SNWS018D December   2006  – June 2016 LMV221

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 2.7-V DC and AC Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Characteristics of the LMV221
      2. 7.3.2 Accurate Power Measurement
        1. 7.3.2.1 Concept of Power Measurements
        2. 7.3.2.2 LOG-Conformance Error
        3. 7.3.2.3 Temperature Drift Error
          1. 7.3.2.3.1 Temperature Compensation
          2. 7.3.2.3.2 Differential Power Errors
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown
        1. 7.4.1.1 Output Behavior in Shutdown
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Functionality and Applications of RF Power Detectors
        1. 8.1.1.1 Functionality of RF Power Detectors
          1. 8.1.1.1.1 Key Characteristics of RF Power Detectors
          2. 8.1.1.1.2 Types of RF Power Detectors
            1. 8.1.1.1.2.1 Diode Detector
            2. 8.1.1.1.2.2 (Root) Mean Square (R)MS) Detector
            3. 8.1.1.1.2.3 Logarithmic Detectors
    2. 8.2 Typical Applications
      1. 8.2.1 Application With Transmit Power Control Loop
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Detector Interfacing
            1. 8.2.1.2.1.1 RF Input
            2. 8.2.1.2.1.2 Output and Reference
              1. 8.2.1.2.1.2.1 Filtering
            3. 8.2.1.2.1.3 Interface to the ADC
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Application With Voltage Standing Wave Ratio (VSWR) Measurement
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Supply Lines
        1. 10.1.1.1 Positive Supply (VDD)
        2. 10.1.1.2 Ground (GND)
      2. 10.1.2 RF Input Interface
      3. 10.1.3 Microstrip Configuration
      4. 10.1.4 GCPW Configuration
      5. 10.1.5 Reference (REF)
      6. 10.1.6 Output (OUT)
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Community Resources
    2. 11.2 Trademarks
    3. 11.3 Electrostatic Discharge Caution
    4. 11.4 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

10 Layout

10.1 Layout Guidelines

As with any other RF device, careful attention must be paid to the board layout. If the board layout is not properly designed, unwanted signals can easily be detected or interference picked up.

Electrical signals (voltages and currents) need a finite time to travel through a trace or transmission line. RF voltage levels at the generator side and at the detector side can therefore be different. This is not only true for the RF strip line, but for all traces on the PCB. Signals at different locations or traces on the PCB are in a different phase of the RF frequency cycle. Phase differences in, for example, the voltage across neighboring lines, may result in crosstalk between lines due to parasitic capacitive, or inductive coupling. This crosstalk is further enhanced by the fact that all traces on the PCB are susceptible to resonance. The resonance frequency depends on the trace geometry. Traces are particularly sensitive to interference when the length of the trace corresponds to a quarter of the wavelength of the interfering signal or a multiple thereof.

10.1.1 Supply Lines

Because the PSRR of the LMV221 is finite, variations of the supply can result in some variation at the output. This can be caused among others by RF injection from other parts of the circuitry or the on/off switching of the PA.

10.1.1.1 Positive Supply (VDD)

In order to minimize the injection of RF interference into the LMV221 through the supply lines, the phase difference between the PCB traces connecting to VDD and GND must be minimized. A suitable way to achieve this is to short both connections for RF. This can be done by placing a small decoupling capacitor between the VDD and GND. It must be placed as close to the device VDD and GND pins as possible as shown in Figure 85. Be aware that the resonance frequency of the capacitor itself must be above the highest RF frequency used in the application, because the capacitor acts as an inductor above its resonance frequency.

Low frequency-supply voltage variations due to PA switching might result in a ripple at the output voltage. The LMV221 has a PSRR of 60 dB for low frequencies.

10.1.1.2 Ground (GND)

The LMV221 must have a ground plane free of noise and other disturbing signals. It is important to separate the RF ground return path from the other grounds. This is due to the fact that the RF input handles large voltage swings. A power level of 0 dBm causes a voltage swing larger than 0.6 VPP over the internal 50-Ω input resistor, resulting in a significant RF return current toward the source. Therefore, TI recommends that the RF ground return path not be used for other circuits in the design. The RF path must be routed directly back to the source without loops.

10.1.2 RF Input Interface

The LMV221 is designed to be used in RF applications having a characteristic impedance of 50 Ω. To achieve this impedance, the input of the LMV221 must be connected via a 50-Ω transmission line. Transmission lines can be easily created on PCBs using microstrip or (grounded) coplanar waveguide (GCPW) configurations. For more details about designing microstrip or GCPW transmission lines, TI recommends a microwave designer handbook is recommended.

10.1.3 Microstrip Configuration

One way to create a transmission line is to use a microstrip configuration. A cross section of the configuration is shown in Figure 83, assuming a two-layer PCB.

LMV221 30014080.gif Figure 83. Microstrip Configuration

A conductor (trace) is placed on the topside of a PCB. The bottom side of the PCB has a fully copper ground plane. The characteristic impedance of the microstrip transmission line is a function of the width W, height H, and the dielectric constant εr.

Characteristics such as height and the dielectric constant of the board have significant impact on transmission line dimensions. A 50-Ω transmission line may result in impractically wide traces. A typical 1.6-mm thick FR4 board results in a trace width of 2.9 mm, for instance. This is impractical for the LMV221 because the pad width of the 6-pin WSON package is 0.25 mm. The transmission line has to be tapered from 2.9 mm to 0.25 mm. Significant reflections and resonances in the frequency transfer function of the board may occur due to this tapering.

10.1.4 GCPW Configuration

A transmission line in a (grounded) coplanar waveguide (GCPW) configuration gives more flexibility in terms of trace width. The GCPW configuration is constructed with a conductor surrounded by ground at a certain distance, S, on the top side. Figure 84 shows a cross section of this configuration. The bottom side of the PCB is a ground plane. The ground planes on both sides of the PCB must be firmly connected to each other by multiple vias. The characteristic impedance of the transmission line is mainly determined by the width W and the distance S. In order to minimize reflections, the width W of the center trace must match the size of the package pad. The required value for the characteristic impedance can subsequently be realized by selection of the proper gap width S.

LMV221 20173781.gif Figure 84. GCPW Configuration

10.1.5 Reference (REF)

The REF pin can be used to compensate for temperature drift of the internal reference voltage as described in Interface to the ADC. The REF pin is directly connected to the inverting input of the transimpedance amplifier. Thus, RF signals and other spurious signals couple directly through to the output. Introduction of RF signals can be prevented by connecting a small capacitor between the REF pin and ground. The capacitor must be placed close to the REF pin as depicted in Figure 85.

10.1.6 Output (OUT)

The OUT pin is sensitive to crosstalk from the RF input, especially at high power levels. The ESD diode between OUT and VDD may rectify the crosstalk, but may add an unwanted inaccurate DC component to the output voltage.

The board layout must minimize crosstalk between the detectors input RFIN and the output of the detector. Using an additional capacitor connected between the output and the positive supply voltage (VDD pin) or GND can prevent this. For optimal performance this capacitor must be placed as close as possible to the OUT pin.

10.2 Layout Example

LMV221 20173701.png Figure 85. Recommended LMV221 Board Layout