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ADS1286UAG4 Arkusz danych(PDF) 10 Page - Texas Instruments |
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ADS1286UAG4 Arkusz danych(HTML) 10 Page - Texas Instruments |
10 / 18 page 10 ® ADS1286 MINIMIZING POWER DISSIPATION In systems that have significant time between conversions, the lowest power drain will occur with the minimum CS LOW time. Bringing CS LOW, transferring data as quickly as possible, and then bringing it back HIGH will result in the lowest current drain. This minimizes the amount of time the device draws power. After a conversion the A/D automati- cally shuts down even if CS is held LOW. If the clock is left running to clock out LSB-data or zero, the logic will draw a small amount of current (see Figure 3). REDUCED REFERENCE OPERATION The effective resolution of the ADS1286 can be increased by reducing the input span of the converter. The ADS1286 exhibits good linearity and gain over a wide range of reference voltages (see Typical Performance Curves “ Change in Linearity vs Reference Voltage” and “Change in Gain vs Reference Voltage”). However, care must be taken when operating at low values of VREF because of the reduced LSB size and the resulting higher accuracy requirement placed on the converter. The following factors must be considered when operating at low VREF values: 1. Offset 2. Noise OFFSET WITH REDUCED V REF The offset of the ADS1286 has a larger effect on the output code. When the ADC is operated with reduced reference voltage. The offset (which is typically a fixed voltage) becomes a larger fraction of an LSB as the size of the LSB is reduced. The Typical Performance Curve “Change in Offset vs Reference Voltage” shows how offset in LSBs is related to reference voltage for a typical value of VOS. For example, a VOS of 122µV which is 0.1 LSB with a 5V reference becomes 0.5LSB with a 1V reference and 2.5LSBs with a 0.2V reference. If this offset is unacceptable, it can be corrected digitally by the receiving system or by offsetting the negative input of the ADS1286. NOISE WITH REDUCED V REF The total input referred noise of the ADS1286 can be reduced to approximately 200 µV peak-to-peak using a ground plane, good bypassing, good layout techniques and minimiz- ing noise on the reference inputs. This noise is insignificant with a 5V reference but will become a larger fraction of an LSB as the size of the LSB is reduced. For operation with a 5V reference, the 200 µV noise is only 0.15LSB peak-to-peak. In this case, the ADS1286 noise will contribute virtually no uncertainty to the output code. How- ever, for reduced references, the noise may become a signifi- cant fraction of an LSB and cause undesirable jitter in the output code. For example, with a 2.5V reference this same 200 µV noise is 0.3LSB peak-to-peak. If the reference is further reduced to 1V, the 200 µV noise becomes equal to 0.8LSBs and a stable code may be difficult to achieve. In this case averaging multiple readings may be necessary. FIGURE 3. Shutdown Current with CS HIGH is Lower than with CS LOW. RC INPUT FILTERING It is possible to filter the inputs with an RC network as shown in Figure 4. For large values of CFILTER (e.g., 1µF), the capacitive input switching currents are averaged into a net DC current. Therefore, a filter should be chosen with a small resistor and large capacitor to prevent DC drops across the resistor. The magnitude of the DC current is approxi- mately IDC = 20pF x VIN/tCYC and is roughly proportional to VIN. When running at the minimum cycle time of 64µs, the input current equals 1.56 µA at V IN = 5V. In this case, a filter resistor of 75 Ω will cause 0.1LSB of full-scale error. If a larger filter resistor must be used, errors can be eliminated by increasing the cycle time. FIGURE 4. RC Input Filtering. R FILTER I DC ADS1286 C FILTER V IN 6.00 5.00 4.00 3.00 2.00 1.00 0.00 0.1 1 10 100 Sample Rate (kHz) T A = 25°C V CC = +5V V REF = +5V f CLK = 16 • fSAMPLE CS = LOW (GND) CS HIGH (V CC) |
Podobny numer części - ADS1286UAG4 |
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Podobny opis - ADS1286UAG4 |
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