SBAS825 April   2017 ADS8578S

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Family 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
    6. 7.6  Timing Requirements: CONVST Control
    7. 7.7  Timing Requirements: Data Read Operation
    8. 7.8  Timing Requirements: Parallel Data Read Operation, CS and RD Tied Together
    9. 7.9  Timing Requirements: Parallel Data Read Operation, CS and RD Separate
    10. 7.10 Timing Requirements: Serial Data Read Operation
    11. 7.11 Timing Requirements: Byte Mode Data Read Operation
    12. 7.12 Timing Requirements: Oversampling Mode
    13. 7.13 Timing Requirements: Exit Standby Mode
    14. 7.14 Timing Requirements: Exit Shutdown Mode
    15. 7.15 Switching Characteristics: CONVST Control
    16. 7.16 Switching Characteristics: Parallel Data Read Operation, CS and RD Tied Together
    17. 7.17 Switching Characteristics: Parallel Data Read Operation, CS and RD Separate
    18. 7.18 Switching Characteristics: Serial Data Read Operation
    19. 7.19 Switching Characteristics: Byte Mode Data Read Operation
    20. 7.20 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Analog Inputs
      2. 8.3.2 Analog Input Impedance
      3. 8.3.3 Input Clamp Protection Circuit
      4. 8.3.4 Programmable Gain Amplifier (PGA)
      5. 8.3.5 Third-Order, Low-Pass Filter (LPF)
      6. 8.3.6 ADC Driver
      7. 8.3.7 Digital Filter and Noise
      8. 8.3.8 Reference
        1. 8.3.8.1 Internal Reference
        2. 8.3.8.2 External Reference
        3. 8.3.8.3 Supplying One VREF to Multiple Devices
      9. 8.3.9 ADC Transfer Function
    4. 8.4 Device Functional Modes
      1. 8.4.1 Device Interface: Pin Description
        1. 8.4.1.1  REFSEL (Input)
        2. 8.4.1.2  RANGE (Input)
        3. 8.4.1.3  STBY (Input)
        4. 8.4.1.4  PAR/SER/BYTE SEL (Input)
        5. 8.4.1.5  CONVSTA, CONVSTB (Input)
        6. 8.4.1.6  RESET (Input)
        7. 8.4.1.7  RD/SCLK (Input)
        8. 8.4.1.8  CS (Input)
        9. 8.4.1.9  OS[2:0]
        10. 8.4.1.10 BUSY (Output)
        11. 8.4.1.11 FRSTDATA (Output)
        12. 8.4.1.12 DB15/BYTE SEL
        13. 8.4.1.13 DB14/HBEN
        14. 8.4.1.14 DB[13:9]
        15. 8.4.1.15 DB8/DOUTB
        16. 8.4.1.16 DB7/DOUTA
        17. 8.4.1.17 DB[6:0]
      2. 8.4.2 Device Modes of Operation
        1. 8.4.2.1 Power-Down Modes
          1. 8.4.2.1.1 Standby Mode
          2. 8.4.2.1.2 Shutdown Mode
        2. 8.4.2.2 Conversion Control
          1. 8.4.2.2.1 Simultaneous Sampling on All Input Channels
          2. 8.4.2.2.2 Simultaneous Sampling Two Sets of Input Channels
        3. 8.4.2.3 Data Read Operation
          1. 8.4.2.3.1 Parallel Data Read
          2. 8.4.2.3.2 Parallel Byte Data Read
          3. 8.4.2.3.3 Serial Data Read
          4. 8.4.2.3.4 Data Read During Conversion
        4. 8.4.2.4 Oversampling Mode of Operation
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
      3. 9.2.3 Application Curve
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    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

9 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.

9.1 Application Information

The ADS8578S enables high-precision measurement of up to eight analog signals simultaneously. The device is a fully integrated data acquisition (DAQ) system based on a 14-bit successive approximation (SAR) analog-to-digital converter (ADC). The device includes an integrated analog front-end for each input channel and an integrated voltage reference with a precision reference buffer. As such, this device does not require any additional active circuits for driving the reference analog input pins of the ADC.

9.2 Typical Application

ADS8578S apps_powergrid_BAS825.gif
1. Decoupling the AVDD capacitor applies to each AVDD pin.
2. REGCAP1 and REGCAP2: each pin requires separate decoupling capacitors.
Figure 84. 8-Channel DAQ for Power Automation Using the ADS8578S

This application example involves the measurement of electrical variables in a power system. The accurate measurement of electrical variables in a power grid is extremely critical because this measurement helps to determine the operating status and running quality of the grid. Such accurate measurements also help to diagnose potential problems with the power network so that these problems can be resolved quickly without having any significant service impact. The key electrical parameters include amplitude, frequency, and phase measurement of the voltage and current on the power lines. These parameters are important to enable metrology in the power automation system to perform harmonic analysis, power factor calculation, power quality assessment, and so forth.

9.2.1 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 PT in Figure 84)
  • Output range of the current transformers (elements labeled CT in Figure 84)
  • Input impedance required from the analog front-end for each channel
  • Fundamental frequency of the power system
  • Number of harmonics that must be acquired
  • Type of signal conditioning required from the analog front end for each channel

9.2.2 Detailed Design Procedure

For the ADS8578S, each channel incorporates an analog front-end composed of a programmable gain amplifier (PGA), analog low-pass filter, and ADC input driver. The analog input for each channel presents a constant resistive impedance of 1 MΩ independent of the ADC sampling frequency and range setting. The high input impedance of the analog front-end circuit allows direct connection to potential transformers (PT) and current transformers (CT). The ADC inputs can support up to ±10-V or ± 5-V bipolar inputs and the integrated signal conditioning eliminates the need for external amplifiers or ADC driver circuits.

The PT and CT used in the system (see Figure 84) have a ±10-V output range. Although the PT and CT provide isolation from the power system, a series resistor must be placed on the analog input channels. The series resistor helps limit the input current to ±10 mA if the input voltages exceed ±15 V. For applications that require protection against overvoltage or fast transient events beyond the specified absolute maximum ratings of the device, an external protection clamp circuit using transient voltage suppressors (TVS) and ESD diodes is recommended.

A low-pass filter is used on each analog input channel to eliminate high-frequency noise pickup and minimize aliasing. Figure 85 shows an example of the recommended configuration for an input RC filter. A balanced RC filter configuration matches the external source resistance on the positive path (AIN_nP) with an equal resistance on the negative path (AIN_nGND). Matching the source impedance in the positive and negative path allows for better common-mode noise rejection and helps maintain the dc accuracy of the system by canceling any additional offset error contributed by the external series resistance.

ADS8578S apps_lowpass_BAS825.gif Figure 85. Input RC Low-Pass Filter

The primary goal of the data acquisition system illustrated in Figure 84 is to measure up to 20 harmonics in a 60-Hz power network. Thus, as shown in Equation 1, the analog front-end must have sufficient bandwidth to detect signals up to 1260 Hz.

Equation 1. ADS8578S apps_eqn1_sbas642.gif

Based on the bandwidth calculated in Equation 1, the ADS8578S is set to simultaneously sample all eight channels at 20 kSPS, which is sufficient throughput to clearly resolve the highest harmonic component of the input signal. The pass band of the low-pass filter configuration shown in Figure 85 is determined by the –3-dB frequency, calculated according to Equation 2.

Equation 2. ADS8578S apps_eqn2_sbas642.gif

The value of CF is selected as 5.6 nF, a standard capacitance value available in 0603-size surface-mount components. In combination with the resistor RF, this low-pass filter provides sufficient bandwidth to accommodate the required 20 harmonics for the input signal of 60 Hz.

The ADS8578S can operate with either the internal voltage reference or an external reference. The Internal Reference section describes the electrical connections and recommended bypass capacitors when using the internal reference. Alternatively for applications that require a higher precision voltage reference, Figure 86 shows an example of an external reference circuit. The REF5025 provides a very low drift, and very accurate external 2.5-V reference. The resistor RFILT and capacitor CFILT form a low-pass filter to reduce the broadband noise and minimize the resulting effect of the reference noise on the system performance.

ADS8578S apps_extref_circuit_sbas642.gif Figure 86. External Reference Circuit for the ADS8578S

9.2.3 Application Curve

Figure 87 shows the frequency spectrum of the data acquired by the ADS8578S for a sinusoidal, ±10-V input at 60 Hz.

The ac performance parameters measured by this design are:

  • SNR = 84.97 dB; SINAD = 84.85 dB
  • THD = –106.5 dB; SFDR = 109.8 dB
ADS8578S D070_SBAS825.gif
Figure 87. Frequency Spectrum for a Sinusoidal ±10-V Signal at 50 Hz