SLAS708A September   2010  – September 2019 ADS7947 , ADS7948 , ADS7949


  1. Features
  2. Applications
  3. Description
    1.     ADS794x Block Diagram
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
    1.     Pin Functions
  7. Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions: ADS794x (12-, 10-, 8-Bit)
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics: ADS7947 (12-Bit)
    6. 7.6  Electrical Characteristics: ADS7948 (10-Bit)
    7. 7.7  Electrical Characteristics: ADS7949 (8-Bit)
    8. 7.8  Timing Requirements
    9. 7.9  Switching Characteristics
    10. 7.10 Typical Characteristics: ADS7947, ADS7948, ADS7949
    11. 7.11 Typical Characteristics: ADS7947 (12-Bit)
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Multiplexer and ADC Input
      2. 8.3.2 Reference
      3. 8.3.3 Clock
      4. 8.3.4 ADC Transfer Function
      5. 8.3.5 Power-Down
    4. 8.4 Device Functional Modes
      1. 8.4.1 Device Operation
    5. 8.5 Programming
      1. 8.5.1 16-Clock Frame
      2. 8.5.2 32-Clock Frame
  9. Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Driving an ADC Without a Driving Op Amp
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Related Links
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

Application Information

The device employs a sample-and-hold stage at the input; see Figure 40 for a typical equivalent circuit of a sample-and-hold stage. The device connects a 32-pF sampling capacitor during sampling. This configuration results in a glitch at the input terminals of the device at the start of the sample. The external circuit must be designed in such a way that the input can settle to the required accuracy during the sampling time chosen. Figure 53 shows a typical driving circuit for the analog inputs.

ADS7947 ADS7948 ADS7949 ai_typ_in_drv_cir_las708.gifFigure 53. Typical Input Driving Circuit

The 470-pF capacitor across the AINxP and AINxN terminals decouples the driving op amp from the sampling glitch. Splitting the series resistance of the input filter in two equal values is recommended, as shown in Figure 53. Both input terminals are recommended to have the same impedance from the external circuit. The low-pass filter at the input limits noise bandwidth of the driving op amp. Select the filter bandwidth so that the full-scale step at the input can settle to the required accuracy during the sampling time. Equation 1, Equation 2, and Equation 3 are useful for filter component selection.

Equation 1. ADS7947 ADS7948 ADS7949 q_filt_tc_las708.gif


    Equation 2. ADS7947 ADS7948 ADS7949 q_filt_t_las708.gif
    Equation 3. ADS7947 ADS7948 ADS7949 q_filt_bw_las708.gif

    Also, make sure the driving op amp bandwidth does not limit the signal bandwidth below filter bandwidth. In many applications, signal bandwidth can be much lower than filter bandwidth. In this case, an additional low-pass filter can be used at the input of the driving op amp. This signal filter bandwidth can be selected in accordance with the input signal bandwidth.