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ADS1258 Datasheet(PDF) 13 Page - Texas Instruments |
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ADS1258 Datasheet(HTML) 13 Page - Texas Instruments |
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13 / 47 page  www.ti.com MULTIPLEXER INPUTS AVSS * 100mV t VREFP or VREFN t AVDD ) 100mV (1) VOLTAGE REFERENCE INPUTS ESD Diodes ESD Diodes 3pF R eff = 40kΩ (f CLK = 16MHz) AVDD AVSS VREFP VREFN ADS1258 SBAS297A – JUNE 2005 – REVISED SEPTEMBER 2005 ESD diodes protect the reference inputs. To keep these diodes from turning on, make sure the voltages A simplified diagram of the input multiplexer is on the reference pins do not go below AVSS by more illustrated in Figure 35. The multiplexer connects one than 100mV, and likewise do not exceed AVDD by of 16 single-ended external inputs, one of eight 100mV, as described in Equation 1: differential external inputs, or one of the on-chip internal variables to the ADC inputs. The output of the channel multiplexer can be routed to external pins A high-quality reference voltage is essential for and then to the input of the ADC. This flexibility achieving the best performance from the ADS1258. allows for use of external signal conditioning. See the Noise and drift on the reference degrade overall External Multiplexer Loop section. system performance. It is especially critical that special care be given to the circuitry that generates ESD diodes protect the analog inputs. To keep these the reference voltages and the layout when operating diodes from turning on, make sure the voltages on in the low-noise settings (that is, with low data rates) the input pins do not go below AVSS by more than to prevent the voltage reference from limiting 100mV, and likewise do not exceed AVDD by more performance. See the Reference Inputs description in than 100mV: the Hardware Considerations segment of the AVSS – 100mV < (Analog Inputs) < AVDD + 100mV. Applications section. The converter supports two modes of channel access through the multiplexer: the Auto-Scan mode and the Fixed-Channel mode. These modes are selected by the MUXMOD bit of register CONFIG0. The Auto-Scan mode scans through the selected channels automatically, with break-before-make switching. The Fixed-Channel mode requires the user to set the channel address for each channel measured. (VREFP, VREFN) The voltage reference for the ADS1258 ADC is the differential voltage between VREFP and VREFN: VREF = VREFP – VREFN. The reference inputs use a structure similar to that of the analog inputs with the circuitry on the reference inputs shown in Figure 34. The load presented by the switched capacitor can be modeled with an effective resistance (Reff) of 40kΩ for fCLK = 16MHz. Note that the effective impedance of Figure 34. Simplified Reference Input Circuit the reference inputs will load an external reference with a non-zero source impedance. 13 |
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