∫ ∫ ∫
SDO
∑
SERIAL
ADC SCK
INTERFACE
CS
VREF DECIMATING FIR
DAC
2400 FD
TEST CIRCUITS
VCC
SDO 3.4k
3.4k CLOAD = 20pF SDO
CLOAD = 20pF
HI-Z TO VOH
VOL TO VOH
VOH TO HI-Z 2400 TA03 HI-Z TO VOL
VOH TO VOL
VOL TO HI-Z 2400 TA04
APPLICATIOUNS INUFORWMATIOUN
Converter Operation Cycle
CONVERT
The LTC2400 is a low power, delta-sigma analog-to-
digital converter with an easy to use 3-wire serial interface.
SLEEP
Its operation is simple and made up of three states. The
converter operating cycle begins with the conversion,
followed by a low power sleep state and concluded with
1 CS AND
the data output (see Figure 1). The 3-wire interface con- SCK
sists of serial data output (SDO), a serial clock (SCK) and
0
a chip select (CS).
DATA OUTPUT
Initially, the LTC2400 performs a conversion. Once the
conversion is complete, the device enters the sleep state. 2400 F01
While in this sleep state, power consumption is reduced by Figure 1. LTC2400 State Transition Diagram
9
LTC2400
APPLICATIOUNS INUFORWMATIOUN
an order of magnitude. The part remains in the sleep state conversion and the output data. Therefore, multiplexing
as long as CS is logic HIGH. The conversion result is held an analog input voltage is easy.
indefinitely in a static shift register while the converter is
The LTC2400 performs offset and full-scale calibrations
in the sleep state.
every conversion cycle. This calibration is transparent to
Once CS is pulled low, the device begins outputting the the user and has no effect on the cyclic operation de-
conversion result. There is no latency in the conversion scribed above. The advantage of continuous calibration is
result. The data output corresponds to the conversion just extreme stability of offset and full-scale readings with re-
performed. This result is shifted out on the serial data out spect to time, supply voltage change and temperature drift.
pin (SDO) under the control of the serial clock (SCK). Data
is updated on the falling edge of SCK allowing the user to Power-Up Sequence
reliably latch data on the rising edge of SCK, see Figure 3.
The LTC2400 automatically enters an internal reset state
The data output state is concluded once 32 bits are read
when the power supply voltage V drops below approxi-
CC
out of the ADC or when CS is brought HIGH. The device
mately 2.2V. This feature guarantees the integrity of the
automatically initiates a new conversion cycle and the
conversion result and of the serial interface mode selec-
cycle repeats.
tion which is performed at the initial power-up. (See the
Through timing control of the CS and SCK pins, the 2-wire I/O sections in the Serial Interface Timing Modes
LTC2400 offers several flexible modes of operation section.)
(internal or external SCK and free-running conversion
When the V voltage rises above this critical threshold,
CC
modes). These various modes do not require program-
the converter creates an internal power-on-reset (POR)
ming configuration registers; moreover, they do not dis-
signal with duration of approximately 0.5ms. The POR
turb the cyclic operation described above. These modes of
signal clears all internal registers. Following the POR
operation are described in detail in the Serial Interface
signal, the LTC2400 starts a normal conversion cycle and
Timing Modes section.
follows the normal succession of states described above.
The first conversion result following POR is accurate
Conversion Clock
within the specifications of the device.
A major advantage delta-sigma converters offer over
conventional type converters is an on-chip digital filter Reference Voltage Range
(commonly known as Sinc or Comb filter). For high
The LTC2400 can accept a reference voltage from 0V to
resolution, low frequency applications, this filter is typi-
V . The converter output noise is determined by the
CC
cally designed to reject line frequencies of 50 or 60Hz plus
thermal noise of the front-end circuits, and as such, its
their harmonics. In order to reject these frequencies in
value in microvolts is nearly constant with reference
excess of 110dB, a highly accurate conversion clock is
voltage. A decrease in reference voltage will not signifi-
required. The LTC2400 incorporates an on-chip highly
cantly improve the converter’s effective resolution. On the
accurate oscillator. This eliminates the need for external
other hand, a reduced reference voltage will improve the
frequency setting components such as crystals or oscilla-
overall converter INL performance. The recommended
tors. Clocked by the on-chip oscillator, the LTC2400
range for the LTC2400 voltage reference is 100mV to V .
CC
rejects line frequencies (50 or 60Hz – 2%) a minimum of
110dB. Input Voltage Range
The converter is able to accommodate system level offset
Ease of Use
and gain errors as well as system level overrange situa-
The LTC2400 data output has no latency, filter settling or
tions due to its extended input range, see Figure 2. The
redundant data associated with the conversion cycle.
LTC2400 converts input signals within the extended input
There is a one-to-one correspondence between the
range of –0.125 • V to 1.125 • V .
REF REF
10