| Technique yields precise calibration of dual-slope ADCs |
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By Greg Sutterlin and Vladimir Vitchev, Maxim Integrated Products, Sunnyvale, CA
Source: Embedded.com
Posted: Jan 18 2008 10:1 |
Rating: 2 (OK)
 
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Handheld devices for measuring toxic gas, blood glucose, and similar applications are increasingly popular, and their low cost has made them throwaway devices, to be discarded when the battery or the sensor expires. Typical devices include a lithium (Li) primary cell, a sensor, an A/D converter (ADC), conditioning circuitry, a microcontroller unit (MCU), and an LCD. To minimize cost, the design often employs simple LED indicators, a low-cost 8-pin MCU, and a discrete dual-slope ADC. This note explains the use of "offset flipping" for on-the-fly calibration of the ADC.
A block diagram of the circuit ( Figure 1) includes a single primary lithium (Li) cell, a millivolt-output bridge sensor, a differential amplifier, and the dual-slope ADC, plus correction circuitry for offset, zero, and span.
Figure 1: Block diagram of the slope-ADC calibration circuit.
(Click to enlarge image)
Component values are selected on assumption that the Li-cell voltage ranges from 2.2 V to 3.6 V. Because that voltage serves as bias for the bridge and also as reference for the ADC, the ADC input and its full-scale output (span) move together as the cell voltage changes. This ratiometric configuration minimizes error and eliminates the need for a precision voltage reference.
The sensor (S 1) produces 20 mV/V at full scale ( Figure 2).
Figure 2: This circuit (depicted in Figure 1) produces offset and span readings, to be stored and used for on-the-fly calibrations of a dual-slope A/D converter
(Click to enlarge image)
For a 3.6 V Li cell, therefore, the output is: 20 mV/V x 3.6 V = 72 mV. The dual op amp (U 2) draws only 18 ìA of quiescent current per amplifier. Its outputs swing rail-to-rail, and operate down to 1.8 V. U 2A is configured as a standard differential amplifier with gain of 30. Operating with a 3.6 V lithium cell, it achieves a full-scale output of 2.160 V. (Note that the resistor network around a differential amplifier loads the input-signal source (sensor), so the sensor should have a low output impedance. If not, you should buffer the sensor with an instrumentation amplifier or equivalent.)
The precision resistor dividers (R N1, R N2, and R N3) are available with accuracies from 0.035% to 0.1% (0.1% was selected for this design), and with divider ratios that exhibit a very low temperature coefficient. The resulting differential-amplifier output is
V OUTA =
VREF + RB/RA [(VINA- - VS1-)] " RC/RD [(VINA+ -VS1+)],
where
VINA- is the negative input of amplifier U2A,
VINA+ is the positive input of amplifier U2A
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ADC article fuck shitty
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Comments:
- dumb ass | 3/25/2008 7:46:00 PM
- The format of this article sucks
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