DN456 - Digitize a $1000 Sensor with a $1 Analog-to-Digital Converter

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
offset and a 0.01% gain error, making it a perfect 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 specifications are
more than adequate for the 270’s 0.05% accuracy.
Figure 1 shows the basic connections, and Figure 2
shows the change in barometic pressure from walking
up and down a flight 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
firmware 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
second by a Sinc1 digital filter. The filter’s effective
bandwidth is approximately 30Hz, which means noise
between 30Hz and 3.9MHz is greatly attenuated. When
combined with a simple 1-pole filter, wideband noise
is generally not a concern.
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LT6660-5
7V TO 10V
1μF
1Ω
OUT
IN
1μF
GND GND
0.1 F
VCC
EXC+
OUT
ISOLATED
24V SUPPLY
(SEE FIGURE 3)
SETRA 270
600mB TO 1100mB
RANGE
0V TO 5V OUTPUT
+
VIN
LTC2450
1k
0.1 F
CS
SCK
SPI
INTERFACE
SDO
GND
DN456 F01
OUT–
EXC
–
4.7 F
Figure 1. 16-Bit Barometric Pressure Measurement
12/08/456
4.102
1010.2
VOLTAGE (V)
4.101
1010.1
BOTTOM STAIR
4.100
1010.0
DOOR
CLOSING
4.099
1009.9
4.098
1009.8
4.097
4.096
1009.7
TOP STAIR
0
500
1000
1500
2000
2500 3000 3500
SAMPLE NUMBER
4000
4500
BAROMETRIC PRESSURE (MILLABARS)
DOOR
OPENING
1009.6
5500
5000
DN456 F02
Figure 2. Stairway Pressure
T1
1,2,3 1:3 12
5V
10μF
1k
0.22μF
D2
VOUT+
24V, 85mA
(TO EXC+)
D1
(PIN 8 TO 10)
(PIN 9 TO 11)
4,5,6
22μF
100μF
VOUT–
(TO EXC–)
7
147k
BIAS
VIN
51.1k
UVLO
RFB
RREF
TC
LT3573
RILIM
6.04k
SW
SS
124k
VC
10k
10nF
TEST GND
10k
100pF
6.8nF
T1: COILTRONICS VPH1-0102 1:3
3 WINDINGS PARALLEL ON PRIMARY,
3 WINDINGS SERIES ON SECONDARY
D1: ZHCS506TA
D2: ZHCS500TA
DN456 F03
Figure 3. 24V Isolated Excitation Supply
A breakthrough feature of the LTC2450 is the proprietary
modulator switching scheme that reduces the average
input current by orders of magnitude compared to
ADCs with similar specifications. Ordinarily, an RC
filter produces offset and gain errors due to the ADC’s
average input current flowing through the resistor.
The very low 50nA average sampling current of the
LTC2450 produces less than a 1LSB error with a 1k,
0.1μF filter. This filter 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.
Data Sheet Download
www.linear.com
Linear Technology Corporation
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 fit in? It fits
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).
For applications help,
call (408) 432-1900
dn456f LT/TP 1208 392K • PRINTED IN THE USA
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