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LTC1287CIJ Datasheet(PDF) 9 Page - Linear Technology |
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LTC1287CIJ Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page LTC1287 9 S APPLICATI I FOR ATIO such as obtained from a voltage regulator (e.g., LT1117). For high frequency bypassing a 0.1 µF ceramic disk placed in parallel with the 22 µF is recommended. Again the leads should be kept to a minimum. Using a battery to power the LTC1287 will help reduce the amount of bypass capacitance required on the VCC pin. A battery placed close to the device will only require 10 µF to adequately bypass the supply pin. Figure 4 shows the effect of poor VCC bypassing. Figure 5 shows the settling of a LT1117 low dropout regulator with a 22 µF bypass capacitor. The noise and ripple is kept around 0.5mV. Figure 6 shows the response of a lithium battery with a 10 µF bypass capacitor. The noise and ripple is kept below 0.5mV. Analog Inputs Because of the capacitive redistribution A/D conversion techniques used, the analog inputs of the LTC1287 have Figure 5. LT1117 Regulator with 22 µF Bypassing on VCC CS 5V/DIV VCC 0.5mV/DIV HORIZONTAL: 20 µs/DIV Figure 6. Lithium Battery with 10 µF Bypassing on VCC CS 5V/DIV VCC 0.5mV/DIV HORIZONTAL: 20 µs/DIV capacitive switching input current spikes. These current spikes settle quickly and do not cause a problem. If large source resistances are used or if slow settling op amps drive the inputs, take care to insure the transients caused by the current spikes settle completely before the conversion begins. Source Resistance The analog inputs of the LTC1287 look like a 100pF capacitor (CIN) in series with a 1.5k resistor (RON). This value for RON is for VCC = 2.7V. With larger supply voltages RON will be reduced. For example, with VCC = 2.7V and V – = – 2.7V, RON becomes 500Ω. CIN gets switched between (+) and (–) inputs once during each conversion cycle. Large external source resistors and capacitances will slow the settling of the inputs. It is important that the overall RC time constant is short enough to allow the analog inputs to settle completely within the allowed time. Figure 7. Analog Input Equivalent Circuit HORIZONTAL: 10 µs/DIV Figure 4. Poor VCC Bypassing. Noise and Ripple Can Cause A/D Errors CS ↑ RON = 1.5k tWHCS + 1/2 CLK CIN = 100pF LTC1287 “+” INPUT RSOURCE + VIN + C1 “–” INPUT RSOURCE – VIN – C2 LTC1287 F07 |
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