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LCL8052CDD Datasheet(PDF) 10 Page - Intersil Corporation |
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LCL8052CDD Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 19 page 3-43 Component Value Selection For optimum performance of the analog section, care must be taken in the selection of values for the integrator capacitor and resistor, auto-zero capacitor, reference voltage, and conversion rate. These values must be chosen to suit the particular application. Integrating Resistor The integrating resistor is determined by the full scale input voltage and the output current of the buffer used to charge the integrator capacitor. This current should be small compared to the output short circuit current such that thermal effects are kept to a minimum and linearity is not affected. Values of 5 µA to 40µA give good results with a nominal of 20 µA. The exact value may be chosen by: NOTE: If gain is used in the buffer amplifier, then: Integrating Capacitor The product of integrating resistor and capacitor is selected to give 9V swing for full scale inputs. This is a compromise between possibly saturating the integrator (at +14V) due to tolerance buildup between the resistor, capacitor and clock and the errors a lower voltage swing could induce due to offsets referred to the output of the comparator. In general, the value of CINT is given by: A very important characteristic of the integrating capacitor is that it has low dielectric absorption to prevent roll-over 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 be read half scale 1.0000, and any deviation is probably due to dielectric absorption. Polypropylene capacitors give undetectable errors at reason- able cost. Polystyrene and polycarbonate capacitors may be used in less critical applications. Auto-Zero and Reference Capacitor The size of the auto-zero capacitor has some influence on the noise of the system, with a larger value capacitor giving less noise. The reference capacitor should be large enough such that stray capacitance to ground from its nodes is negligible. When gain is used in the buffer amplifier the reference capacitor should be substantially larger than the auto-zero capacitor. As a rule of thumb, the reference capacitor should be approximately the gain times the value of the auto-zero capacitor. The dielectric absorption of the reference cap and auto-zero cap are only important at power-on or when the circuit is recovering from an overload. Thus, smaller or cheaper caps can be used here 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 = 2VREF. 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 an external high quality reference be used where ambient temperature is not controlled or where high-accuracy absolute measurements are being made. Buffer Gain At the end of the auto-zero interval, the instantaneous noise voltage on the auto-zero capacitor is stored and subtracted from the input voltage while adding to the reference voltage during the next cycle. The result of this is that the noise voltage is effectively somewhat greater than the input noise voltage of the buffer itself during integration. By introducing some voltage gain into the buffer, the effect of the auto-zero noise (referred to the input) can be reduced to the level of the inherent buffer noise. This generally occurs with a buffer gain of between 3 and 10. Further increase in buffer gain merely increases the total offset to be handled by the auto- zero loop, and reduces the available buffer and integrator swings, without improving the noise performance of the system. The circuit recommended for doing this with the ICL8068/ICL71C03 is shown in Figure 6. R INT Full Scale Voltage (See Note) 20 µA ------------------------------------------------------------------------------- = R INT Buffe rGain () (Full Scale Voltage) 20 µA -------------------------------------------------------------------------------------------- = C INT 10,000(4-1/2 Digit) 1000(3-1/2 Digit) Clock Period × 20 µA () × Integrator Output Voltage Swing ------------------------------------------------------------------------------------------------------------------------- = FIGURE 6. ADDING BUFFER GAIN TO ICL8068 A2 + - A3 + - INTEG. COMP. A1 + - BUFFER 14 11 9 INT OUT -INT IN BUF OUT 10 -BUF IN -1.2V 2 -15V 1 -15V 7 8 +15V 12 +INT IN 13 ICL8068 INT. REF. 6 3 +BUF IN 5 REF OUT 10k Ω 1k Ω 300pF COMP OUT 100k Ω 10-50K TO ICL7104 ICL8052/ICL71C03, ICL8068/ICL71C03 |
Similar Part No. - LCL8052CDD |
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Similar Description - LCL8052CDD |
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