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. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 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 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008