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ADS1625IPAPRG4 Datasheet(PDF) 18 Page - Texas Instruments |
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ADS1625IPAPRG4 Datasheet(HTML) 18 Page - Texas Instruments |
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18 / 37 page  ADS1625 ADS1626 SBAS280E − JUNE 2003 − REVISED MAY 2007 www.ti.com 18 INPUT CIRCUITRY The ADS1625/6 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 actual implementation is different. The timing for switches S1 and S2 is shown in Figure 9. S 1 S 2 10pF AINP ADS1625 ADS1626 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 ADS1625/6 inputs must be able to handle the load presented by the switching capacitors within the ADS1625/6. Input switches S1 in Figure 9 are closed approximately onehalf of the sampling period, tsample, allowing only ≈12ns for the internal capacitors to be charged by the inputs, when fCLK = 40MHz. Figure 10 and Figure 11 show the recommended circuits when using single-ended or differential op amps, respectively. The analog inputs must be driven differentially to achieve optimum performance. The external capacitors, 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 inputs and use good capacitor bypass techniques on their supplies; usually a smaller high-quality ceramic capacitor in parallel with a larger capacitor. Keep the resistances 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 ADS1625/6 inputs, a simple RC filter can set the input common-mode voltage. The ADS1625/6 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 ADS1625/6. 392 Ω OPA 2822 AD S162 5 AD S162 6 AGN D OPA 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 F 0.01 F 392 Ω 1F 392 Ω 1k Ω 1k Ω 1F 392 Ω 49.9 Ω 49.9 Ω ≤ µ µ µ µ Figure 10. Recommended Driver Circuit Using the OPA2822 ADS1625 THS4503 ADS1626 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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