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TC7117AIJL データシート(PDF) 11 Page - TelCom Semiconductor, Inc |
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TC7117AIJL データシート(HTML) 11 Page - TelCom Semiconductor, Inc |
11 / 14 page 3-213 TELCOM SEMICONDUCTOR, INC. 7 6 5 4 3 1 2 8 3-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTERS WITH HOLD TC7116 TC7116A TC7117 TC7117A COMPONENT VALUE SELECTION Auto-Zero Capacitor The size of the auto-zero capacitor has some influ- ence on system noise. For 200 mV full scale, where noise is very important, a 0.47 µF capacitor is recommended. On the 2V scale, a 0.047 µF capacitor increases the speed of recovery from overload and is adequate for noise on this scale. Reference Capacitor A 0.1 µF capacitor is acceptable in most applications. However, where a large common-mode voltage exists (i.e., the V – IN pin is not at analog common), and a 200-mV scale is used, a larger value is required to prevent roll-over error. Generally, 1 µF will hold the roll-over error to 0.5 count in this instance. Integrating Capacitor The integrating capacitor should be selected to give the maximum voltage swing that ensures tolerance build-up will not saturate the integrator swing (approximately 0.3V from either supply). In the TC7116/TC7116A or the TC7117/ TC7117A, when the analog common is used as a reference, a nominal ±2V full- scale integrator swing is acceptable. For the TC7117/TC7117A, with ±5V supplies and analog com- mon tied to supply ground, a ±3.5V to ±4V swing is nominal. For 3 readings per second (48 kHz clock), nominal values for CINT are 0.22 µ1F and 0.10 µF, respectively. If different oscillator frequencies are used, these values should be changed in inverse proportion to maintain the output swing. The integrating capacitor must have low dielectric ab- sorption to prevent roll-over errors. Polypropylene capaci- tors are recommended for this application. Integrating Resistor Both the buffer amplifier and the integrator have a class A output stage with 100 µA of quiescent current. They can supply 20 µA of drive current with negligible nonlinearity. The integrating resistor should be large enough to remain in this very linear region over the input voltage range, but small enough that undue leakage requirements are not placed on the PC board. For 2V full scale, 470 k Ω is near optimum and, similarly, 47 k Ω for 200 mV full scale. Oscillator Components For all frequency ranges, a 100-k Ω resistor is recom- mended; the capacitor is selected from the equation: For a 48 kHz clock (3 readings per second), C = 100 pF. Figure 10. Negative Power Supply Generation With TC7660 VREF + V + TC7117 TC7117A 36 10 µF VIN + VIN – COM GND 32 31 30 21 10 µF VIN V – (–5V) 5 3 8 2 4 +5V 35 TC7660 26 + + + – TC04 LED DRIVE Reference Voltage To generate full-scale output (2000 counts), the analog input requirement is VIN = 2 VREF. Thus, for the 200 mV and 2V scale, VREF should equal 100 mV and 1V, respectively. In many applications, where the ADC is connected to a transducer, a scale factor exists between the input voltage and the digital reading. For instance, in a measuring system the designer might like to have a full-scale reading when the voltage from the transducer is 700 mV. Instead of dividing the input down to 200 mV, the designer should use the input voltage directly and select VREF = 350 mV. Suitable values for integrating resistor and capacitor would be 120 k Ω and 0.22 µF. This makes the system slightly quieter and also avoids a divider network on the input. The TC7117/TC7117A, with ±5V supplies, can accept input signals up to ±4V. Another advantage of this system is when a digital reading of zero is desired for VIN ≠ 0. Temperature and weighing systems with a variable tare are examples. This offset reading can be conveniently generated by connecting the voltage transducer between V + IN and analog common, and the variable (or fixed) offset voltage between analog com- mon and V – IN. TC7117/TC7117A POWER SUPPLIES The TC7117/TC7117A are designed to operate from ±5V supplies. However, if a negative supply is not available, it can be generated with a TC7660 DC-to-DC converter and two capacitors. Figure 10 shows this application. In selected applications, a negative supply is not re- quired. The conditions for using a single +5V supply are: (1) The input signal can be referenced to the center of the common-mode range of the converter. (2) The signal is less than ±1.5V. (3) An external reference is used. 45 RC f = . |
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