LINEAR TECHNOLOGY LTC2450 Technical data
Digitize a $1000 Sensor with a $1 Analog-to-Digital Converter
Design Note 456
Mark Thoren
Introduction
The LTC2450 is a 16-bit, single-ended input, deltasigma ADC in a 2mm × 2mm DFN package, but don’t
let its small size and low cost fool you. The LTC2450
has impressive DC specs, including 2LSB INL, 2LSB
off set and a 0.01% gain error, making it a perfe ct match
for many high-end industrial sensors, as well as a wide
variety of data acquisition, measurement, control and
general purpose voltage monitoring applications.
Digitize an Accurate Sensor with an
Accurate ADC
The Setra Model 270 is an exquisitely accurate
barometric pressure sensor often used in weather
stations and semiconductor manufacturing. A 600 to
1100 millibar input range corresponds to a 0V to 5V
output. Despite its high accuracy, it does not require an
expensive ADC to take full advantage of this sensor’s
performance. The LTC2450’s DC specifi cations are
more than adequate for the 270’s 0.05% accuracy.
Figure 1 shows the basic connections, and Figure 2
sh ow s t he c ha ng e in ba rom et ic p re ssur e f rom wa lk in g
up and down a fl ight of stairs, taking a short break on
each stair. After sample 2500, the sensor is resting on
the bottom stair measuring only the change in ambient
pressure. The spikes at the end of the graph are from
a door opening and closing. The Setra 270 must be
treated as a 4-terminal device, as the negative output
is nominally 5V above the negative excitation. Figure
3 shows an isolated supply for the sensor’s excitation,
and the 4.7μF capacitor keeps switching noise from
affecting the measurement.
Not So Obvious Features
Mixed signal designers often try to extract more resolution from an ADC by averaging samples. Averaging
reduces the signal bandwidth and improves resolution,
but it consumes processing resources, complicates
fi rmware and necessitates the use of a fast ADC even
though the application may require a much slower data
rate. It also does not improve accuracy. The LTC2450
is inherently very accurate, and it does the averaging
for you. The front-end sample rate of the LTC2450
is 3.9Msps, which is decimated to 30 samples per
1
second by a Sinc
digital fi lter. The fi lter’s effective
bandwidth is approximately 30Hz, which means noise
between 30Hz and 3.9MHz is greatly attenuated. When
combined with a simple 1-pole fi lter, wideband noise
is generally not a concern.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
12/08/456
ISOLATED
24V SUPPLY
(SEE FIGURE 3)
7V TO 10V
1μF
+
EXC
SETRA 270
600mB TO 1100mB
RANGE
0V TO 5V OUTPUT
–
EXC
4.7 F
OUT
OUT
LT6660-5
IN
+
1k
0.1
–
OUT
GND
GND
V
IN
F
V
LTC2450
GND
1Ω
0.1 F
CC
CS
SCK
SDO
DN456 F01
Figure 1. 16-Bit Barometric Pressure Measurement
1μF
SPI
INTERFACE
4.102
4.101
4.100
4.099
VOLTAGE (V)
4.098
BOTTOM STAIR
DOOR
OPENING
DOOR
CLOSING
1010.2
BAROMETRIC PRESSURE (MILLABARS)
1010.1
1010.0
1009.9
1009.8
4.097
4.096
0
TOP STAIR
SAMPLE NUMBER
2500 00540051
Figure 2. Stairway Pressure
5V
10k124k
10nF
10μF
6.8nF
BIAS
UVLO
TC
R
ILIM
SS
VC GNDTEST
10k
LT3573
100pF
0.22μF
1k
D2
V
R
147k
IN
51.1k
R
FB
REF
6.04k
SW
T1: COILTRONICS VPH1-0102 1:3
3 WINDINGS PARALLEL ON PRIMARY,
3 WINDINGS SERIES ON SECONDARY
D1: ZHCS506TA
D2: ZHCS500TA
Figure 3. 24V Isolated Excitation Supply
A break through feature of the LTC2450 is the propri etary
modulator switching scheme that reduces the average
input current by orders of magnitude compared to
ADCs with similar specifi cations. Ordinarily, an RC
fi lter produces offset and gain errors due to the ADC’s
average input current fl owing through the resistor.
The very low 50nA average sampling current of the
LTC2450 produces less than a 1LSB error with a 1k,
0.1μF fi lter. This fi lter is more than adequate for limiting
the wideband noise of most active devices driving the
ADC, as well as preventing sampling current “spikes”
from the modulator from affecting the source.
1009.7
1009.6
55003500500 2000 000500040001 3000
DN456 F02
100
V
24V, 85mA
(TO EXC
μF
V
(TO EXC–)
OUT
OUT
+
+
)
–
1,2,3
4,5,6
T1
1:3
12
(PIN 8 TO 10)
(PIN 9 TO 11)
7
D1
22
μF
DN456 F03
Conclusion
With manufacturers claiming up to 32 bits of resolution on their precision ADCs, and a confusing array
of choices between 8 and 32 bits, often with highly
obfuscated data sheets, where does a “medium speed,
medium resolution” part like the LTC2450 fi t in? It fi ts
pretty much anywhere where you would use a 4-1/2
digit (40,000 count) digital voltmeter to test your circuit. At the other end of the performance spectrum,
the LTC2450 is more economical than many 12-bit
ADCs. The next time you are searching for an ADC,
consider the LTC2450. It may be just what you need
for performance, while occupying a tiny amount of
board space that matches its tiny price ($1.15 each in
1000-piece quantities).
Data Sheet Download
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(408) 432-1900
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dn456f LT/TP 1208 392K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2008
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