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TC820 数据表(PDF) 10 Page - Microchip Technology |
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TC820 数据表(HTML) 10 Page - Microchip Technology |
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10 / 28 page  TC820 DS21476B-page 10 © 2002 Microchip Technology Inc. 3.2.3 SIGNAL INTEGRATION PHASE Upon completion of the auto-zero phase, the auto-zero loop is opened and the internal differential inputs con- nect to VIN+ and VIN-. The differential input signal is then integrated for a fixed time period, which is 2000 counts (4000 clock periods). The externally set clock frequency is divided by two before clocking the internal counters. The integration time period is: EQUATION 3-3: The differential input voltage must be within the device's Common mode range when the converter and measured system share the same power supply com- mon (ground). If the converter and measured system do not share the same power supply common, as in battery powered applications, VIN- shouldbetiedto analog common. Polarity is determined at the end of signal integration phase. The sign bit is a "true polarity" indication, in that signals less than 1LSB are correctly determined. This allows precision null detection that is limited only by device noise and auto-zero residual offsets. 3.2.4 REFERENCE INTEGRATE (DE-INTEGRATE) PHASE The reference capacitor, which was charged during the auto-zero phase, is connected to the input of the inte- grating amplifier. The internal sign logic ensures the polarity of the reference voltage is always connected in the phase opposite to that of the input voltage. This causes the integrator to ramp back to zero at a constant rate, determined by the reference potential. The amount of time required (tDEINT) for the integrating amplifier to reach zero is directly proportional to the amplitude of the voltage that was put on the integrating capacitor (VINT) during the integration phase. EQUATION 3-4: The digital reading displayed by the TC820 is: The oscillator frequency is divided by 2 prior to clock- ing the internal decade counters. The four-phase mea- surement cycle takes a total of 8000 (4000) counts or 16,000 clock pulses. The 8000 count phase is indepen- dent of input signal magnitude or polarity. Each phase of the measurement cycle has the follow- ing length: TABLE 3-2: MEASUREMENT CYCLE PHASE LENGTH Note 1: This time period is fixed. The integration period for theTC820 is: tINT (TC820) = 4000/FOSC = 2000 counts. Where FOSC is the clock oscillator frequency. 2: Times shown are the RANGE/FREQ at logic low (normal operation). When RANGE/FREQ is logic high, signal integrate times are 200 counts. See Section 3.2.7, “10:1 Range Change”. 3.2.5 INPUT OVER RANGE When the analog input is greater than full scale, the LCD will display "OL" and the "OVER RANGE" LCD annunciator will be on. 3.2.6 PEAK READING HOLD The TC820 provides the capability of holding the high- est (or peak) reading. Connecting the PK HOLD input to VDD enables the peak hold feature. At the end of each conversion, the contents of the TC820 counter are compared to the contents of the display register. If the new reading is higher than the reading being dis- played, the higher reading is transferred to the display register. A "higher" reading is defined as the reading with the higher absolute value. The peak reading is held in the display register, so the reading will not "droop" or slowly decay with time. The held reading will be retained until a higher reading occurs, the PK HOLD input is disconnected from VDD, or power is removed. The peak signal to be measured must be present dur- ing the TC820 signal integrate period. The TC820 does not perform transient peak detection of the analog input signal. However, in many cases, such as measuring temperature or electric motor starting current, the TC820 "acquisition time" will not be a limitation. If true peak detection is required, a simple circuit will suffice. See the applications section for details. The peak reading function is also available when the TC820 is in the Frequency Counter mode. The counter auto-ranging feature is disabled when peak reading hold is selected. tINT = 4000 FOSC tDEINT = RINTCINTVINT VREF VIN+VIN- Digital Count = 2000 VREF Conversion Phase Counts 1) Auto-Zero 1500 2) Signal Integrate (Notes 1, 2) 2000 3) Reference Integrate 1 to 4001 4) Integrator Output Zero 499 to 4499 |
类似零件编号 - TC820 |
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类似说明 - TC820 |
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