SLAS574B September   2013  – November 2025 ADS5474-SP

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Timing Characteristics
    7. 5.7 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
      1. 6.1.1 Input Configuration
      2. 6.1.2 Clock Inputs
      3. 6.1.3 Digital Outputs
  8. 7 Application and Implementation
    1. 7.1 Power Supply Recommendations
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Definition of Specifications
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

6.1.1 Input Configuration

The analog input for the ADS5474 consists of an analog pseudo-differential buffer followed by a bipolar transistor T&H. The analog buffer isolates the source driving the input of the ADC from any internal switching and presents a high impedance that is easy to drive at high input frequencies, compared to an ADC without a buffered input. The input common-mode is set internally through a 500Ω resistor, connected from 3.1V to each of the inputs (common-mode is ≅2.4V on 12- and 13-bit members of this family). The configuration results in a differential input impedance of 1kΩ.

ADS5474-SP Analog Input Equivalent CircuitFigure 6-1 Analog Input Equivalent Circuit

For a full-scale differential input, each of the differential lines of the input signal (pins 16 and 17) swings symmetrically between (3.1V + 0.55V) and (3.1V – 0.55V). This range means that each input has a maximum signal swing of 1.1VPP for a total differential input signal swing of 2.2VPP. Operation below 2.2VPP is allowable, with the characteristics of performance versus input amplitude demonstrated in Figure 5-19 and Figure 5-20. For instance, for performance at 1.1VPP rather than 2.2VPP, refer to the SNR and SFDR at –6dBFS (0dBFS = 2.2 VPP). The maximum swing is determined by the internal reference voltage generator, eliminating the need for any external circuitry for this purpose.

Top performance for the ADS5474 performs is when the analog inputs are driven differentially. The circuit in Figure 6-2 shows one possible configuration using an RF transformer with termination either on the primary or on the secondary of the transformer. In addition, the configuration of the evaluation module is with two back-to-back transformers, further demonstrating good performance. If voltage gain is required, use a step-up transformer.

ADS5474-SP Converting a Single-Ended Input to a Differential Signal Using an RF TransformerFigure 6-2 Converting a Single-Ended Input to a Differential Signal Using an RF Transformer

In addition to the transformer configurations, Texas Instruments offers a wide selection of single-ended operational amplifiers that can be selected depending on the application. Use an RF gain-block amplifier, such as THS9001, for high-input-frequency applications. Use the configuration shown in Figure 6-3 for large voltage gains at intermediate-frequencies in the range from 50MHz to 400MHz. Tune the component values for different intermediate frequencies. The example Figure 6-3 shows is located on the evaluation module and is tuned for an IF of 170MHz. More information regarding this configuration is found in the ADS5474 EVM User's Guide and the THS9001 50MHz to 350MHz Cascadeable Amplifier data sheet.

ADS5474-SP Using the THS9001 if Amplifier With the ADS5474Figure 6-3 Using the THS9001 if Amplifier With the ADS5474

For applications requiring DC-coupling with the signal source, a differential input or differential output amplifier, such as the THS4509 (shown in Figure 6-4) provides good harmonic performance and low noise over a wide range of frequencies.

ADS5474-SP Using the THS4509 or THS4520 With the ADS5474Figure 6-4 Using the THS4509 or THS4520 With the ADS5474

In this configuration, the THS4509 amplifier circuit provides 10dB of gain, converts the single-ended input to differential, and sets the proper input common-mode voltage to the ADS5474 using the VCM output pin of the ADC. The 50Ω resistors and 18pF capacitor between the THS4509 outputs and ADS5474 inputs (along with the input capacitance of the ADC) limit the bandwidth of the signal to approximately 70MHz (–3dB). Input termination is accomplished through the 78.9Ω resistor and 0.22μF capacitor to ground, in conjunction with the input impedance of the amplifier circuit.

A 0.22μF capacitor and 49.9Ω resistor are inserted to ground across the 78.9Ω resistor and 0.22μF capacitor on the alternate input to balance the circuit. Gain is a function of the source impedance, termination, and 348Ω feedback resistor. See the THS4509 Wideband, Low-Noise, Low-Distortion, Fully-Differential Amplifier data sheet for further component values to set proper 50Ω termination for other common gains.

Because the ADS5474 recommended input common-mode voltage is 3.1V, the THS4509 operates from a single power-supply input with VS+ = 5V and VS– = 0V (ground). This configuration has the potential to slightly exceed the recommended output voltage from the THS4509 of 3.6V due to the ADC input common-mode of 3.1V and the +0.55V full-scale signal. Exceeding the voltage does not harm the THS4509 but can result in degradation in the harmonic performance of the THS4509.

An amplifier with a wider recommended output voltage range is the THS4520, which is optimized for low noise and low distortion in the range of frequencies up to ≅20MHz. Applications that are not sensitive to harmonic distortion can consider either device at higher frequencies.