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ADS1258IRTCTG4 Datasheet(PDF) 16 Page - Texas Instruments |
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ADS1258IRTCTG4 Datasheet(HTML) 16 Page - Texas Instruments |
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16 / 54 page  t SAMPLE ON OFF S 1 S 2 OFF ON S 1 S 1 AVSS + 1.3V R AIN = ReffB || 2ReffA AVSS + 1.3V R effA = 190kΩ R effB = 78kΩ (f CLK = 16MHz) R effA = 190kΩ ADCINN ADCINP C A1 = 0.65pF C B = 1.6pF C A2 = 0.65pF ADCINN S 2 AVSS + 1.3V S 2 AVSS + 1.3V ADCINP Equivalent Circuit R eff = tSAMPLE/CX NOTE: ESD input diodes not shown. ADS1258 SBAS297G – JUNE 2005 – REVISED MARCH 2011 www.ti.com ADC INPUTS As with the multiplexer and reference inputs, ESD diodes protect the ADC inputs. To keep these diodes The ADS1258 ADC inputs (ADCINP, ADCINN) from turning on, make sure the voltages on the input measure the input signal using internal capacitors pins do not go below AVSS by more than 100mV, that are continuously charged and discharged. The and likewise do not exceed AVDD by more than left side of Figure 37 shows a simplified schematic of 100mV. the ADC input circuitry; the right side of Figure 37 shows the input circuitry with the capacitors and switches replaced by an equivalent circuit. Figure 36 shows the ON/OFF timings of the switches shown in Figure 37. S1 switches close during the input sampling phase. With S1 closed, CA1 charges to ADCINP, CA2 charges to ADCINN, and CB charges to (ADCINP – ADCINN). For the discharge phase, S1 opens first and then S2 closes. CA1 and CA2 discharge to approximately AVSS + 1.3V and CB discharges to 0V. This two-phase sample/discharge cycle repeats with a period of tSAMPLE = 2/fCLK. Figure 36. S1 and S2 Switch Timing for Figure 37 The charging of the input capacitors draws a transient current from the source driving the ADS1258 ADC inputs. The average value of this current can be used to calculate an effective impedance (Reff) where Reff = VIN/IAVERAGE. These impedances scale inversely with fCLK. For example, if fCLK is reduced by a factor of two, the impedances will double. Figure 37. Simplified ADC Input Structure 16 Submit Documentation Feedback Copyright © 2005–2011, Texas Instruments Incorporated Product Folder Link(s): ADS1258 |
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