 |
 |
|
| LTC6802-2 |
|
|||||||||||||||||
 LINER
|
| |||||||||||||||||
|
30 page 
LTC6802-2 0 68022fa applicaTions inForMaTion ADVANTAGES OF DELTA-SIGMA ADCS The LTC6802-2 employs a delta-sigma analog-to-digital converter for voltage measurement. The architecture of delta-sigma converters can vary considerably, but the common characteristic is that the input is sampled many times over the course of a conversion and then filtered or averaged to produce the digital output code. In contrast, a SAR converter takes a single snapshot of the input voltage and then performs the conversion on this single sample. For measurements in a noisy environment, a delta-sigma converter provides distinct advantages over a SAR converter. While SAR converters can have high sample rates, the full- power bandwidth of a SAR converter is often greater than 1MHz, which means the converter is sensitive to noise out to this frequency. And many SAR converters have much higher bandwidths—up to 50MHz and beyond. It is pos- sible to filter the input, but if the converter is multiplexed V+ C12 S12 C11 S11 C10 S10 C9 S9 C8 S8 C7 S7 C6 S6 C5 S5 C4 S4 C3 S3 C2 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 0V TO 5.5V 5.5V 3.1V 1.5V 1.5V 0V 0V 3.6V 3.6V 7.2V CSBI SDO SDI SCKI A3 A2 A1 A0 GPIO2 GPIO1 WDTB MMB TOS VREG VREF VTEMP2 VTEMP1 NC V– S1 C1 S2 43.2V 43.2V 43.2V 39.6V 39.6V 36V 36V 32.4V 32.4V 28.8V 28.8V 25.2V 25.2V 21.6V 21.6V 18V 18V 14.4V 14.4V 10.8V 10.8V 7.2V LTC6802-2 68022 F17 Figure 17. Typical Pin Voltages for 12 3.6V Cells to measure several input channels a separate filter will be required for each channel. A low frequency filter cannot reside between a multiplexer and an ADC and achieve a high scan rate across multiple channels. Another conse- quence of filtering a SAR ADC is that any noise reduction gained by filtering the input cancels the benefit of having a high sample rate in the first place, since the filter will take many conversion cycles to settle. For a given sample rate, a delta-sigma converter can achieve excellent noise rejection while settling completely inasingleconversion—somethingthatafilteredSARcon- verter cannot do. Noise rejection is particularly important in high voltage switching controllers, where switching noise will invariably be present in the measured voltage. Other advantages of delta sigma converters are that they are inherently monotonic, meaning they have no missing codes, and they have excellent DC specifications. |
|
