SNOSCY1 March   2014 LDC1041

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Terminal Configuration and Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 Handling Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Inductive Sensing
      2. 7.3.2 Measuring Rp with LDC1041
      3. 7.3.3 Measuring Inductance with LDC1041
    4. 7.4 Device Functional Modes
      1. 7.4.1 Power Modes
      2. 7.4.2 INTB Pin Modes
        1. 7.4.2.1 Comparator Mode
        2. 7.4.2.2 Wake-Up Mode
        3. 7.4.2.3 DRDYB Mode
    5. 7.5 Programming
      1. 7.5.1 SPI Description
        1. 7.5.1.1 Extended SPI Transactions
    6. 7.6 Register Map and Description
  8. Applications and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Calculation of Rp_MIN and Rp_MAX
        1. 8.1.1.1 Setting Rp_MAX
        2. 8.1.1.2 Setting Rp_MIN
      2. 8.1.2 Output Data Rate
      3. 8.1.3 Choosing Filter Capacitor (CFA and CFB Terminals)
    2. 8.2 Typical Applications
      1. 8.2.1 Axial Distance Sensing Using a PCB Sensor with LDC1041
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Sensor and Target
          2. 8.2.1.2.2 Calculating Sensor Capacitor
          3. 8.2.1.2.3 Choosing Filter Capacitor
          4. 8.2.1.2.4 Setting Rp_MIN and Rp_MAX
          5. 8.2.1.2.5 Calculating Minimum Sensor Frequency
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Linear Position Sensing Application Diagram
      3. 8.2.3 Angular Position Sensing Application Diagram
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Trademarks
    2. 11.2 Electrostatic Discharge Caution
    3. 11.3 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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7 Detailed Description

7.1 Overview

The LDC1041 is an Inductance-to-Digital Converter that simultaneously measures the impedance and resonant frequency of an LC resonator. It accomplishes this task by regulating the oscillation amplitude in a closed loop configuration to a constant level, while monitoring the energy dissipated by the resonator. By monitoring the amount of power injected into the resonator, the LDC1041 can determine the value of Rp; it returns this as a digital value which is inversely proportional to Rp. In addition, the LDC1041 also measure the oscillation frequency of the LC circuit; this frequency is used to determine the inductance of the LC circuit. The device outputs a digital value that is inversely proportional to frequency.

The threshold detector block provides a comparator with hysteresis, with the threshold registers programed and comparator enabled, proximity data register is compared with threshold registers and INTB pin indicates the output.

The device has a simple 4-wire SPI interface. The INTB pin provides multiple functions which are programmable with SPI.

The device has separate supplies for Analog and I/O, with analog operating at 5V and I/O at 1.8-5V. The integrated LDO needs a 56nF capacitor connected from CLDO pin to GND.

7.2 Functional Block Diagram

blockdiagram_snoscy1.gif

7.3 Feature Description

7.3.1 Inductive Sensing

An AC current flowing through a coil will generate an AC magnetic field. If a conductive material, such as a metal target, is brought into the vicinity of the coil, this magnetic field will induce circulating currents (eddy currents) on the surface of the target. These eddy currents are a function of the distance, size, and composition of the target. The eddy currents then generate their own magnetic field, which opposes the original field generated by the coil. This mechanism is best compared to a transformer, where the coil is the primary core and the eddy current is the secondary core. The inductive coupling between both cores depends on distance and shape. Hence the resistance and inductance of the secondary core (eddy current), shows up as a distant dependent resistive and inductive component on the primary side (coil). Figure 5 to Figure 8 show a simplified circuit model.

inductor_modeled_resistor_snoscx2.gifFigure 5. Inductor With A Metal Target

Eddy currents generated on the surface of the target can be modeled as a transformer as shown in Figure 6. The coupling between the primary and secondary coils is a function of the distance and the conductor’s characteristics. In Figure 6, the inductance Ls is the coil’s inductance, and Rs is the coil’s parasitic series resistance. The inductance L(d), which is a function of distance d, is the coupled inductance of the metal target. Likewise, R(d) is the parasitic resistance of the eddy currents and is also a function of distance.

metal_target_eddy_currents_snoscx2.gifFigure 6. Metal Target Modeled As L And R With Circulating Eddy Currents

Generating an alternating magnetic field with just an inductor will consume a large amount of power. This power consumption can be reduced by adding a parallel capacitor, turning it into a resonator as shown in Figure 7 . In this manner the power consumption is reduced to the eddy and inductor losses Rs+R(d) only.

LC_tank_oscillator_snoscx2.gifFigure 7. LC Tank Connected To Oscillator

The LDC1041 doesn’t measure the series resistance directly; instead it measures the equivalent parallel resonance impedance Rp (see Figure 8). This representation is equivalent to the one shown in Figure 8 , where the parallel resonance impedance Rp(d) is given by:

Equation 1. eq01_snoscy1.gif
equivalent_res_parallel_LC_tank_snoscx2.gifFigure 8. Equivalent Resistance Of Rs in Parallel With LC Tank

Figure 9 below shows the variation in Rp as a function of distance for a 14mm diameter PCB coil (Sensor Details:Table 19). The target in this example is a section of a 2mm thick stainless steel disk.

C002_snoscy1.pngFigure 9. Typical Rp vs Distance With 14mm PCB Coil

7.3.2 Measuring Rp with LDC1041

The LDC1041 supports a wide range of LC combinations, with oscillation frequencies ranging from 5kHz to 5MHz and Rp ranging from 798Ω to 3.93MΩ. This range of Rp can be viewed as the maximum input range of an ADC. As illustrated in Figure 9, the range of Rp is typically much smaller than the maximum input range supported by the LDC1041. To get better resolution in the desired sensing range, the LDC1041 offers a programmable input range through the Rp_MIN and Rp_MAX registers. Refer to Calculation of Rp_MIN and Rp_MAX below for how to set these registers.

When the sensor’s resonance impedance Rp drops below the programed Rp_MIN, the LDC’s Rp output will clip at its full scale output. This situation could, for example, happen when a target comes too close to the coil.

C003_snoscy1.pngFigure 10. Transfer Characteristics Of LDC1041 With Rp_MIN= 1.347 kΩ And Rp_MAX= 38.785 kΩ

The resonance impedance can be calculated from the digital output code as follows:

Equation 2. eq02_snoscy1.gif

where

  • Y=Proximity Data/27
  • Rp_MAX and Rp_MIN are the maximum and minimum Rp values selected in the respective registers
  • Proximity data is the LDC output, register address 0x22.

Example: If Proximity data (address 0x22) is 50, Rp_MIN is 2.394 kΩ, and Rp_MAX is 38.785 kΩ, the resonance impedance is given by:

Y=50/27 = 0.3906

Rp=(38785*2394)/(2394×(1-0.3906) + 38785×0.3906) =(92851290)/(15149.421 + 1458.9036)

Rp = 5.59 kΩ

7.3.3 Measuring Inductance with LDC1041

LDC1041 measures the sensor’s frequency of oscillation using a frequency counter. The frequency counter timing is set by an external clock applied on TBCLK terminal. The sensor frequency can be calculated from the frequency counter register value (see registers 0x23 through 0x25) as follows:

Equation 3. eq03_snoscy1.gif

where

  • Fext is the frequency of the external clock
  • Fcount is the value obtained from the Frequency Counter Data register(address 0x23,0x24,0x25)
  • Response Time is the programmed response time (see LDC configuration register, address 0x04)

The sensor inductance can be determined by:

Equation 4. eq04_snoscy1.gif

where

  • C is the parallel capacitance of the resonator

Example: If Fext=6MHz, Response time=6144, C=100pF and measured Fcount= 3000 (dec) (address 0x23 through 0x25)

fsensor=(1/3)*(6000000/3000)*(6144)= 4.096MHz

Now using,eq04_snoscy1.gif

Inductance, L = 15.098 µH

The accuracy of measurement largely depends upon the choice of the external time-base clock (TBCLK). A higher frequency will provide better measurement accuracy.

7.4 Device Functional Modes

7.4.1 Power Modes

The LDC1041 has two power modes:

  1. Active Mode : In this mode the Proximity data and frequency data conversion is enabled.
  2. Stand-by Mode: This is the default mode on device power-up. In this mode conversion is disabled.

7.4.2 INTB Pin Modes

The INTB terminal is a configurable output terminal which can be used to drive an interrupt on an MCU. The LDC1041 provides three different modes on INTB terminal:

  1. Comparator Mode
  2. Wake-Up Mode
  3. DRDY Mode

LDC1041 has built-in High and Low trigger threshold registers which can be used as a comparator with programmable hysteresis or in a special mode which can be used to wake-up an MCU. These modes are explained in detail below.

7.4.2.1 Comparator Mode

In the Comparator mode, the INTB terminal is asserted or deasserted when the proximity register value increases above Threshold High or decreases below Threshold Low registers respectively. In this mode, the LDC1041 essentially behaves as a proximity switch with programmable hysteresis.

INTB_comparator_mode_snoscx2.gifFigure 11. Behavior Of INTB Terminal In Comparator Mode

7.4.2.2 Wake-Up Mode

In Wake-Up mode, the INTB terminal is asserted when proximity register value increases above Threshold High and de-asserted when wake-up mode is disabled in INTB terminal mode register.

This mode can be used to wake-up an MCU from sleep, to conserve power.

INTB_wakeup_mode_snoscx2.gifFigure 12. Behavior Of INTB Terminal In Wake-Up Mode

7.4.2.3 DRDYB Mode

In DRDY(Data Ready) mode, the INTB terminal is asserted every time the conversion data is available and de-asserted once the read command on register 0x22 is registered internally; if the read is in progress, the terminal is pulsed instead. The valid condition for new data availability is CSB high and DRDYB falling edge.

td_snoscy1.gifFigure 13. Behavior of INTB Terminal in DRDYB Mode with SPI Extending Beyond Subsequent Conversions
INTB_DRDYB_mode_snoscy1.gifFigure 14. Behavior Of INTB Terminal In DRDYB Mode with SPI Reading The Data Within Subsequent Conversion

7.5 Programming

The LDC1041 utilizes a 4-wire SPI to access control and data registers. The LDC1041 is an SPI slave device and does not initiate any transactions.

7.5.1 SPI Description

A typical serial interface transaction begins with an 8-bit instruction, which is comprised of a read/write bit (MSB, R=1) and a 7 bit address of the register, followed by a data field which is typically 8 bits. However, the data field can be extended to a multiple of 8 bits by providing sufficient SPI clocks. Refer to the Extended SPI Transactions section below.

td_serial_interface_protocol_snoscx2.gifFigure 15. Serial Interface Protocol

Each assertion of CSB starts a new register access. The R/Wb bit in the command field configures the direction of the access; a value of 0 indicates a write operation and a value of 1 indicates a read operation. All output data is driven on the falling edge of the serial clock (SCLK), and all input data is sampled on the rising edge of the serial clock (SCLK). Data is written into the register on the rising edge of the 16th clock. It is required to deassert CSB after the 16th clock; if CSB is deasserted before the 16th clock, no data write will occur.

7.5.1.1 Extended SPI Transactions

A transaction may be extended to multiple registers by keeping the CSB asserted beyond the initial 16 clocks. In this mode, the register addresses increment automatically. CSB must be asserted during 8*(1+N) clock cycles of SCLK, where N is the amount of bytes to write or read during the transaction.

During an extended read access, SDO outputs the register contents every 8 clock cycles after the initial 8 clocks of the command field. During an extended write access, the data is written to the registers every 8 clock cycles after the initial 8 clocks of the command field.

Extended transactions can be used to read 8-bits of Proximity data and 24-bits of frequency data in a single SPI transaction by initiating a read from the register 0x22.

7.6 Register Map and Description

Table 1. Register Map(1)(2)(3)

Register Name Address Direction Default Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Device ID 0x00 RO 0x84 Device ID
Rp_MAX 0x01 R/W 0x0E Rp Maximum
Rp_MIN 0x02 R/W 0x14 Rp Minimum
Watchdog Timer Frequency 0x03 R/W 0x45 Min Sensor Frequency
LDC Configuration 0x04 R/W 0x1B Reserved(000) Amplitude Response Time
Reserved 0x05 RO 0x01 Reserved(00000001)
Reserved 0x06 R/W 0xFF Reserved
Comparator Threshold High MSB 0x07 R/W 0xFF Threshold High MSB
Reserved 0x08 R/W 0x00 Reserved
Comparator Threshold Low MSB 0x09 R/W 0x00 Threshold Low MSB
INTB Terminal Configuration 0x0A R/W 0x00 Reserved(00000) INTB_MODE
Power Configuration 0x0B R/W 0x00 Reserved(0000000) PWR_MODE
Status 0x20 RO OSC Dead DRDYB Wake-up Comparator Do Not Care
Reserved 0x21 RO Reserved(00000000)
Proximity Data 0x22 RO Proximity Data
Frequency Counter Data LSB 0x23 RO FCOUNT LSB
Frequency Counter Data Mid-Byte 0x24 RO FCOUNT Mid Byte
Frequency Counter Data MSB 0x25 RO FCOUNT MSB
(1) Values of register fields which are unused should be set to default values only.
(2) Registers 0x01 through 0x05 are Read Only when the part is awake (PWR_MODE bit is SET)
(3) R/W: Read/Write. RO: Read Only. WO: Write Only.

Table 2. Revision ID

Address = 0x00, Default=0x84, Direction=RO
Bit Field Field Name Description
7:0 Revision ID Revision ID of Silicon.

Table 3. Rp_MAX

Address = 0x01, Default=0x0E, Direction=R/W
Bit Field Field Name Description
7:0 Rp Maximum Maximum Rp that LDC1041 needs to measure. Configures the input dynamic range of LDC1041. See Table 4 for register settings.

Table 4. Register Settings for Rp_MAX

Register setting Rp (kΩ)
0x00 3926.991
0x01 3141.593
0x02 2243.995
0x03 1745.329
0x04 1308.997
0x05 981.748
0x06 747.998
0x07 581.776
0x08 436.332
0x09 349.066
0x0A 249.333
0x0B 193.926
0x0C 145.444
0x0D 109.083
0x0E 83.111
0x0F 64.642
0x10 48.481
0x11 38.785
0x12 27.704
0x13 21.547
0x14 16.160
0x15 12.120
0x16 9.235
0x17 7.182
0x18 5.387
0x19 4.309
0x1A 3.078
0x1B 2.394
0x1C 1.796
0x1D 1.347
0x1E 1.026
0x1F 0.798

Table 5. Rp_MIN

Address = 0x02, Default=0x14, Direction=R/W
Bit Field Field Name Description
7:0 Rp Minimum Minimum Rp that LDC1041 needs to measure. Configures the input dynamic range of LDC1041. See Table 6 for register settings.(1)
(1) This Register needs a mandatory write as it defaults to 0x14.

Table 6. Register Settings for Rp_MIN

Register setting Rp (kΩ)
0x20 3926.991
0x21 3141.593
0x22 2243.995
0x23 1745.329
0x24 1308.997
0x25 981.748
0x26 747.998
0x27 581.776
0x28 436.332
0x29 349.066
0x2A 249.333
0x2B 193.926
0x2C 145.444
0x2D 109.083
0x2E 83.111
0x2F 64.642
0x30 48.481
0x31 38.785
0x32 27.704
0x33 21.547
0x34 16.160
0x35 12.120
0x36 9.235
0x37 7.182
0x38 5.387
0x39 4.309
0x3A 3.078
0x3B 2.394
0x3C 1.796
0x3D 1.347
0x3E 1.026
0x3F 0.798

Table 7. Watchdog Timer Frequency

Address = 0x03, Default=0x45, Direction=R/W
Bit Field Field Name Description
7:0 Min Sensor Frequency Sets the watchdog timer. The Watchdog timer is set based on the lowest sensor frequency.
Equation 5. eq07_snoscy1.gif

where

  • F is the sensor frequency
Example:
If Sensor frequency is 1Mhz
Min Sensor Frequency=68.94*log10(1M/2500)=Round to nearest integer(179.38)=179

Table 8. LDC Configuration

Address = 0x04, Default=0x1B, Direction=R/W
Bit Field Field Name Description
7:5 Reserved Reserved to 0
4:3 Amplitude Sets the oscillation amplitude
00:1V
01:2V
10:4V
11:Reserved
2:0 Response Time 000: Reserved
001: Reserved
010: 192
011: 384
100: 768
101: 1536
110: 3072
111: 6144

Table 9. Comparator Threshold High MSB

Address = 0x07, Default=0xFF, Direction=R/W
Bit Field Field Name Description
7:0 Threshold High Threshold High Register.

Table 10. Comparator Threshold Low MSB

Address = 0x09, Default=0x00, Direction=R/W
Bit Field Field Name Description
7:0 Threshold Low Threshold Low Register.

Table 11. INTB Terminal Configuration

Address = 0x0A, Default=0x00, Direction=R/W
Bit Field Field Name Description
7:3 Reserved Reserved to 0
2:0 Mode 000: All modes disabled
001: Wake-up Enabled on INTB terminal
010: INTB terminal indicates the status of Comparator output
100: DRDYB Enabled on INTB terminal
All other combinations are Reserved

Table 12. Power Configuration

Address = 0x0B, Default=0x00, Direction=R/W
Bit Field Field Name Description
7:1 Reserved Reserved to 0
0 PWR_MODE 0:Stand-By mode
1:Active Mode. Conversion is Enabled
Refer to Power Modes for more details.

Table 13. Status

Address = 0x20, Default=NA, Direction=RO
Bit Field Field Name Description
7 OSC status 1:Indicates oscillator overloaded and stopped
0:Oscillator working
6 Data Ready 1:No new data available
0:Data is ready to be read
5 Wake-up 1:Wake-up disabled
0:Wake-up triggered. Proximity data is more than Threshold High value.
4 Comparator 1:Proximity data is less than Threshold Low value
0:Proximity data is more than Threshold High value
3:0 Do not Care

Table 14. Proximity Data

Address = 0x22, Default=NA, Direction=RO
Bit Field Field Name Description
7:0 Proximity data Proximity data

Table 15. Frequency Counter LSB

Address = 0x23, Default=NA, Direction=RO
Bit Field Field Name Description
7:0 FCOUNT LSB (FCOUNT[7:0]) LSB of Frequency Counter. Sensor frequency can be calculated using the output data rate. Please refer to the Measuring Inductance with LDC1041.

Table 16. Frequency Counter Mid-Byte

Address = 0x24, Default=NA, Direction=RO
Bit Field Field Name Description
7:0 FCOUNT Mid byte (FCOUNT[15:8]) Middle Byte of Output data rate

Table 17. Frequency Counter MSB

Address = 0x25, Default=NA, Direction=RO
Bit Field Field Name Description
7:0 FCOUNT MSB (FCOUNT[23:16]) MSB of Output data rate

Conversion data is updated to these registers only when a read is initiated on 0x22 register. If the read is delayed between subsequent conversions, these registers are not updated until another read is initiated on 0x22.