SBAS761A February   2020  – February 2020 ADS8355

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Block Diagram
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1      Absolute Maximum Ratings
    2. 6.2      ESD Ratings
    3. 6.3      Recommended Operating Conditions
    4. 6.4      Thermal Information
    5. 6.5      Electrical Characteristics
    6. Table 1. Timing Requirements
    7. Table 2. Switching Characteristics
    8. 6.6      Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Reference
      2. 7.3.2 Analog Inputs
        1. 7.3.2.1 Analog Input: Full-Scale Range Selection
        2. 7.3.2.2 Analog Input: Single-Ended and Pseudo-Differential Configurations
      3. 7.3.3 Transfer Function
    4. 7.4 Device Functional Modes
      1. 7.4.1 Conversion Data Read: Dual-SDO Mode (Default)
      2. 7.4.2 Conversion Data Read: Single-SDO Mode
      3. 7.4.3 Low-Power Modes
        1. 7.4.3.1 STANDBY Mode
        2. 7.4.3.2 PD (Power-Down) Mode
    5. 7.5 Programming
      1. 7.5.1 Register Read/Write Operation
    6. 7.6 Register Map
      1. 7.6.1 ADS8355 Registers
        1. 7.6.1.1  PD_STANDBY Register (Offset = 4h) [reset = 0h]
          1. Table 9. PD_STANDBY Register Field Descriptions
        2. 7.6.1.2  PD_KEY Register (Offset = 5h) [reset = 0h]
          1. Table 10. PD_KEY Register Field Descriptions
        3. 7.6.1.3  SDO_CTRL Register (Offset = Dh) [reset = 0h]
          1. Table 11. SDO_CTRL Register Field Descriptions
        4. 7.6.1.4  DATA_OUT_CTRL Register (Offset = 11h) [reset = 0h]
          1. Table 12. DATA_OUT_CTRL Register Field Descriptions
        5. 7.6.1.5  REF_SEL Register (Offset = 20h) [reset = 0h]
          1. Table 13. REF_SEL Register Field Descriptions
        6. 7.6.1.6  REFDAC_A_LSB Register (Offset = 24h) [reset = 0h]
          1. Table 14. REFDAC_A_LSB Register Field Descriptions
        7. 7.6.1.7  REFDAC_A_MSB Register (Offset = 25h) [reset = 0h]
          1. Table 15. REFDAC_A_MSB Register Field Descriptions
        8. 7.6.1.8  REFDAC_B_LSB Register (Offset = 26h) [reset = 0h]
          1. Table 16. REFDAC_B_LSB Register Field Descriptions
        9. 7.6.1.9  REFDAC_B_MSB Register (Offset = 27h) [reset = 0h]
          1. Table 17. REFDAC_B_MSB Register Field Descriptions
        10. 7.6.1.10 INPUT_CONFIG Register (Offset = 28h) [reset = 0h]
          1. Table 18. INPUT_CONFIG Register Field Descriptions
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Input Amplifier Selection
      2. 8.1.2 Charge Kickback Filter
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
      3. 8.2.3 Application Curve
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Community Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.1.2 Charge Kickback Filter

Converting analog-to-digital signals requires sampling an input signal at a constant rate. Any higher frequency content in the input signal beyond half the sampling frequency is digitized and folded back into the low-frequency spectrum. This process is called aliasing. Therefore, an analog, charge kickback filter must be used to remove the harmonic content from the input signal before being sampled by the ADC. A charge kickback filter is designed as a low-pass, RC filter, for which the 3-dB bandwidth is optimized based on specific application requirements. For DC signals with fast transients (including multiplexed input signals), a high-bandwidth filter is designed to allow accurately settling the signal at the ADC inputs during the small acquisition time window. For AC signals, keep the filter bandwidth low to band-limit the noise fed into the ADC input, thereby increasing the signal-to-noise ratio (SNR) of the system.

A filter capacitor, CFLT, connected across the ADC inputs (see Figure 43), filters the noise from the front-end drive circuitry, reduces the sampling charge injection, and provides a charge bucket to quickly charge the internal sample-and-hold capacitors during the acquisition process. As a rule of thumb, the value of this capacitor must be at least 10 times the specified value of the ADC sampling capacitance. For these devices, the input sampling capacitance is equal to 40 pF. Thus, the value of CFLT must be greater than 400 pF. The capacitor must be a COG- or NPO-type because these capacitor types have a high-Q, low-temperature coefficient, and stable electrical characteristics under varying voltages, frequency, and time.

ADS8355 apps_chargekickback_BAS761.gifFigure 43. Charge Kickback Filter

Driving capacitive loads can degrade the phase margin of the input amplifiers, thus making the amplifier marginally unstable. To avoid amplifier stability issues, series isolation resistors (RFLT) are used at the output of the amplifiers. A higher value of RFLT is helpful from the amplifier stability perspective, but adds distortion as a result of interactions with the nonlinear input impedance of the ADC. Distortion increases with source impedance, input signal frequency, and input signal amplitude. Therefore, the selection of RFLT requires balancing the stability and distortion of the design. For more information on ADC input R-C filter component selection, see the TI Precision Labs on ti.com.