LINEAR TECHNOLOGY LTC2440 Technical data
LTC2440
24-Bit High Speed
FEATURES
n
Up to 3.5kHz Output Rate
n
Selectable Speed/Resolution
n
2μV
n
200nV
Noise at 880Hz Output Rate
RMS
Noise at 6.9Hz Output Rate with
RMS
Simultaneous 50/60Hz Rejection
n
0.0005% INL, No Missing Codes
n
Autosleep Enables 20μA Operation at 6.9Hz
n
<5μV Offset (4.5V < VCC < 5.5V, –40°C to 85°C)
n
Differential Input and Differential Reference with
GND to V
n
No Latency, Each Conversion is Accurate Even After
Common Mode Range
CC
an Input Step
n
Internal Oscillator—No External Components
n
Pin Compatible with the LTC2410
n
24-Bit ADC in Narrow 16-Lead SSOP Package
APPLICATIONS
n
High Speed Multiplexing
n
Weight Scales
n
Auto Ranging 6-Digit DVMs
n
Direct Temperature Measurement
n
High Speed Data Acquisition
Differential
Δ∑
ADC with
Selectable Speed/Resolution
DESCRIPTION
The LTC®2440 is a high speed 24-bit No Latency Δ∑TM
ADC with 5ppm INL and 5μV offset. It uses proprietary
delta-sigma architecture enabling variable speed and resolution with no latency. Ten speed/resolution combinations
(6.9Hz/200nV
to 3.5kHz/25μV
RMS
through a simple serial interface. Alternatively, by tying a
single pin HIGH or LOW, a fast (880Hz/2μV
noise (6.9Hz, 200nV
, 50/60Hz rejection) speed/reso-
RMS
lution combination can be easily selected. The accuracy
(offset, full-scale, linearity, drift) and power dissipation
are independent of the speed selected. Since there is no
latency, a speed/resolution change may be made between
conversions with no degradation in performance.
Following each conversion cycle, the LTC2440 automatically enters a low power sleep state. Power dissipation
may be reduced by increasing the duration of this sleep
state. For example, running at the 3.5kHz conversion speed
but reading data at a 100Hz rate draws 240μA average
current (1.1mW) while reading data at a 7Hz output rate
draws only 25μA (125μW).The LTC2440 communicates
through a fl exible 3-wire or 4-wire digital interface that is
compatible with the LTC2410 and is available in a narrow
16-lead SSOP package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
No Latency Δ∑ is a trademark of Linear Technology Corporation. All other trademarks are
the property of their respective owners.
) are programmed
RMS
) or ultralow
RMS
TYPICAL APPLICATION
Simple 24-Bit 2-Speed Acquisition System
4.5V TO 5.5V
V
BUSY
CC
LTC2440
+
REFERENCE VOLTAGE
0.1V TO V
ANALOG INPUT
TO 0.5V
–0.5V
REF
REF
REF
4
CC
REF
IN
IN
GND
f
O
–
SCK
+
SDO
–
CS
SDI
EXT
3-WIRE
SPI INTERFACE
2440 TA01
V
CC
6.9Hz, 200nV NOISE,
50/60Hz REJECTION
10-SPEED SERIAL
PROGRAMMABLE
880Hz OUTPUT RATE,
2μV NOISE
2440 TA01
Speed vs RMS Noise
100
VCC = 5V
V
= 5V
REF
+
V
= V
IN
10
200nV AT 6.9Hz
RMS NOISE (μV)
1
(50/60Hz REJECTION)
0.1
1
–
= 0V
IN
2μV AT 880Hz
10 100
CONVERSION RATE (Hz)
1000 10000
2440 TA02
2440fc
1
LTC2440
(Notes 1,2)
PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC) to GND ....................... –0.3V to 6V
Analog Input Pins Voltage
to GND ......................................–0.3V to (V
+ 0.3V)
CC
Reference Input Pins Voltage
to GND ......................................–0.3V to (V
Digital Input Voltage to GND .........–0.3V to (V
Digital Output Voltage to GND .......–0.3V to (V
+ 0.3V)
CC
+ 0.3V)
CC
+ 0.3V)
CC
Operating Temperature Range
LTC2440C ............................................... 0°C to 70°C
LTC2440I ............................................. –40°C to 85°C
Storage Temperature Range ...................–65°C to 150°C
Lead Temperature (Soldering, 10 sec) ..................300°C
GND
REF
REF
GND
V
IN
IN
SDI
CC
TOP VIEW
1
2
+
3
–
4
+
5
–
6
7
8
GN PACKAGE
16-LEAD PLASTIC SSOP
T
= 125°C, θJA = 110°C/W
JMAX
16
GND
15
BUSY
14
f
O
13
SCK
12
SDO
11
CS
10
EXT
9
GND
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC2440CGN#PBF LTC2440CGN#TRPBF 2440 Narrow 16-Lead SSOP 0°C to 70°C
LTC2440IGN#PBF LTC2440IGN#TRPBF 2440I Narrow 16-Lead SSOP –40°C to 85°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based fi nish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. (Notes 3, 4)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Resolution (No Missing Codes) 0.1V ≤ V
Integral Nonlinearity V
= 5V, REF+ = 5V, REF– = GND, V
CC
+
REF
= 2.5V, REF– = GND, V
≤ VCC, –0.5 • V
REF
≤ VIN ≤ 0.5 • V
REF
INCM
= 1.25V, (Note 6)
INCM
, (Note 5)
REF
= 2.5V, (Note 6)
Offset Error 2.5V ≤ REF+ ≤ VCC, REF– = GND, GND ≤ IN+ = IN– ≤ VCC (Note 12)
+
Offset Error Drift 2.5V ≤ REF
Positive Full-Scale Error REF
REF
≤ VCC, REF– = GND, GND ≤ IN+ = IN– ≤ V
+
= 5V, REF– = GND, IN+ = 3.75V, IN– = 1.25V
+
= 2.5V, REF– = GND, IN+ = 1.875V, IN– = 0.625V
CC
Positive Full-Scale Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND, IN+ = 0.75REF+, IN– = 0.25 • REF
+
Negative Full-Scale Error REF
= 5V, REF– = GND, IN+ = 1.25V, IN– = 3.75V
+
REF
= 2.5V, REF– = GND, IN+ = 0.625V, IN– = 1.875V
Negative Full-Scale Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND, IN+ = 0.25 • REF+, IN– = 0.75 • REF
Total Unadjusted Error 5V ≤ V
5V ≤ V
REF
≤ 5.5V, REF+ = 2.5V, REF– = GND, V
CC
≤ 5.5V, REF+ = 5V, REF– = GND, V
CC
+
= 2.5V, REF– = GND, V
= 1.25V, (Note 6)
INCM
Input Common Mode Rejection DC 2.5V ≤ REF+ ≤ VCC, REF– = GND, GND ≤ IN– = IN+ ≤ V
INCM
INCM
= 1.25V
= 2.5V
CC
l
24 Bits
l
l
5
15 ppm of V
3
ppm of V
REF
REF
2.5 5 μV
20 nV/ °C
l
l
+
l
l
+
101030
50
ppm of V
ppm of V
0.2 ppm of V
101030
50
ppm of V
ppm of V
0.2 ppm of V
15
15
15
ppm of V
ppm of V
ppm of V
REF
REF
REF
REF
/°C
REF
REF
/°C
REF
REF
REF
120 dB
2440fc
2
LTC2440
ANALOG INPUT AND REFERENCE
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
+
IN
–
IN
V
IN
+
REF
–
REF
V
REF
C
S(IN+)
C
S(IN–)
C
S(REF+)
C
S(REF–)
I
DC_LEAK(IN+, IN–,
+
, REF–)
REF
I
SAMPLE(IN+, IN–,
+
, REF–)
REF
Absolute/Common Mode IN+ Voltage
Absolute/Common Mode IN– Voltage
Input Differential Voltage Range (IN+ – IN–)
Absolute/Common Mode REF+ Voltage
Absolute/Common Mode REF– Voltage
Reference Differential Voltage Range (REF+
–
– REF
)
IN+ Sampling Capacitance 3.5 pF
IN– Sampling Capacitance 3.5 pF
REF+ Sampling Capacitance 3.5 pF
REF– Sampling Capacitance 3.5 pF
Leakage Current, Inputs and Reference CS = VCC, IN+ = GND, IN– = GND,
Average Input/Reference Current During
Sampling
= 25°C. (Note 3)
A
+
REF
= 5V, REF– = GND
l
GND – 0.3V VCC + 0.3V V
l
GND – 0.3V VCC + 0.3V V
l
–V
/2 V
REF
l
l
l
l
0.1 V
GND VCC – 0.1V V
0.1 V
–100 10 100 nA
/2 V
REF
CC
CC
Varies, See Applications Section
V
V
DIGITAL INPUTS AND DIGITAL OUTPUTS
The l denotes the specifi cations which apply over the full
operating temperature range, otherwise specifi cations are at TA = 25°C. (Note 3)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
IN
V
IL
V
IN
High Level Input Voltage
CS, f
O
Low Level Input Voltage
CS, f
O
High Level Input Voltage
4.5V ≤ VCC ≤ 5.5V
4.5V ≤ VCC ≤ 5.5V
4.5V ≤ VCC ≤ 5.5V (Note 8)
SCK
V
IL
Low Level Input Voltage
4.5V ≤ VCC ≤ 5.5V (Note 8)
SCK
I
IN
I
IN
Digital Input Current
CS, f
O
Digital Input Current
0V ≤ VIN ≤ V
CC
0V ≤ VIN ≤ VCC (Note 8)
SCK
C
IN
C
IN
Digital Input Capacitance
CS, f
O
Digital Input Capacitance
(Note 8) 10 pF
SCK
V
OH
High Level Output Voltage
IO = –800μA
SDO, BUSY
V
OL
Low Level Output Voltage
IO = 1.6mA
SDO, BUSY
V
OH
High Level Output Voltage
IO = –800μA (Note 9)
SCK
V
OL
Low Level Output Voltage
IO = 1.6mA (Note 9)
SCK
I
OZ
Hi-Z Output Leakage
SDO
l
l
l
l
l
l
25 V
0.8 V
25 V
0.8 V
–10 10 μA
–10 10 μA
10 pF
l
VCC – 0.5V V
l
l
VCC – 0.5V V
l
l
–10 10 μA
0.4V V
0.4V V
2440fc
3
LTC2440
POWER REQUIREMENTS
The l denotes the specifi cations which apply over the full operating temperature
range, otherwise specifi cations are at T
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
CC
I
CC
Supply Voltage
Supply Current
Conversion Mode
Sleep Mode
TIMING CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating temperature
range, otherwise specifi cations are at TA = 25°C. (Note 3)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
f
EOSC
t
HEO
t
LEO
t
CONV
f
ISCK
D
ISCK
f
ESCK
t
LESCK
t
HESCK
t
DOUT_ISCK
t
DOUT_ESCK
t
1
t
2
t
3
t
4
t
KQMAX
t
KQMIN
t
5
t
7
t
8
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 GND.
Note 3: V
V
REF
V
IN
Note 4: f
f
EOSC
= 4.5 to 5.5V unless otherwise specifi ed.
CC
= REF+ – REF–, V
= IN+ – IN–, V
pin tied to GND or to external conversion clock source with
O
= 10MHz unless otherwise specifi ed.
Note 5: Guaranteed by design, not subject to test.
Note 6: Integral nonlinearity is defi ned 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.
External Oscillator Frequency Range
External Oscillator High Period
External Oscillator Low Period
Conversion Time OSR = 256 (SDI = 0)
Internal SCK Frequency Internal Oscillator (Note 9)
Internal SCK Duty Cycle (Note 9)
External SCK Frequency Range (Note 8)
External SCK Low Period (Note 8)
External SCK High Period (Note 8)
Internal SCK 32-Bit Data Output Time Internal Oscillator (Notes 9, 11)
External SCK 32-Bit Data Output Time (Note 8)
↑
CS to SDO Low Z (Note 12)
CS ↑ to SDO High Z
↑
CS to SCK (Note 9)
↑
CS to SCK ↑
SCK to SDO Valid
↑
↑
SDO Hold After SCK (Note 5)
SCK Set-Up Before CS
SDI Setup Before SCK ↑
SDI Hold After SCK ↑
= (REF+ + REF–)/2;
REFCM
= (IN+ + IN–)/2.
INCM
= 25°C. (Note 3)
A
↑
↑
CS = 0V (Note 7)
CS = V
(Note 7)
CC
OSR = 32768 (SDI = 1)
External Oscillator (Note 10, 13)
External Oscillator (Notes 9, 10)
External Oscillator (Notes 9, 10)
(Note 12)
(Notes 8, 12)
(Note 5)
(Note 5)
Note 7: The converter uses the internal oscillator.
Note 8: The converter is in external SCK mode of operation such that the
SCK pin is used as a digital input. The frequency of the clock signal driving
SCK during the data output is f
Note 9: The converter is in internal SCK mode of operation such that the
SCK pin is used as a digital output. In this mode of operation, the SCK pin
has a total equivalent load capacitance of C
Note 10: The external oscillator is connected to the f
oscillator frequency, f
Note 11: The converter uses the internal oscillator. fO = 0V.
Note 12: Guaranteed by design and test correlation.
Note 13: There is an internal reset that adds an additional 1μs (typical) to
the conversion time.
l
4.5 5.5 V
l
l
l
0.1 20 MHz
l
25 10000 ns
l
25 10000 ns
l
0.99
l
126
l
l
0.8 0.9
l
45 55 %
l
l
25 ns
l
25 ns
l
30.9 35.3
l
l
l
l
l
25 ns
l
l
15 ns
l
50 ns
l
10 ns
l
10 ns
, is expressed in kHz.
EOSC
40 • OSR + 170
025ns
025ns
and is expressed in Hz.
ESCK
f
EOSC
8
8
1.13
145
(kHz)
11
30
1.33
170
mA
μA
ms
ms
ms
1
f
/10
EOSC
20 MHz
41.6 μs
320/f
EOSC
32/f
ESCK
5μs
25 ns
= 20pF.
LOAD
pin. The external
O
2440fc
s
s
4
TYPICAL PERFORMANCE CHARACTERISTICS
Integral Nonlinearity
Integral Nonlinearity f
10
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
)
REF
–
= GND
V
REF
5
V
f
T
INCM
O
A
= 2.5V
= GND
= 25°C
= 3.5kHz
OUT
f
= 1.76kHz Integral Nonlinearity f
OUT
10
VCC = 5V
= 5V
V
REF
+
V
REF
)
REF
–
V
REF
5
= 5V
= GND
V
INCM
= GND
f
O
= 25°C
T
A
= 2.5V
)
REF
10
5
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
–
= GND
V
REF
LTC2440
OUT
V
= 2.5V
INCM
= GND
f
O
= 25°C
T
A
= 880Hz
0
–5
INL ERROR (ppm OF V
–10
–2.5
–1.5
–2
–1
Integral Nonlinearity f
10
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
–10
–
= GND
V
REF
5
0
–5
–2
–1.5
–2.5
–1
)
REF
INL ERROR (ppm OF V
–0.5
V
INCM
= GND
f
O
T
A
–0.5
0
VIN (V)
= 2.5V
= 25°C
0
VIN (V)
0
–5
INL ERROR (ppm OF V
1.5
0.5
1
= 440Hz Integral Nonlinearity f
OUT
2
2440 G01
2.5
–10
10
)
REF
–2.5
5
–2
VCC = 5V
= 5V
V
REF
+
V
REF
–
V
REF
–1.5
= 5V
= GND
0
–5
INL ERROR (ppm OF V
–10
–2
–1.5
0.5
1.5
1
2
2440 G04
2.5
–2.5
0
–5
INL ERROR (ppm OF V
–10
–2
–1.5
0
–0.5
–1
0.5
1.5
1
2.5
2
–2.5
VIN (V)
2440 G02
= 220Hz Integral Nonlinearity f
V
INCM
= GND
f
O
T
= 25°C
A
OUT
= 2.5V
)
REF
10
5
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
–
= GND
V
REF
0
–1
–0.5
0.5
1.5
1
2.5
2
VIN (V)
2440 G03
= 110Hz
OUT
V
= 2.5V
INCM
= GND
f
O
= 25°C
T
A
0
–5
INL ERROR (ppm OF V
–10
–2
–1.5
0
–1
–0.5
0.5
1.5
1
2.5
2
–2.5
VIN (V)
2440 G05
0
0.5
1
1.5
2.5
–1
–0.5
2
VIN (V)
2440 G06
Integral Nonlinearity f
10
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
)
REF
INL ERROR (ppm OF V
–10
–
= GND
V
REF
5
0
–5
–2
–1.5
–2.5
–1
V
INCM
= GND
f
O
= 25°C
T
A
–0.5
= 2.5V
0
VIN (V)
= 55Hz Integral Nonlinearity f
OUT
10
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
)
REF
–
= GND
V
REF
5
0
–5
INL ERROR (ppm OF V
–10
–2
–1.5
0.5
1
1.5
2.5
2
2440 G07
–2.5
Integral Nonlinearity
= 27.5Hz
OUT
V
= 2.5V
INCM
= GND
f
O
= 25°C
T
A
0
0.5
1
1.5
2.5
–1
–0.5
2
VIN (V)
2440 G08
f
= 13.75Hz
OUT
10
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
)
REF
–
= GND
V
REF
5
0
–5
INL ERROR (ppm OF V
–10
–2
–1.5
–2.5
V
= 2.5V
INCM
= GND
f
O
= 25°C
T
A
0
0.5
1
1.5
2.5
–1
–0.5
2
VIN (V)
2440 G09
2440fc
5
LTC2440
TYPICAL PERFORMANCE CHARACTERISTICS
Integral Nonlinearity
f
= 6.875Hz
OUT
10
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
)
REF
–
= GND
V
REF
5
0
–5
INL ERROR (ppm OF V
–10
–2
–1.5
–2.5
–1
Integral Nonlinearity
vs Temperature
10
VCC = 5V
= 2.5V
V
REF
+
= 2.5V
V
REF
–5
5
0
–1.25
V
REF
–
= GND
TA = –55°C
TA = 25°C
–0.75
)
REF
INL ERROR (ppm OF V
–10
V
= 2.5V
INCM
= GND
f
O
= 25°C
T
A
0
–0.5
VIN (V)
V
= 1.25V
INCM
OSR = 32768
= GND
f
O
TA = 125°C
–0.25
VIN (V)
0.5
0.25
Integral Nonlinearity
vs Conversion Rate Integral Nonlinearity vs V
10
V
= 3.75V
)
REF
INCM
5
0
V
= 1.25V
INCM
–5
VCC = 5V
INL ERROR (ppm OF V
–10
–1.25
= 2.5V
V
REF
+
= 2.5V
V
REF
–
= GND
V
REF
–0.75
V
INCM
OSR = 32768
= GND
f
O
= 25°C
T
A
–0.25
VIN (V)
)
10.0
REF
7.5
VCC = 5V
= 5V
V
REF
+
V
REF
–
V
REF
= 5V
= GND
–2.5V ≤ V
= 2.5V
V
INCM
= GND
f
O
= 25°C
T
A
≤ 2.5V
IN
5.0
2.5
INL ERROR (ppm OF V
0
0
1.5
1
2.5
2
500 1000 1500 2000
2500 3000 3500
CONVERSION RATE (Hz)
2440 G10
2440 G11
0.25
= 2.5V
INCM
0.75
1.25
2440 G12
Integral Nonlinearity
0.75
2440 G13
1.25
vs Temperature –Full-Scale Error vs V
10
VCC = 5V
= 5V
V
REF
+
= 5V
V
REF
)
REF
–
= GND
V
REF
5
0
TA = 125°C
TA = –25°C
–5
INL ERROR (ppm OF V
–10
–2
–1.5
–2.5
V
INCM
OSR = 32768
= GND
f
O
–1
–0.5
= 2.5V
0
VIN (V)
TA = 25°C
0.5
1
1.5
2
2440 G14
2.5
20
)
REF
10
0
–10
–FULL-SCALE ERROR (ppm OF V
–20
1
0
2
V
REF
(V)
REF
3
4
5
2440 G15
+Full-Scale Error vs V
20
)
REF
10
0
–10
+FULL-SCALE ERROR (ppm OF V
–20
1
0
6
REF
–Full-Scale Error vs V
10
9
)
REF
8
CC
7
6
5
4
3
V
= 2.5V
REF
+
2
V
REF
FULL-SCALE ERROR (ppm OF V
3
4
2
V
(V)
REF
5
2440 G16
–
V
REF
1
V
INCM
0
4.5
= 2.5V
= GND
= 1.25V
4.7
OSR = 32768
fO = GND
= 25°C
T
A
5.1
4.9
VCC (V)
5.3
5.5
2440 G17
+Full-Scale Error vs V
0
V
= 2.5V
REF
+
–1
V
)
REF
–
V
REF
REF
–2
V
INCM
–3
–4
–5
–6
–7
–8
FULL-SCALE ERROR (ppm OF V
–9
–10
4.5
= 2.5V
= GND
= 1.25V
4.7
OSR = 32768
= GND
f
O
= 25°C
T
A
4.9
VCC (V)
5.1
CC
5.3
5.5
2440 G18
2440fc
TYPICAL PERFORMANCE CHARACTERISTICS
LTC2440
Negative Full-Scale Error
vs Temperature
20
)
15
REF
10
5
0
5.5V
–5
–10
FULL-SCALE ERROR (ppm OF V
–15
–20
–55
4.5V
5V
–25 5
VCC = 4.5V
= 4.5V
V
REF
+
= 4.5V
V
REF
–
= GND
V
REF
= 2.25V
V
INCM
OSR = 32768
= GND
f
O
35
= 5.5V, 5V
V
CC
= 5V
V
REF
+
= 5V
V
REF
–
= GND
V
REF
= 2.5V
V
INCM
OSR = 32768
= GND
f
O
65 95
125
Positive Full-Scale Error
vs Temperature Offset Error vs V
20
)
15
REF
10
5.5V
5
0
–5
4.5V
–10
FULL-SCALE ERROR (ppm OF V
–15
–20
–55
–25 5
TEMPERATURE (°C)
2440 G19
Offset Error vs Conversion Rate Offset Error vs V
5.0
VCC = 5V
= 5V
V
REF
+
V
)
REF
–
V
REF
REF
2.5
= 5V
= GND
V
f
T
0
–2.5
OFFSET ERROR (ppm OF V
–5.0
0
500 1000 1500 2000
CONVERSION RATE (Hz)
+
IN
= GND
O
= 25°C
A
= V
IN
–
= GND
2500 3000 3500
2440 G22
5.0
VCC = 5V
= 5V
V
REF
2.5
+
V
REF
–
V
REF
)
REF
0
–2.5
OFFSET ERROR (ppm OF V
–5.0
0
INL vs Output Rate
(OSR = 128) External Clock Sweep
Offset Error vs Temperature
5.0
2.5
VCC = 5V
0
OFFSET ERROR (μV)
–2.5
–5.0
–55
VCC = 5.5V
VCC = 4.5V
= 2.5V
V
REF
+
= 2.5V
V
REF
–
= GND
V
REF
+
= V
V
IN
IN
OSR = 256
= GND
f
O
53565
–25
TEMPERATURE (°C)
–
= GND
VCC = 4.5V
= 5.5V, 5V
V
CC
= 5V
V
REF
+
= 5V
V
REF
–
= GND
V
REF
+
= V
V
IN
OSR = 256
= GND
f
O
–
= GND
IN
95 125
2440 G25
10MHz to 20MHz
20
18
16
14
EXTERNAL CLOCK 10MHz
12
(OR INTERNAL OSCILLATOR)
10
8
LINEARITY (BITS)
6
V
4
REF
TEMP = 25°C
2
SWEEP (V
0
2000
5V
VCC = 4.5V
= 4.5V
V
REF
+
= 4.5V
V
REF
–
= GND
V
REF
= 2.25V
V
INCM
OSR = 32768
= GND
f
O
35
= 5.5V, 5V
V
CC
= 5V
V
REF
+
= 5V
V
REF
–
= GND
V
REF
= 2.5V
V
INCM
OSR = 32768
= GND
f
O
65 95
TEMPERATURE (°C)
2440 G20
INCM
+
–
V
= V
= V
IN
IN
(V)
/2) TO V
INCM
3
EXTERNAL
CLOCK 20MHz
/2
REF
4
2440 G23
2440 G26
OSR = 32768
= 5V
= GND
= GND
f
O
= 25°C
T
A
1
2
V
INCM
= VCC = 5V
– V
IN
REF
2500 3000 3500 4000
OUTPUT RATE (Hz)
125
5.0
)
REF
2.5
0
–2.5
OFFSET ERROR (ppm OF V
–5.0
3.5
3.0
2.5
2.0
RMS NOISE (μV)
1.5
1.0
5
0.5
5
4
3
2
RMS NOISE (μV)
1
0
2000
CC
–
= GND
OSR = 32768
= GND
f
O
= 25°C
T
A
4.9
VCC (V)
5.1
4.5
V
= 2.5V
REF
+
= 2.5V
V
REF
–
= GND
V
REF
+
= V
V
IN
IN
4.7
RMS Noise vs Temperature
VCC = 4.5V
VCC = 5V
VCC = 5.5V
–55
VCC = 4.5V
= 2.5V
V
REF
+
= 2.5V
V
REF
–
= GND
V
REF
+
–
= V
V
IN
IN
OSR = 256
= GND
f
O
53565
–25
= GND
= 5.5V, 5V
V
CC
= 5V
V
REF
+
= 5V
V
REF
–
= GND
V
REF
+
= V
V
IN
OSR = 256
= GND
f
O
TEMPERATURE (°C)
RMS Noise vs Output Rate
(OSR = 128) External Clock
Sweep 10MHz to 20MHz
V
= VCC = 5V
REF
TEMP = 25°C
±V
/2
V
IN
REF
2500 3000 3500 4000
OUTPUT RATE (Hz)
5.3
2440 G21
–
= GND
IN
95 125
5.5
2440 G24
2440 G27
2440fc
7
LTC2440
PIN FUNCTIONS
GND (Pins 1, 8, 9, 16): Ground. Multiple ground pins
internally connected for optimum ground current fl ow and
decoupling. Connect each one of these pins to a ground
V
CC
plane through a low impedance connection. All four pins
must be connected to ground for proper operation.
(Pin 2): Positive Supply Voltage. Bypass to GND
V
CC
(Pin 1) with a 10μF tantalum capacitor in parallel with 0.1μF
ceramic capacitor as close to the part as possible.
+
(Pin 3), REF– (Pin 4): Differential Reference Input.
REF
The voltage on these pins can have any value between
GND and V
as long as the reference positive input, REF+,
CC
is maintained more positive than the reference negative
–
input, REF
+
IN
(Pin 5), IN– (Pin 6): Differential Analog Input. The
, by at least 0.1V.
voltage on these pins can have any value between GND
– 0.3V and V
bipolar input range (V
• (V
) to 0.5 • (V
REF
+ 0.3V. Within these limits the converter
CC
= IN+ – IN–) extends from –0.5
IN
). Outside this input range the
REF
converter produces unique overrange and underrange
output codes.
SDI (Pin 7): Serial Data Input. This pin is used to select
the speed/resolution of the converter. If SDI is grounded
(pin compatible with LTC2410) the device outputs data at
880Hz with 21 bits effective resolution. By tying SDI HIGH,
the converter enters the ultralow noise mode (200nV
RMS
)
with simultaneous 50/60Hz rejection at 6.9Hz output
rate. SDI may be driven logic HIGH or LOW anytime during the conversion or sleep state in order to change the
speed/resolution. The conversion immediately following
the data output cycle will be valid and performed at the
newly selected output rate/resolution. SDI may also be
programmed by a serial input data stream under control of
SCK during the data output cycle. One of ten speed/resolution ranges (from 6.9Hz/200nV
to 3.5kHz/21μV
RMS
RMS
)
may be selected. The fi rst conversion following a new
selection is valid and performed at the newly selected
speed/resolution.
EXT (Pin 10): Internal/External SCK Selection Pin. This pin
is used to select internal or external SCK for outputting
data. If EXT is tied low (pin compatible with the LTC2410),
the device is in the external SCK mode and data is shifted
out the device under the control of a user applied serial
clock. If EXT is tied high, the internal serial clock mode
is selected. The device generates its own SCK signal and
outputs this on the SCK pin. A framing signal BUSY (Pin 15)
goes low indicating data is being output.
CS (Pin 11): Active LOW Digital Input. A LOW on this pin
enables the SDO digital output and wakes up the ADC.
Following each conversion the ADC automatically enters
the Sleep mode and remains in this low power state as
long as CS is HIGH. A LOW-to-HIGH transition on CS
during the Data Output transfer aborts the data transfer
and starts a new conversion.
SDO (Pin 12): Three-State Digital Output. During the Data
Output period, this pin is used as serial data output. When
the chip select CS is HIGH (CS = V
) the SDO pin is in a
CC
high impedance state. During the Conversion and Sleep
periods, this pin is used as the conversion status output. The
conversion status can be observed by pulling CS LOW.
SCK (Pin 13): Bidirectional Digital Clock Pin. In Internal
Serial Clock Operation mode, SCK is used as digital output
for the internal serial interface clock during the Data Output
period. In External Serial Clock Operation mode, SCK is used
as digital input for the external serial interface clock during
the Data Output period. The Serial Clock Operation mode is
determined by the logic level applied to the EXT pin.
fO (Pin 14): Frequency Control Pin. Digital input that controls the internal conversion clock. When fO is connected to
VCC or GND, the converter uses its internal oscillator running
at 9MHz. The conversion rate is determined by the selected
OSR such that t
(t
= 1.137ms at OSR = 256, t
CONV
32768). The fi rst null is located at 8/ t
(in ms) = (40 • OSR + 170)/9000
CONV
= 146ms at OSR =
CONV
, 7kHz at OSR =
CONV
256 and 55Hz (simultaneous 50/60Hz) at OSR = 32768.
When f
kHz), the conversion time becomes t
170)/f
is driven by an oscillator with frequency f
O
= (40 • OSR +
CONV
(in ms) and the fi rst null remains 8/t
EOSC
EOSC
CONV
(in
.
BUSY (Pin 15): Conversion in Progress Indicator. For
compatibility with the LTC2410, this pin should not be
tied to ground. This pin is HIGH while the conversion
is in progress and goes LOW indicating the conversion
is complete and data is ready. It remains low during the
sleep and data output states. At the conclusion of the data
output state, it goes HIGH indicating a new conversion
has begun.
2440fc
8
FUNCTIONAL BLOCK DIAGRAM
LTC2440
V
CC
GND
+
REF
REF
IN
–
IN
+
–
+
–
TEST CIRCUITS
SDO
DAC
1.69k
Hi-Z TO V
VOL TO V
VOH TO Hi-Z
–+
OH
OH
C
LOAD
= 20pF
2440 TA03
ADC
Figure 1. Functional Block Diagram
AUTOCALIBRATION
AND CONTROL
DECIMATING FIR
2440 F01
SDO
Hi-Z TO V
VOH TO V
VOL TO Hi-Z
INTERNAL
OSCILLATOR
f
O
(INT/EXT)
SDO
SCK
SERIAL
INTERFACE
V
CC
1.69k
= 20pF
C
LOAD
OL
OL
2440 TA04
CS
SDI
BUSY
EXT
APPLICATIONS INFORMATION
CONVERTER OPERATION
Converter Operation Cycle
The LTC2440 is a high speed, delta-sigma analog-to-digital
converter with an easy to use 4-wire serial interface (see
Figure 1). Its operation is made up of three states. The
converter operating cycle begins with the conversion,
followed by the low power sleep state and ends with the
data output (see Figure 2). The 4-wire interface consists
of serial data input (SDI), serial data output (SDO), serial
clock (SCK) and chip select (CS). The interface, timing,
operation cycle and data out format is compatible with
the LTC2410.
CONVERT
SLEEP
FALSE
CS = LOW
AND
SCK
TRUE
DATA OUTPUT
2440 F02
Figure 2. LTC2440 State Transition Diagram
2440fc
9
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