SBAS558C December   2012  – December 2015 ADS42B49

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Description (continued)
  6. 6 ADS424x and ADS422x Family Comparison
  7. 7 Pin Configuration and Functions
  8. 8 Specifications
    1. 8.1  Absolute Maximum Ratings
    2. 8.2  ESD Ratings
    3. 8.3  Recommended Operating Conditions
    4. 8.4  Thermal Information
    5. 8.5  Electrical Characteristics: ADS42B49 (250 MSPS)
    6. 8.6  Electrical Characteristics: General
    7. 8.7  Digital Characteristics
    8. 8.8  Timing Requirements: LVDS and CMOS Modes
    9. 8.9  Serial Interface Timing Characteristics
    10. 8.10 Reset Timing (Only When Serial Interface is Used)
    11. 8.11 LVDS Timings at Lower Sampling Frequencies
    12. 8.12 CMOS Timings at Lower Sampling Frequencies
    13. 8.13 Typical Characteristics
      1. 8.13.1 ADS42B49
      2. 8.13.2 Contour
  9. 9 Parameter Measurement Information
  10. 10Detailed Description
    1. 10.1 Overview
    2. 10.2 Functional Block Diagram
    3. 10.3 Feature Description
      1. 10.3.1 Migrating from the ADS62P49 and ADS4249
      2. 10.3.2 Digital Functions
      3. 10.3.3 Gain for SFDR and SNR Trade-Off
      4. 10.3.4 Offset Correction
    4. 10.4 Device Functional Modes
      1. 10.4.1 Power-Down
        1. 10.4.1.1 Global Power-Down
        2. 10.4.1.2 Channel Standby
        3. 10.4.1.3 Input Clock Stop
      2. 10.4.2 Digital Output Information
        1. 10.4.2.1 Output Interface
        2. 10.4.2.2 DDR LVDS Outputs
        3. 10.4.2.3 LVDS Buffer
        4. 10.4.2.4 Parallel CMOS Interface
        5. 10.4.2.5 CMOS Interface Power Dissipation
        6. 10.4.2.6 Multiplexed Mode of Operation
        7. 10.4.2.7 Output Data Format
      3. 10.4.3 Parallel Configuration Details
    5. 10.5 Programming
      1. 10.5.1 Parallel Configuration Only
      2. 10.5.2 Serial Interface Configuration Only
      3. 10.5.3 Using Both Serial Interface and Parallel Controls
      4. 10.5.4 Serial Interface Details
        1. 10.5.4.1 Register Initialization
        2. 10.5.4.2 Serial Register Readout
    6. 10.6 Register Maps
      1. 10.6.1 Register Description
  11. 11Application and Implementation
    1. 11.1 Application Information
      1. 11.1.1 Driving Circuit
        1. 11.1.1.1 Drive Circuit Requirements
      2. 11.1.2 Clock Input
    2. 11.2 Typical Application
      1. 11.2.1 Design Requirements
      2. 11.2.2 Detailed Design Procedure
        1. 11.2.2.1 Analog Input
        2. 11.2.2.2 Clock Driver
        3. 11.2.2.3 Digital Interface
      3. 11.2.3 Application Curves
  12. 12Power Supply Recommendations
    1. 12.1 Using DC/DC Power Supplies
    2. 12.2 Power Supply Bypassing
  13. 13Layout
    1. 13.1 Layout Guidelines
      1. 13.1.1 Grounding
      2. 13.1.2 Supply Decoupling
      3. 13.1.3 Exposed Pad
      4. 13.1.4 Routing Analog Inputs
    2. 13.2 Layout Example
  14. 14Device and Documentation Support
    1. 14.1 Device Support
      1. 14.1.1 Device Nomenclature
    2. 14.2 Documentation Support
      1. 14.2.1 Related Documentation
    3. 14.3 Community Resources
    4. 14.4 Trademarks
    5. 14.5 Electrostatic Discharge Caution
    6. 14.6 Glossary
  15. 15Mechanical, Packaging, and Orderable Information

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）
サーマルパッド・メカニカル・データ
発注情報

11 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.

11.1 Application Information

The analog input pins have analog buffers (running off the AVDD_BUF supply) that internally drive the differential sampling circuit. As a result of the analog buffer, the input pins present high input impedance to the external driving source (10-kΩ dc resistance and 2.5-pF input capacitance). The buffer helps to isolate the external driving source from the switching currents of the sampling circuit. This buffering makes driving the buffered inputs easier than when compared to an ADC without the buffer.

The input common-mode is set internally using a 5-kΩ resistor from each input pin to VCM so the input signal can be ac-coupled to the pins. Each input pin (INP, INM) must swing symmetrically between VCM + 0.5 V and VCM – 0.5 V, resulting in a 2-V PP differential input swing.

The input sampling circuit has a high 3-dB bandwidth that extends up to 700 MHz (measured with 50-Ω source driving 50-Ω termination between INP and INM).

The dynamic offset of the first-stage sub-ADC limits the maximum analog input frequency to approximately 400 MHz (with 2-VPP amplitude) and to approximately 500 MHz (with 1.6-VPP amplitude) before the performance degrades. This offset is separate from the full-power analog bandwidth of 700 MHz, which is only an indicator of signal amplitude versus frequency.

11.1.1 Driving Circuit

Example driving circuit configuration is shown in Figure 53. Notice that the board circuitry is simplified compared to the non-buffered ADS4249.

To optimize even-harmonic performance at high input frequencies (greater than the first Nyquist), the use of back-to-back transformers is recommended, as shown in Figure 53. Note that the drive circuit is terminated by 50 Ω near the ADC side. The ac-coupling capacitors allow the analog inputs to self-bias around the required common-mode voltage.

ADS42B49 ai_drvr_hi_bas558.gif Figure 53. Drive Circuit for High Input Frequencies

The mismatch in the transformer parasitic capacitance (between the windings) results in degraded even-order harmonic performance. Connecting two identical RF transformers back-to-back helps minimize this mismatch and good performance is obtained for high-frequency input signals. An additional termination resistor pair may be required between the two transformers, as shown in Figure 53. The center point of this termination is connected to ground to improve the balance between the P (positive) and M (negative) sides. The values of the terminations between the transformers and on the secondary side must be chosen to obtain an effective 50 Ω (for a 50-Ω source impedance).

11.1.1.1 Drive Circuit Requirements

For optimum performance, the analog inputs must be driven differentially. This technique improves the common-mode noise immunity and even-order harmonic rejection. A small resistor (5 Ω to 10 Ω) in series with each input pin is recommended to damp out ringing caused by package parasitics.

Figure 54, Figure 55, and Figure 56 show the differential impedance (ZIN = RIN || CIN) at the ADC input pins. The presence of the analog input buffer results in an almost constant input capacitance up to 1 GHz.

ADS42B49 ai_adc_equiv_cir_bas558.gif
1. X = A or B.
2. ZIN = RIN || (1/jωCIN).
Figure 54. ADC Equivalent Input Impedance
ADS42B49 G033_Input_Resistance.png Figure 55. ADC Analog Input Resistance (RIN) Across Frequency
ADS42B49 G034_Input_Capacitance.png Figure 56. ADC Analog Input Capacitance (CIN) Across Frequency

11.1.2 Clock Input

The ADS42B49 clock inputs can be driven differentially (sine, LVPECL, or LVDS) or single-ended (LVCMOS), with little or no difference in performance between them. The common-mode voltage of the clock inputs is set to VCM using internal 5-kΩ resistors. This setting allows the use of transformer-coupled drive circuits for sine-wave clock or ac-coupling for LVPECL and LVDS clock sources are shown in Figure 57, Figure 58 and Figure 59. See Figure 60 details the internal clock buffer.

ADS42B49 ai_dif_sinewave_clk_bas550.gif

NOTE:

RT = termination resistor, if necessary.
Figure 57. Differential Sine-Wave Clock Driving Circuit
ADS42B49 ai_lvds_clk_drv_bas550.gif Figure 58. LVDS Clock Driving Circuit
ADS42B49 ai_lvpecl_clk_drv_bas550.gif Figure 59. LVPECL Clock Driving Circuit
ADS42B49 ai_intclk_buffer_bas550.gif

NOTE:

CEQ is 1 pF to 3 pF and is the equivalent input capacitance of the clock buffer.
Figure 60. Internal Clock Buffer

A single-ended CMOS clock can be ac-coupled to the CLKP input, with CLKM connected to ground with a 0.1-μF capacitor, as shown in Figure 61. For best performance, the clock inputs must be driven differentially, thereby reducing susceptibility to common-mode noise. For high input frequency sampling, TI recommends using a clock source with very low jitter. Band-pass filtering of the clock source can help reduce the effects of jitter. There is no change in performance with a non-50% duty cycle clock input.

ADS42B49 ai_drv_cir_1end_bas550.gif Figure 61. Single-Ended Clock Driving Circuit

11.2 Typical Application

ADS42B49 Typical_Application_Schematic_ADS42B49.gif Figure 62. Example Schematic for ADS42B49

11.2.1 Design Requirements

Example design requirements are listed in Table 11 for the ADC portion of the signal chain. These do not necessary reflect the requirements of an actual system, but rather demonstrate why the ADS42B49 may be chosen for a system based on a set of requirements.

Table 11. Design Requirements for ADS42B49

DESIGN PARAMETER EXAMPLE DESIGN REQUIREMENT ADS42B49 CAPABILITY
Sampling rate ≥ 200 Msps to allow 125 MHz of unaliased bandwidth Max sampling rate: 250 Msps
Input frequency > 200 MHz to accommodate full 2nd nyquist zone Large signal –3-dB bandwidth: 400-MHz operation
SNR > 68 dBFS at –1 dFBS, 170 MHz 70.7 dBFS at –1 dBFS, 170 MHz (0-dB gain)
SFDR > 80 dBc at –1 dFBS, 170 MHz 85 dBc at –1 dBFS, 170 MHz (0-dB gain)
Input full scale voltage 1.5 Vpp 1.5 Vpp
Overload recovery time < 3 clock cycles 1 clock cycle
Digital interface DDR LVDS DDR LVDS
Power consumption < 500 mW per channel 425 mW per channel

11.2.2 Detailed Design Procedure

11.2.2.1 Analog Input

The analog input of the ADS42B49 is typically driven by a fully-differential amplifier. The amplifier must have sufficient bandwidth for the frequencies of interest. The noise and distortion performance of the amplifier affects the combined performance of the ADC and amplifier. The amplifier is often AC coupled to the ADC to allow both the amplifier and ADC to operate at the optimal common-mode voltages. The user can DC couple the amplifier to the ADC if required. An alternate approach is to drive the ADC using transformers. DC coupling cannot be used with the transformer approach.

11.2.2.2 Clock Driver

The ADS42B49 should be driven by a high performance clock driver, such as a clock jitter cleaner. The clock must have low noise to maintain optimal performance. LVPECL is the most common clocking interface, but LVDS and LVCMOS can also be used. Do not drive the clock input from an FPGA, unless the noise degradation can be tolerated, such as for input signals near DC where the clock noise impact is minimal.

11.2.2.3 Digital Interface

The ADS42B49 supports both LVDS and CMOS interfaces. The LVDS interface should be used for best performance when operating at maximum sampling rate. The LVDS outputs can be connected directly to the FPGA without any additional components. When using CMOS outputs, resistors should be placed in series with the outputs to reduce the output current spikes and limit the performance degradation. The resistors should be large enough to limit current spikes, but not so large as to significantly distort the digital output waveform. An external CMOS buffer should be used when driving distances greater than a few inches, to reduce ground bounce within the ADC.

11.2.3 Application Curves

Figure 63 and Figure 64 show performance obtained at 100-MHz and 280-Mhz input frequencies, respectively, using appropriate driving circuit.

ADS42B49 100MHz_IF.png Figure 63. 100-MHz Input Frequency
ADS42B49 280MHz_IF.png Figure 64. 280-MHz Input Frequency