Datasheet LTC1287 (Analog Devices) - 9
| Hersteller | Analog Devices |
| Beschreibung | 3V Single Chip 12-Bit Data Acquisition System |
| Seiten / Seite | 16 / 9 — APPLICATI. S I FOR ATIO. Figure 6. Lithium Battery with 10. F Bypassing … |
| Dateiformat / Größe | PDF / 328 Kb |
| Dokumentensprache | Englisch |
APPLICATI. S I FOR ATIO. Figure 6. Lithium Battery with 10. F Bypassing on VCC. Analog Inputs. Source Resistance
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LTC1287
O U U W U APPLICATI S I FOR ATIO
such as obtained from a voltage regulator (e.g., LT1117). For high frequency bypassing a 0.1µF ceramic disk placed in parallel with the 22µF is recommended. Again the leads 5V/DIV CS 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 10µF to adequately bypass the 0.5mV/DIV VCC supply pin. Figure 4 shows the effect of poor VCC bypassing. Figure 5 shows the settling of a LT1117 low dropout regulator with a 22µF bypass capacitor. The noise and ripple is kept around 0.5mV. Figure 6 shows the response HORIZONTAL: 20µs/DIV of a lithium battery with a 10µF bypass capacitor. The
Figure 6. Lithium Battery with 10
µ
F Bypassing on VCC
noise and ripple is kept below 0.5mV. capacitive switching input current spikes. These current
Analog Inputs
spikes settle quickly and do not cause a problem. If large Because of the capacitive redistribution A/D conversion source resistances are used or if slow settling op amps techniques used, the analog inputs of the LTC1287 have 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 – VERTICAL: 0.5mV/DIV = – 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 HORIZONTAL: 10µs/DIV time constant is short enough to allow the analog inputs
Figure 4. Poor VCC Bypassing. Noise and
to settle completely within the allowed time.
Ripple Can Cause A/D Errors
“+” INPUT RSOURCE + V LTC1287 IN + CS↑ 5V/DIV CS C1 RON = 1.5k “–” CIN = INPUT R t 100pF SOURCE – WHCS + V 1/2 CLK IN – 0.5mV/DIV V C2 CC LTC1287 F07
Figure 7. Analog Input Equivalent Circuit
HORIZONTAL: 20µs/DIV
Figure 5. LT1117 Regulator with 22
µ
F Bypassing on VCC
1287fa 9
O U U W U APPLICATI S I FOR ATIO
such as obtained from a voltage regulator (e.g., LT1117). For high frequency bypassing a 0.1µF ceramic disk placed in parallel with the 22µF is recommended. Again the leads 5V/DIV CS 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 10µF to adequately bypass the 0.5mV/DIV VCC supply pin. Figure 4 shows the effect of poor VCC bypassing. Figure 5 shows the settling of a LT1117 low dropout regulator with a 22µF bypass capacitor. The noise and ripple is kept around 0.5mV. Figure 6 shows the response HORIZONTAL: 20µs/DIV of a lithium battery with a 10µF bypass capacitor. The
Figure 6. Lithium Battery with 10
µ
F Bypassing on VCC
noise and ripple is kept below 0.5mV. capacitive switching input current spikes. These current
Analog Inputs
spikes settle quickly and do not cause a problem. If large Because of the capacitive redistribution A/D conversion source resistances are used or if slow settling op amps techniques used, the analog inputs of the LTC1287 have 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 – VERTICAL: 0.5mV/DIV = – 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 HORIZONTAL: 10µs/DIV time constant is short enough to allow the analog inputs
Figure 4. Poor VCC Bypassing. Noise and
to settle completely within the allowed time.
Ripple Can Cause A/D Errors
“+” INPUT RSOURCE + V LTC1287 IN + CS↑ 5V/DIV CS C1 RON = 1.5k “–” CIN = INPUT R t 100pF SOURCE – WHCS + V 1/2 CLK IN – 0.5mV/DIV V C2 CC LTC1287 F07
Figure 7. Analog Input Equivalent Circuit
HORIZONTAL: 20µs/DIV
Figure 5. LT1117 Regulator with 22
µ
F Bypassing on VCC
1287fa 9