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AME811 Datasheet(PDF) 11 Page - Analog Microelectronics |
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AME811 Datasheet(HTML) 11 Page - Analog Microelectronics |
11 / 15 page Analog Microelectronics, Inc. 11 AME811/AME811A/AME811R 3-1/2 Digit A/D Converter - Low Power With HOLD And Differential Reference Inputs Integrating Capacitor (Cint) Cint should be selected to maximize the integrator out- put voltage swing without causing output saturation. A ± 2V full-scale integrator output swing is recommended if “ANALOG COMMON” is used as signal reference. For 3 readings/second (fosc = 48 KHz) a 0.22 µF value is suggested. If a different oscillator frequency is used, Cint must be changed in inverse proportion to maintain the nominal ± 2V integrator swing. An exact expres- sion for Cint is: Cint = [(4000)(1/fosc)(Vfs/Rint)] / Vint where: fosc= Oscillator clock frequency Vfs = Full-scale input voltage Rint = Integrating resistor Vint = Desired full-scale integrator output swing Cint must have low dielectric absorption to minimize rollover error. A polypropylene capacitor is recom- mended. Integrating Resistor (Rint) The input buffer amplifier and integrator both have a class A output stage with 100 µA quiescent current. The integrator and buffer can supply 20 µA drive cur- rents with negligible linearity errors. Rint is chosen to keep the output stage in the linear region. For a 200mV full-scale, it is 47K Ω; 2.0V full-scale requires 470KΩ. Summary of component selection: Note: fosc = 48 KHz Oscillator Components R-C Oscillator A 100 K Ω Rosc is recommended for all frequencies. Cosc is selected by using the equation: fosc = 0.45/(RC) For fosc of 48KHz, Cosc is 100pF nominally. To achieve maximum line noise rejection, the signal- integrate period should be a multiple of line period. The optimum oscillator frequencies for 60 Hz and 50 Hz rejection are listed as follows: For 60 Hz rejection: 40KHz, 48KHz, 60KHz etc. For 50 Hz rejection: 40KHz, 50KHz, 66-2/3KHz etc. Reference Voltage Selection A full-scale reading (2000 counts) requires the input signal be twice the reference voltage. In some applications a scale factor other than unity may exist between a transducer output voltage and the re- quired digital reading. Assume, for example, a pres- sure transducer output is 600 mV for 2000 Ib/in2. Rather than dividing the input voltage by three the reference voltage should be set to 300 mV. This permits the trans- ducer input to be used directly. The integrator resistor would be 120K Ω. In some temperature and weighting system with variable tare, the offset reading can be generated by connecting the voltage transducer be- tween INHI and COMMON and the variable offset volt- age between COMMON and INLO. Low Battery Flag (LB) The low battery flag is set when the supply voltage (V+ to V-) is lower than seven volts, typical. Once the LB is set, the waveform of the LB will be out of phase with the BP (Back Plane) to turn on a low battery annuncia- tor for AME811. F u l l s c a l e 200. 0m V 2 . 000V Ca z 0 . 4 7 µ F 0 . 047 µ F Ri n t 47 K Ω 470 K Ω C i nt 0. 22 µ F 0 . 22 µ F V r ef 100. 0m V 1 . 000V F u l l -S c a l e V o l tag e V r e f 20 0. 0 m V 10 0. 0 m V 2. 000 V 1 . 000 V |
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