SBAS543C August   2011  – February 2016 ADS8528 , ADS8548 , ADS8568

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics: General
    6. 7.6  Electrical Characteristics: ADS8528
    7. 7.7  Electrical Characteristics: ADS8548
    8. 7.8  Electrical Characteristics: ADS8568
    9. 7.9  Serial Interface Timing Requirements
    10. 7.10 Parallel Interface Timing Requirements (Read Access)
    11. 7.11 Parallel Interface Timing Requirements (Write Access)
    12. 7.12 Typical Characteristics
  8. 8 Parameter Measurement information
    1. 8.1 Equivalent Circuits
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Analog
        1. 9.3.1.1 Analog Inputs
        2. 9.3.1.2 Analog-to-Digital Converter (ADC)
        3. 9.3.1.3 Conversion Clock
        4. 9.3.1.4 CONVST_x
        5. 9.3.1.5 Data Readout and BUSY/INT Signal
        6. 9.3.1.6 Sequential Operation
        7. 9.3.1.7 Reference
      2. 9.3.2 Digital
        1. 9.3.2.1 Device Configuration
        2. 9.3.2.2 Parallel Interface
        3. 9.3.2.3 Serial Interface
        4. 9.3.2.4 Output Data Format
    4. 9.4 Device Functional Modes
      1. 9.4.1 Hardware Mode
      2. 9.4.2 Software Mode
      3. 9.4.3 Daisy-Chain Mode
      4. 9.4.4 Reset and Power-Down Modes
    5. 9.5 Register Maps
      1. 9.5.1 Configuration (CONFIG) Register
        1. 9.5.1.1 CONFIG: Configuration Register (default = 000003FFh)
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
      3. 10.2.3 Application Curve
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Documentation Support
      1. 13.1.1 Related Documentation
    2. 13.2 Related Links
    3. 13.3 Community Resources
    4. 13.4 Trademarks
    5. 13.5 Electrostatic Discharge Caution
    6. 13.6 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

10.1 Application Information

The ADS85x8 enables high-precision measurement of up to eight analog signals simultaneously. The Typical Application section summarizes some of the typical use cases for the ADS85x8 and the main steps and components used around the analog-to-digital converter (ADC).

10.2 Typical Application

The accurate measurement of electrical variables in a power grid is extremely critical because this measurement helps determine the operating status and running quality of the grid. Such accurate measurements also help diagnose problems with the power network, thereby enabling prompt solutions and minimizing down time. The key electrical variables measured in 3-phase power systems are the three line voltages, the neutral voltage at the load, the three line currents, and the neutral return current; see Figure 45. These variables enable metrology and power automation systems to determine the amplitude, frequency, and phase information in order to perform harmonic analysis, power factor calculations, and power quality assessment, among others.

ADS8528 ADS8548 ADS8568 ADS85x8_APPLIC_DIAG.gif Figure 45. Simultaneous Acquisition of Voltage and Current in a 3-Phase Power System

10.2.1 Design Requirements

To begin the design process, a few parameters must be decided upon. The designer must know the following:

  • Output range of the potential transformers (elements labeled PT1, PT2, and PT3 in Figure 45)
  • Output range of the current transformers (elements labeled CT1, CT2, and CT3 in Figure 45)
  • Input impedance required from the analog front-end for each channel
  • Fundamental frequency of the power system
  • Number of harmonics that must be acquired, and
  • Type of signal conditioning required from the analog front-end for each channel

10.2.2 Detailed Design Procedure

Figure 46 shows the topology chosen to meet the design requirements.

NOTE

A feedback capacitor CF is included in order to provide a low-pass filter characteristic and attenuate signals outside the band of interest.
ADS8528 ADS8548 ADS8568 ADS8568_OP_AMP_INV.gif Figure 46. Op Amp in an Inverting Configuration

The potential transformers and current transformers used in the system depicted in Figure 45 provide the eight inputs required. These transformers have a ±10-V output range. Although the PT and CT elements provide isolation from the power system, the value of RIN is selected as 100 kΩ in order to provide an additional, high-impedance safety element in the current path leading up to the input of the ADC. Moreover, selecting a low-frequency gain of –1 V/V (as shown in Equation 5) provides a ±10-V output that can be fed into the ADS8568; therefore, the value of RF is selected as 100 kΩ too.

Equation 5. ADS8528 ADS8548 ADS8568 ADS8555_OP_AMP_G_eq.gif

The primary goal of the acquisition system depicted in Figure 45 is to measure up to 20 harmonics in a 60-Hz power network. With this goal in mind, the analog front-end must have sufficient bandwidth to measure signals up to 1260 Hz as shown in Equation 6.

Equation 6. ADS8528 ADS8548 ADS8568 ADS8555_f_max.gif

Based on the bandwidth from in Equation 6, the ADS8568 is set to simultaneously sample all six channels at 15.36 kSPS, thus providing enough samples to clearly resolve even the highest harmonic required.

The passband of the configuration shown in Figure 46 is determined by the –3-dB frequency according to Equation 7. The value of CF is selected as 820 pF because CF is a standard capacitance value available in 0603 size (surface-mount component) and such values, combined with that of RF, result in sufficient bandwidth to accommodate the required 20 harmonics (at 60 Hz).

Equation 7. ADS8528 ADS8548 ADS8568 ADS8555_f_3dB.gif

The value of R1 is selected as the parallel combination of RIN and RF to prevent the input bias current of the operational amplifier from generating an offset error.

The value of component C1 is chosen as 0.1 µF to provide a low-impedance path for noise signals that can be picked up by R1, thus improving the EMI robustness and noise performance of the system.

The OPA2277 is chosen for its low input offset voltage, low drift, bipolar swing, sufficient gain-bandwidth product, and low quiescent current. For additional information on the procedure to select SAR ADC input drivers, see the TIPD151 verified design guide, 16-Bit 100-KSPS 4-Channel Multiplexed Data Acquisition System Design Guide.

The charge injection damping circuit is composed by R2 (49.9 Ω) and C2 (370 pF); these components reject high-frequency noise and meet the settling requirements of the ADS8568 input.

Figure 47 shows the reference block used in this design.

ADS8528 ADS8548 ADS8568 ADS8555_REF_BLOCK.gif Figure 47. Reference Block

For more information on the design of charge injection damping circuits and reference driving circuits for SAR ADCs, see the TIPD149 verified design reference guide, Power-Optimized 16-Bit 1-MSPS Data Acquisition Block for Lowest Distortion and Noise.

10.2.3 Application Curve

Figure 48 shows the frequency spectrum of the data acquired by the ADS8568 for a sinusoidal, 20-VPP input at 60 Hz.

ADS8528 ADS8548 ADS8568 ADS8568_10Vp_60Hz_FFT.png Figure 48. Frequency Spectrum for a Sinusoidal 20-VPP Signal at 60 Hz

The ac performance parameters are:

  • SNR: 91.16 dB
  • THD: –94.34 dB
  • SNDR: 89.45 dB
  • SFDR: 96.56 dB