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TC835 Datasheet(PDF) 9 Page - TelCom Semiconductor, Inc |
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TC835 Datasheet(HTML) 9 Page - TelCom Semiconductor, Inc |
9 / 12 page 3-73 TELCOM SEMICONDUCTOR, INC. 7 6 5 4 3 1 2 8 TC835 PERSONAL COMPUTER DATA ACQUISITION A/D CONVERTER The stability of the reference voltage is a major factor in the overall absolute accuracy of the converter. For this reason, it is recommended that a high-quality reference be used where high-accuracy absolute measurements are being made. Suitable references are: Part Type Manufacturer TC04A TelCom Semiconductor TC9491 TelCom Semiconductor APPLICATIONS INFORMATION Component Value Selection The integrating resistor is determined by the full-scale input voltage and the output current of the buffer used to charge the integrator capacitor. Both the buffer amplifier and the integrator have a class A output stage, with 100 µA of quiescent current. A 20 µA drive current gives negligible linearity errors. Values of 5 µA to 40 µA give good results. The exact value of an integrating resistor for a 20 µA current is easily calculated. RINT = Integrating Capacitor The product of integrating resistor and capacitor should be selected to give the maximum voltage swing that ensures the tolerance buildup will not saturate the integrator swing (approximately 0.3V from either supply). For ±5V supplies and ANALOG COMMON tied to supply ground, a ±3.5V to ±4V full-scale integrator swing is adequate. A 0.10 µF to 0.47 µF is recommended. In general, the value of CINT is given by: CINT = = [10,000 × clock period] × IINT Integrator output voltage swing (10,000) (clock period) (20 µA) Integrator output voltage swing A very important characteristic of the integrating capaci- tor is that it has low dielectric absorption to prevent rollover or ratiometric errors. A good test for dielectric absorption is to use the capacitor with the input tied to the reference. This ratiometric condition should read half-scale 0.9999, any deviation is probably due to dielectric absorption. Polypro- pylene capacitors give undetectable errors at reasonable cost. Polystyrene and polycarbonate capacitors may also be used in less critical applications. Auto-Zero and Reference Capacitors The size of the auto-zero capacitor has some influence on the noise of the system. A large capacitor reduces the noise. The reference capacitor should be large enough such that stray capacitance to ground from its nodes is negligible. The dielectric absorption of the reference capacitor and auto-zero capacitor are only important at power-on, or when the circuit is recovering from an overload. Smaller or cheaper capacitors can be used if accurate readings are not required for the first few seconds of recovery. Reference Voltage The analog input required to generate a full-scale output is VIN = 2 VREF. Conversion Timing Line Frequency Rejection A signal integration period at a multiple of the 60 Hz line frequency will maximize 60 Hz "line noise" rejection. A 200 kHz clock frequency will reject 60 Hz and 400 Hz noise. This corresponds to five readings per second. full-scale voltage 20 µA Conversion Rate vs Clock Frequency Oscillator Frequency Conversion Rate (kHz) (Conv/Sec) 100 2.5 120 3 200 5 300 7.5 400 10 800 20 1200 30 Oscillator Frequency (kHz) 60 Hz 50 Hz 400 Hz 50.000 • • • 53.333 — — • 66.667 • — • 80.000 — — • 83.333 — • • 100.000 • • • 125.000 — • • 133.333 — — • 166.667 — — • 200.000 • — • 250.000 — • • The conversion rate is easily calculated: Conversion Rate (Readings 1/sec) = Line Frequency Rejection Clock Frequency (Hz) 4000 |
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