This action correctly arranges A2's input configuration. In operation, comparator A1 controls the state of switches S B and S C, causing the polarity of A2's feedback loop to match the polarity of the external "PRTD" excitation. Optical isolation of the PWM signal is performed by the dual-channel optoisolator U4. Either polarity of dc excitation is accommodated, and all necessary operating power (less than 10 ♚ at 10 V) for the isolated part of the DAC circuitry is derived from photovoltaic optoisolator U1. 138) over the full range of the DIN 43 760 standard of −200☌ to 850☌. 112) and algorithms (see "Efficient Algorithms Improve The Linearization Of Platinum RTDs," electronic design, Oct. This feature makes it possible to thoroughly optimize PRTD linearization (see "Precision Thermometer Linearizes, Digitally Calibrates Platinum RTDs," Electronic Design, May 29, 2000, p. The circuit can therefore be used in automated digital calibration of precision PRTD signal conditioning. It can be used to digitally simulate a 1000-Ω (at 0☌) platinum-resistance temperature detector (PRTD). Here, PWM is the 0 to 1.0 duty cycle of the control signal produced by the HC12 microcontroller. Instead, it outputs an optically isolated pure ohmic resistance in response to a PWM control input with 16-bit resolution (R O = R REF/PWM). This DAC circuit affords a fine example because it produces neither an analog voltage nor an analog current output ( see the figure). Sometimes, however, an application comes along with a satisfyingly weird wrinkle. But mostly that's about as exciting as the topic ever gets. Occasionally the conversation may become slightly enlivened by a requirement for such nonstandard features as optical isolation or a pulse-width-modulated (PWM) digital input. It's usually confined to a boring discussion of settling time, the number of bits of resolution needed, the virtues of serial or parallel interfaces, and whether current or voltage output is best. The subject of digital-to-analog converter (DAC) applications is generally rather tame.
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