SBOS748 September   2016 REF6225 , REF6230 , REF6233 , REF6241 , REF6245 , REF6250


  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      Typical Application
      2.      Reference Droop comparison (1 LSB = 19.07 µV, With ADS8881 at 1 MSPS)
  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
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Parameter Measurement Information
    1. 8.1 Solder Heat Shift
    2. 8.2 Thermal Hysteresis
    3. 8.3 Reference Droop Measurements
    4. 8.4 1/f Noise Performance
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Integrated ADC Drive Buffer
      2. 9.3.2 Temperature Drift
      3. 9.3.3 Load Current
      4. 9.3.4 Stability
    4. 9.4 Device Functional Modes
  10. 10Applications 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
        1. Results
      3. 10.2.3 Application Curves
  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 Receiving Notification of Documentation Updates
    4. 13.4 Community Resources
    5. 13.5 Trademarks
    6. 13.6 Electrostatic Discharge Caution
    7. 13.7 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

Integrated ADC Drive Buffer

Many ADC data sheets specify a few microamps of average current draw from the REF pin. Almost all voltage references provide these few microamps of average current; but not all voltage references are practical for driving a high-resolution, high-throughput SAR ADC because the peak current drawn can be very high when the capacitors are switched on the REF pin. The worst-case demand for the voltage reference is during a burst-mode conversion, when the ADC is idle for a very long time, before a conversion is initiated, and the first sample converted is expected to be precise. Usually, a large capacitor is connected between the REF pin and ground pin (or sometimes between the REFP and REFM pins) of the ADC to smoothen the current load and reduce the burden on the voltage reference. The voltage reference must then be capable of providing the average current required to completely charge the reference capacitor, but without causing the reference voltage to droop significantly. Most voltage references lack the ability to completely charge the reference capacitor, and settle when the binary-weighted capacitors are being switched onto the REF pin because of the large output impedance. Usually, voltage references have output impedances in the range of 10's of ohms at frequencies higher than 100 Hz. The output voltage of the voltage reference must be buffered with a low output impedance (usually high bandwidth) amplifier to achieve excellent linearity and distortion performance.

The key amplifier specifications to be considered when designing a reference buffer for a high-precision ADC are: low offset, low drift, wide bandwidth, and low output impedance. While it is possible to select an amplifier that sufficiently meets all these requirements, the amplifier comes at a cost of excessive power consumption. For example, the OPA350 is a 38-MHz bandwidth amplifier with a maximum offset of 0.5 mV, and low offset drift of 4 µV/ºC, but consumes a quiescent current of 5.2mA. This is because (from an amplifier design perspective) offset and drift are dc specifications, whereas bandwidth, low output impedance, and high capacitive drive capability are high-frequency specifications. Therefore, achieving all the performance in one amplifier requires power. However, a more efficient design to meet the low power budget is to use a composite reference buffer, which uses an amplifier with superior high-frequency specifications in the feedback loop of a dc precision amplifier to get the overall performance at much lower power consumption. Figure 48 shows such a composite amplifier design with the OPA333 (dc precision amplifier) and THS4281 (high-bandwidth amplifier). This reference buffer design requires three devices, and a large number of external components. This solution still consumes close to 2 mA of quiescent current.

REF6225 REF6230 REF6233 REF6241 REF6245 REF6250 Composite_Solution_SBOS708.gifFigure 48. Composite Amplifier Reference Buffer

The REF62xx family of voltage references have an integrated low output impedance buffer (ADC drive buffer); therefore, there is no need for an external buffer while driving the REF pin of high-precision, high-throughput SAR ADCs, as shown in Figure 49. The ADC drive buffer of the REF62xx is capable of replenishing a charge of 70 pC on a 47-µF capacitor in 1 µs, without allowing the voltage on the capacitor to droop more than 1 LSB at 18-bit precision. The REF62xx are trimmed at multiple temperatures in production, achieving a max drift of just 3 ppm/°C between 0°C and 70 °C for both the voltage reference and the buffer combined, while operating at a typical quiescent current of 820 µA. The reference drift is guaranteed from 0°C and 70 °C. The REF62xx can operate from -55°C to 125°C without getting damaged.Figure 50 compares the output impedance of a regular voltage reference (REF20xx) and a voltage reference with integrated ADC drive buffer (REF62xx). Figure 51 compares the burst-mode, reference-settling performance of a regular voltage reference and the REF62xx.

REF6225 REF6230 REF6233 REF6241 REF6245 REF6250 REF62xx_TIPD_BOS748.gifFigure 49. REF62xx Driving REF Pin of ADS8881 SAR ADC
REF6225 REF6230 REF6233 REF6241 REF6245 REF6250 C063_SBOS748.png
Figure 50. Output Impedance Comparison
REF6225 REF6230 REF6233 REF6241 REF6245 REF6250 C042_SBOS748.png
1 LSB = 19.07 µV, with ADS8881 at 1 MSPS
Figure 51. Reference Droop Comparison