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ADS1222IPWT bảng dữ liệu(PDF) 9 Page - Burr-Brown (TI) |
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ADS1222IPWT bảng dữ liệu(HTML) 9 Page - Burr-Brown (TI) |
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9 / 21 page  ADS1222 SBAS314A − APRIL 2004 − REVISED SEPTEMBER 2004 www.ti.com 9 Analog Input Measurement without the Input Buffer With the buffer disabled by setting the BUFEN pin low, the ADS1222 measures the input signal using internal capacitors that are continuously charged and discharged. Figure 14 shows a simplified schematic of the ADS1222 input circuitry, with Figure 15 showing the on/off timings of the switches. The S1 switches close during the input sampling phase. With S1 closed, CA1 charges to AINP, CA2 charges to AINN, and CB charges to (AINP – AINN). For the discharge phase, S1 opens first and then S2 closes. CA1 and CA2 discharge to approximately VDD/2 and CB discharges to 0V. This two-phase sample/discharge cycle repeats with a frequency of fCLK/32 (62.5kHz for fCLK = 2MHz). Mux VDD VDD/2 VDD/2 S 1 S 1 AINN AINP S 2 C A1 3pF C B 6pF C A2 3pF VDD ESD Protection AINPx AINNx S 2 Figure 14. Simplified Input Structure with the Buffer Turned Off ON OFF ON S 1 S 2 OFF t SAMPLE = 32/fCLK Figure 15. S1 and S2 Switch Timing for Figure 14 The constant charging of the input capacitors presents a load on the inputs that can be represented by effective impedances. Figure 16 shows the input circuitry with the capacitors and switches of Figure 14 replaced by their effective impedances. These impedances scale inversely with fCLK frequency. For example, if fCLK frequency is reduced by a factor of 2, the impedances will double. Zeff A =tSAMPLE/CA1 =6MΩ (1) Zeff B =tSAMPLE/CB =3MΩ (1) Zeff A =tSAMPLE/CA2 =6MΩ (1) NOTE: (1) fCLK =2MHz. AINPx AINNx VDD/2 VDD/2 Figure 16. Effective Analog Input Impedances with the Buffer Off ESD diodes protect the inputs. To keep these diodes from turning on, make sure the voltages on the input pins do not go below GND by more than 100mV, and likewise do not exceed VDD by 100mV: GND – 100mV < (AINP, AINN) < VDD + 100mV Analog Input Measurement with the Input Buffer When the buffer is enabled by setting the BUFEN pin high, a low-drift, chopper-stabilized input buffer is used to achieve very high input impedance. The buffer charges the input sampling capacitors, thus removing the load from the measurement. Because the input buffer is chopper-stabilized, the charging of parasitic capacitances causes the charge to be carried away, as if by resistance. The input impedance can be modeled by a single resistor, as shown in Figure 17. The impedance scales inversely with fCLK frequency, as in the nonbuffered case. AINP AINN 1.2G Ω (1) NOTE: (1) f CLK =2MHz. Figure 17. Effective Analog Input Impedances with the Buffer On Note that the analog inputs (listed in the Electrical Characteristics table as Absolute Input Range) must remain between GND + 0.05V to VDD − 1.5V. Exceeding this range degrades linearity and results in performance outside the specified limits. |
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