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ADS1606IPAPR 数据表(PDF) 18 Page - Texas Instruments |
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ADS1606IPAPR 数据表(HTML) 18 Page - Texas Instruments |
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18 / 38 page  ADS1605 ADS1606 SBAS274H − MARCH 2003 − REVISED MAY 2007 www.ti.com 18 INPUT CIRCUITRY The ADS1605/6 uses switched-capacitor circuitry to measure the input voltage. Internal capacitors are charged by the inputs and then discharged internally with this cycle repeating at the frequency of CLK. Figure 8 shows a conceptual diagram of these circuits. Switches S2 represent the net effect of the modulator circuitry in discharging the sampling capacitors; the ac- tual implementation is different. The timing for switches S1 and S2 is shown in Figure 9. S 1 S 2 10pF AINP ADS1605 ADS1606 8pF VMID S 1 S 2 10pF AINN 8pF VMID AGND Figure 8. Conceptual Diagram of Internal Circuitry Connected to the Analog Inputs On S1 S2 Off On Off t SAMPLE =1/fCLK Figure 9. Timing for the Switches in Figure 2 DRIVING THE INPUTS The external circuits driving the ADS1605/6 inputs must be able to handle the load presented by the switching capacitors within the ADS1605/6. The input switches S1 in Figure 8 are closed approximately one half of the sampling period, tsample, allowing only ≈12ns for the in- ternal capacitors to be charged by the inputs, when fCLK = 40MHz. Figure 10 and Figure 11 show the recommended cir- cuits when using single-ended or differential op amps, respectively. The analog inputs must be driven differen- tially to achieve optimum performance. The external ca- pacitors, between the inputs and from each input to AGND, improve linearity and should be placed as close to the pins as possible. Place the drivers close to the in- puts and use good capacitor bypass techniques on their supplies; usually a smaller high-quality ceramic capaci- tor in parallel with a larger capacitor. Keep the resistanc- es used in the driver circuits low—thermal noise in the driver circuits degrades the overall noise performance. When the signal can be ac-coupled to the ADS1605/6 inputs, a simple RC filter can set the input common mode voltage. The ADS1605/6 is a high-speed, high- performance ADC. Special care must be taken when selecting the test equipment and setup used with this device. Pay particular attention to the signal sources to ensure they do not limit performance when measuring the ADS1605/6. 392 Ω O P A 2822 ADS 1 6 05 ADS 1 6 06 AG N D O P A 2822 40pF V CM (1) V CM (1) V CM (1) 100pF AINP AINN 100pF (3) 392 Ω 40pF 100pF (1) Recommended V CM =2.0V. (2) Optional ac−coupling circuit provides common−mode input voltage. (3) Increase to 390pF when f IN ≤ 100kHz for improved SNR and THD. (2) (2) (2) (2) − V IN 2 V IN 2 392 Ω 392 Ω 392 Ω 0.01 µF 0.01 µF 392 Ω 1 µF 392 Ω 1k Ω 1k Ω 1 µF 392 Ω 49.9 Ω 49.9 Ω Figure 10. Recommended Driver Circuit Using the OPA2822 ADS1605 THS4503 ADS1606 22pF −V IN +V IN 100pF 100pF AINP AINN 100pF 24.9 Ω 392 Ω 392 Ω 24.9 Ω 392 Ω 392 Ω 22pF V CM Figure 11. Recommended Driver Circuits Using the THS4503 Differential Amplifier |
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