SNAS648B October   2014  – August 2015 TDC1000 , TDC1000-Q1

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  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 Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Switching Characteristics
    8. 6.8 Typical Characteristics
  7. Parameter Measurement Information
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Transmitter Signal Path
      2. 8.3.2 Receiver Signal Path
      3. 8.3.3 Low Noise Amplifier (LNA)
      4. 8.3.4 Programmable Gain Amplifier (PGA)
      5. 8.3.5 Receiver Filters
      6. 8.3.6 Comparators for STOP Pulse Generation
        1. 8.3.6.1 Threshold Detector and DAC
        2. 8.3.6.2 Zero-cross Detect Comparator
        3. 8.3.6.3 Event Manager
      7. 8.3.7 Common-mode Buffer (VCOM)
      8. 8.3.8 Temperature Sensor
        1. 8.3.8.1 Temperature Measurement with Multiple RTDs
        2. 8.3.8.2 Temperature Measurement with a Single RTD
    4. 8.4 Device Functional Modes
      1. 8.4.1 Time-of-Flight Measurement Mode
        1. 8.4.1.1 Mode 0
        2. 8.4.1.2 Mode 1
        3. 8.4.1.3 Mode 2
      2. 8.4.2 State Machine
      3. 8.4.3 TRANSMIT Operation
        1. 8.4.3.1 Transmission Pulse Count
        2. 8.4.3.2 TX 180° Pulse Shift
        3. 8.4.3.3 Transmitter Damping
      4. 8.4.4 RECEIVE Operation
        1. 8.4.4.1 Single Echo Receive Mode
        2. 8.4.4.2 Multiple Echo Receive Mode
      5. 8.4.5 Timing
        1. 8.4.5.1 Timing Control and Frequency Scaling (CLKIN)
        2. 8.4.5.2 TX/RX Measurement Sequencing and Timing
      6. 8.4.6 Time-of-Flight (TOF) Control
        1. 8.4.6.1 Short TOF Measurement
        2. 8.4.6.2 Standard TOF Measurement
        3. 8.4.6.3 Standard TOF Measurement with Power Blanking
        4. 8.4.6.4 Common-mode Reference Settling Time
        5. 8.4.6.5 TOF Measurement Interval
      7. 8.4.7 Averaging and Channel Selection
      8. 8.4.8 Error Reporting
    5. 8.5 Programming
      1. 8.5.1 Serial Peripheral Interface (SPI)
        1. 8.5.1.1 Chip Select Bar (CSB)
        2. 8.5.1.2 Serial Clock (SCLK)
        3. 8.5.1.3 Serial Data Input (SDI)
        4. 8.5.1.4 Serial Data Output (SDO)
    6. 8.6 Register Maps
      1. 8.6.1 TDC1000 Registers
        1. 8.6.1.1  CONFIG_0 Register (address = 0h) [reset = 45h]
        2. 8.6.1.2  CONFIG_1 Register (address = 1h) [reset = 40h]
        3. 8.6.1.3  CONFIG_2 Register (address = 2h) [reset = 0h]
        4. 8.6.1.4  CONFIG_3 Register (address 3h) [reset = 3h]
        5. 8.6.1.5  CONFIG_4 Register (address = 4h) [reset = 1Fh]
        6. 8.6.1.6  TOF_1 Register (address = 5h) [reset = 0h]
        7. 8.6.1.7  TOF_0 Register (address = 6h) [reset = 0h]
        8. 8.6.1.8  ERROR_FLAGS Register (address = 7h) [reset = 0h]
        9. 8.6.1.9  TIMEOUT Register (address = 8h) [reset = 19h]
        10. 8.6.1.10 CLOCK_RATE Register (address = 9h) [reset = 0h]
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Level and Fluid Identification Measurements
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1 Level Measurements
          2. 9.2.1.2.2 Fluid Identification
        3. 9.2.1.3 Application Curves
      2. 9.2.2 Water Flow Metering
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
          1. 9.2.2.2.1 Regulations and Accuracy
          2. 9.2.2.2.2 Transit-Time in Ultrasonic Flow-Meters
          3. 9.2.2.2.3 ΔTOF Accuracy Requirement Calculation
          4. 9.2.2.2.4 Operation
        3. 9.2.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
      2. 12.1.2 Development Support
    2. 12.2 Documentation Support
    3. 12.3 Related Links
    4. 12.4 Community Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

9 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.

9.1 Application Information

The TDC1000 is an analog front-end for ultrasonic sensing applications. The device is typically used for the driving and sensing of ultrasonic transducers to perform accurate time-of-flight measurements. Ultrasonic time-of-flight sensing allows for fluid level measurements, fluid identification or concentration, flow measurements, and proximity/distance sensing.

9.2 Typical Applications

9.2.1 Level and Fluid Identification Measurements

TDC1000 TDC1000-Q1 app_lvl_concent_NAS648.gifFigure 51. Level and Concentration Measurement Application Diagram

9.2.1.1 Design Requirements

Table 15. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
Fluid Level
Range 2 – 10 cm
Fluid Identification
Accuracy 0.5% concentration variation
Distance 5.08 cm

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Level Measurements

For level sensing applications, the total time-of-flight (TOF) of the sound wave in the fluid is measured. The pulses transmitted by transducer B travel through the fluid, typically from the bottom, to the surface of the fluid. The discontinuity between the fluid and air generates a reflected wave which returns back to transducer B.

At the beginning of a measurement cycle, the transducer is connected to a transmit channel of the AFE, and the transmit burst excites the transducer to generate an ultrasound wave. Synchronous to the TX burst, a START pulse is generated by the TDC1000 to indicate the start of a measurement. After the transmission is completed, and depending on the device configuration, the transducer is connected to a receive channel of the AFE.

When a valid echo is received, the TDC1000 generates a STOP pulse. Generation of multiple STOP pulses is possible through register configuration of the device. The START and STOP signal times are compared to determine the TOF.

The level of the fluid can be determined using the following equation:

Equation 6. TDC1000 TDC1000-Q1 eq_lvl_d_NAS648.gif

where

  • d is the fluid level in meters (m)
  • TOF is the time-of-flight in seconds (s)
  • c is the speed of sound in the fluid in meters per second (m/s)
TDC1000 TDC1000-Q1 lvl_meter_pulses_NAS648.gifFigure 52. Relation Between Transmit and Receive Pulses in Level Measurements

Level measurements have 2 main criteria: resolution and range (maximum height). Resolution accuracies of 1-2 mm are achievable but are impractical due to any environmental disturbances, such as tank vibrations, creating millimeter level surface waves. Ranges of up to 1 m are measurable utilizing VDD level excitation pulses, but surface disturbances and signal loss over longer distances make the reliable echo reception an issue. Greater level measurement reception can be achieved by mechanical means (level guide tube) and/or electronic means (level shifting the TX pulses to greater voltages; see TIDA-00322).

9.2.1.2.2 Fluid Identification

The TDC1000 can be used to measure the time-of-flight for a known distance to calculate the speed of sound (cmedium) in the fluid. This application utilizes the same circuitry as the level example but a transducer in a different configuration connected to the second channel. In this example, the speed of sound in the fluid (cmedium) is measured by using transducer A.

The temperature can also be measured to compensate for the temperature variation of sound. With the known distance, TOF and temperature, the speed of sound in the fluid can be determined and the identity of the medium verified.

After measuring the time-of-flight for the fixed distance, the speed of sound can be calculated as follows:

Equation 7. TDC1000 TDC1000-Q1 eq_lvl_c_NAS648.gif

where

  • cmedium is the speed of sound in the fluid in meters per second (m/s)
  • d is the level in meters (m)
  • TOF is the time of flight in seconds (s)

The measurement process is identical to the level example above. The speed of sound can be used to uniquely identify a variety of fluids. In this example, the concentration of diesel exhaust fluid (DEF) is measured with a desired accuracy resolution of 0.5% of concentration variation. For most fluids, the speed of sound varies over temperature, so every application will be different. In this example, all samples were all at ambient temperature of 23°C.

9.2.1.3 Application Curves

The data used in the following level and fluid identification graphs was collected using ultrasonic test cells. The test cells are acrylic plastic containers with width of 2.54 cm and ultrasonic transducers attached to the outside using cyanoacrylate glue. The transducers in this experiment are STEMiNC 1MHz piezo electric ceramic discs (SMD10T2R111). Equivalent transducers with the following characteristics could be used:

  • Piezo material: SM111
  • Dimensions: 10mm diameter x 2mm thickness
  • Resonant frequency: 1050 kHz (thickness mode)

TDC1000 TDC1000-Q1 D015_SNAS648.gif

Fluid Height in Tank Time-of-Flight (µs)
Full (10 cm) 145
Full – 1 (9 cm) 131
Full – 2 (8 cm) 118
3 cm 50
2 cm 35
Figure 53. Time-of-Flight for Fluid Height in Tank
TDC1000 TDC1000-Q1 D019_SNAS648.gif

Fluid Speed of sound (m/s)
Distilled water 1481.87
Tap water 1483.13
Figure 55. Speed of Sound in Distilled Water and Tap Water
TDC1000 TDC1000-Q1 D018_SNAS648.gif

Fluid Speed of sound (m/s)
Distilled water 1481.87
Tap water 1483.13
DEF 10.0% 1530.49
DEF 20.0% 1576.42
DEF 30.0% 1620.00
DEF 31.5% 1627.37
DEF 32.0% 1629.15
DEF 32.5% 1630.00
Figure 54. Speed of Sound for Various Fluids and Diesel Exhaust Fluid (DEF) Concentration
TDC1000 TDC1000-Q1 D020_SNAS648.gif

Fluid Speed of sound (m/s)
DEF 30.0% 1620.00
DEF 31.5% 1627.37
DEF 32.0% 1629.15
DEF 32.5% 1630.00
Figure 56. Speed of Sound of Various Diesel Exhaust Fluid (DEF) Concentrations

9.2.2 Water Flow Metering

TDC1000 TDC1000-Q1 app_water_fm_NAS648.gifFigure 57. Water Flow-meter Application Simplified Diagram

9.2.2.1 Design Requirements

The parameters in Table 16 are considered for this example.

Table 16. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
Pipe diameter 15 mm
Distance between transducers 60 mm
Minimum flow rate 0.015 m3/h
Accuracy at minimum flow rate 5%

9.2.2.2 Detailed Design Procedure

The design of flow-meters requires a thorough technical assessment of the system where the device will be used. The following is a list of areas to consider:

  • Minimum and maximum flow rate at maximum allowable error in the system
  • Transitional flow rate
  • Instantaneous and total quantity pumped over time
  • Accuracy of the meter within prescribed limits per applicable standards
  • Pressure in the system
  • Operating temperature range

The appropriate ultrasonic sensor and the proper electronics for interfacing to the sensor are determined based on the system requirements. The following is a list of specifications applicable to the senor/assembly used in the system:

  • Excitation frequency
  • Excitation source voltage
  • Pipe diameter
  • Distance between the transducers (or reflectors)

9.2.2.2.1 Regulations and Accuracy

If the flow meter is intended for residential applications, it must be designed to meet the required standards. For example, per the INTERNATIONAL ORGANIZATION OF LEGAL METROLOGY (OIML), the metrological requirements of water meters are defined by the values of Q1, Q2, Q3 and Q4, which are described in Table 17.

Table 17. Flow-rate Zones per OIML

FLOW-RATE ZONE DESCRIPTION
Q1 Lowest flow rate at which the meter is to operate within the maximum permissible errors.
Q2 Flow rate between the permanent flow rate and the minimum flow rate that divides the flow rate range into two zones, the upper flow rate zone and the lower flow rate zone, each characterized by its own maximum permissible errors.
Q3 Highest flow rate within the rated operating condition at which the meter is to operate within the maximum permissible errors.
Q4 Highest flow rate at which the meter is to operate for a short period of time within the maximum permissible errors, while maintaining its metrological performance when it is subsequently operating within the rated operating conditions.

A water meter is designated by the numerical value of Q3 in m3/h and the ratio Q3/Q1. The value of Q3 and the ratio of Q3/Q1 are selected from the lists provided in the OIML standards.

Water meters have to be designed and manufactured such that their errors do not exceed the maximum permissible errors (MPE) defined in the standards. For example, in OIML standards, water meters need to be designated as either accuracy class 1 or accuracy class 2, according to the requirements.

For class 1 water meters, the maximum permissible error in the upper flow rate zone (Q2 ≤ Q ≤ Q4) is ±1%, for temperatures from 0.1°C to 30°C, and ±2% for temperatures greater than 30°C. The maximum permissible error for the lower flow-rate zone (Q1 ≤ Q < Q2) is ±3%, regardless of the temperature range.

For class 2 water meters, the maximum permissible error for the upper flow rate zone (Q2 ≤ Q ≤ Q4) is ±2%, for temperatures from 0.1°C to 30°C, and ±3% for temperatures greater than 30°C. The maximum permissible error for the lower flow rate zone (Q1 ≤ Q < Q2) is ±5% regardless of the temperature range.

The flow meter accuracy specified in the standards dictates the required accuracy in the electronics used for driving the ultrasonic transducers, circuits in the receiver path, and time measurement sub circuits. The stringent accuracy required at lower flow rates would require a very low noise signal chain in the transmitter and receiver circuits used in ultrasonic flow meters, as well as the ability to measure picosecond time intervals.

9.2.2.2.2 Transit-Time in Ultrasonic Flow-Meters

Transit-time ultrasonic flow meters works based on the principle that sound waves in a moving fluid travel faster in the direction of flow (downstream), and slower in the opposite direction of flow (upstream).

The system requires at least two transducers. The first transducer operates as a transmitter during the upstream cycle and as a receiver during the downstream cycle, and the second transducer operates as a receiver during the upstream cycle and as a transmitter during the downstream cycle. An ultrasonic flow meter operates by alternating transmit and receive cycles between the pair of transducers and accurately measuring the time-of-flight both directions.

TDC1000 TDC1000-Q1 transmit_rec_pulses_NAS648.gifFigure 58. Relation Between Transmit and Receive Pulses Upstream/Downstream

In this example, the upstream TOF is defined as:

Equation 8. TDC1000 TDC1000-Q1 eq_T_BA_NAS648.gif

where

  • l is the path length between the two transducers in meters (m)
  • c is the speed of sound in water in meters per second (m/s)
  • v is the velocity of the water in the pipe in meters per second (m/s)

In this example, the downstream TOF is defined as:

Equation 9. TDC1000 TDC1000-Q1 eq_T_AB_NAS648.gif

where

  • l is the path length between the two transducers in meters (m)
  • c is the speed of sound in water in meters per second (m/s)
  • v is the velocity of the water in the pipe in meters per second (m/s)

The difference of TOF is defined as:

Equation 10. TDC1000 TDC1000-Q1 eq_dTOF_NAS648.gif

where

  • tBA is the upstream TOF from transducer B to transducer A in seconds (s)
  • tAB is the downstream TOF from transducer A to transducer B in seconds (s)

After the difference in time-of-flight (ΔTOF) is calculated, the water velocity inside the pipe can be related to the ΔTOF using the following equation:

Equation 11. TDC1000 TDC1000-Q1 eq_V_NAS648.gif

where

  • c is the speed of sound in water in meters per second (m/s)
  • l is the path length between the two transducers in meters (m)

Finally, the mass flow rate can be calculated as follows:

Equation 12. TDC1000 TDC1000-Q1 eq_Q_kvA_NAS648.gif

where

  • k is the flow-meter constant
  • v is the velocity of the water in the pipe in meters per second (m/s)
  • A is the cross-section area of the pipe in meters-squared (m2)

9.2.2.2.3 ΔTOF Accuracy Requirement Calculation

Based on the minimum mass flow requirement and accuracy requirements in Table 16, the ΔTOF accuracy needed can be calculated as follows:

  1. Convert the mass flow rate to m3/s:
  2. TDC1000 TDC1000-Q1 eq_calc01_NAS648.gif
  3. Calculate the flow velocity assuming k = 1:
  4. TDC1000 TDC1000-Q1 eq_calc02_NAS648.gif
  5. Calculate the ΔTOF for the given speed of sound. In this example, a speed of sound c = 1400 m/s is assumed:
  6. TDC1000 TDC1000-Q1 eq_calc03_NAS648.gif
  7. The requirement of 5% accuracy for minimum flow will result in a ΔTOF accuracy of:
  8. TDC1000 TDC1000-Q1 eq_calc04_NAS648.gif

For this reason, this system requires a high accuracy timer/stopwatch that can measure the lower flow rate state.

9.2.2.2.4 Operation

The TDC1000 is used to drive the transmitter, amplify and filter the received signal and conditioning the echo for START and STOP pulse generation. The TDC7200 ps-accurate timer is used to measure the time interval between the rising edge of the START pulse and the rising edge of the STOP pulse produced by the TDC1000.

The microcontroller should first configure the TDC7200 and the TDC1000 for the measurement. When the microcontroller issues a start command to the TDC7200 via the SPI interface, the TDC7200 sends a trigger pulse to the TRIGGER pin of the TDC1000. When the TDC1000 drives the transmit transducer, a synchronous START pulse is produced on the START pin, which commands the TDC7200 to start its counters. When a valid echo pulse is received on the receive transducer, the TDC1000 generates a STOP pulse on the STOP pin, which commands the TDC7200 to stop its counters. This procedure is repeated for the upstream and downstream cycles.

A temperature measurement can be performed and the result can be used to correct for temperature dependency of the speed of sound.

9.2.2.3 Application Curves

The following figures show data and histograms created with data collected under a zero flow condition at room temperature. A simple offset calibration has been applied, where the overall average of the data is subtracted from the data.

TDC1000 TDC1000-Q1 D021_SNAS648.gifFigure 59. Calibrated Raw and Averaged Delta Time-of-Flight Data
TDC1000 TDC1000-Q1 D022_SNAS648.gifFigure 60. Raw Calibrated Data Histogram
TDC1000 TDC1000-Q1 D023_SNAS648.gifFigure 61. 10x Running Average Data Histogram