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[FAQ] TDC1000: Everything You Need to Know for Ultrasonic ToF (Liquid-Coupled) [Collateral, Tools, Designs, FAQ]

Part Number: TDC1000

For liquid-coupled ultrasonic sensing applications including level sensing, fluid contamination and identification sensing, and liquid/gas flow sensing the TDC1000 Ultrasonic AFE is the recommended device. Typically, the TDC1000 is paired with the TDC7200 digital timer, but depending on the accuracy needed, a microcontroller such as the MSP430 or something more powerful such as the C2000 can be used in place of the TDC7200. All official collateral for the TDC1000 and TDC7200 are available on their associated product pages.

Getting Started and Evaluation Flow

Device Selection

To begin evaluation of the TDC device family, the first step is to choose a device based on the intended application. Each device in the family is listed below with a brief description, and general applications:

TDC1000http://www.ti.com/product/TDC1000

Dual channel ultrasonic time-of-flight sensing AFE

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification
  • Liquid/gas flow sensing

 

TDC1011http://www.ti.com/product/TDC1011

Single channel ultrasonic time-of-flight sensing AFE

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification

 

TDC7200http://www.ti.com/product/TDC7200

Time to digital converter

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification
  • Liquid/gas flow sensing
  • LIDAR time-of-flight measurement

TDC7201http://www.ti.com/product/TDC7201

Time to digital converter with combined mode for short time measurements

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification
  • Liquid/gas flow sensing
  • LIDAR time-of-flight measurement

EVM Selection

Once a device or devices have been chosen, next an EVM needs to be selected for the evaluation. There are several EVMs incorporating the TDC device family for a variety of applications listed below:

TDC1000-TDC7200EVMwww.ti.com/.../tdc1000-tdc7200evm

Includes the TDC1000 as the ultrasonic AFE, the TDC7200 to measure the time of flight, and an MSP430F5528 to configure the TDC devices and communicate with a PC GUI. Onboard bandpass filter suitable for use with 1-2Mhz transducers.

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification
  • Liquid flow sensing (highest accuracy)

TDC1000-C2000EVM: www.ti.com/.../tdc1000-c2000evm

Includes the TDC1000 as the ultrasonic AFE and a C2000 microprocessor to measure time of flight, configure the TDC1000, and communicate with a PC GUI. Both the TDC1000, and some versions of the C2000 microcontroller are qualified for use in automotive applications. Onboard bandpass filter suitable for use with 1-2Mhz transducers.

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification
  • Liquid flow sensing (lower accuracy)
  • Automotive applications

TDC1000-GASEVMwww.ti.com/.../tdc1000-gasevm

Includes the TDC1000 as the ultrasonic AFE, the TDC7200 to measure the time of flight, and an MSP430F5528 to configure the TDC devices and communicate with a PC GUI. Also includes the BSTEVM, which is a daughtercard that boosts the output voltage of the TDC1000 up to 30V for use with high voltage transducers in gas-coupled applications. This EVM has the same functionality as the TDC1000-TDC7200EVM, however the frequency range of the onboard bandpass filter is designed for 200-500kHz transducers.

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification
  • Liquid flow sensing (highest accuracy)
  • Gas flow sensing

TDC7200EVMwww.ti.com/.../tdc7200evm

Includes the TDC7200 on an MSP430 boosterpack. Designed to be compatible with the MSP-EXP430F5529LP launch pad (must be purchased separately). Must be connected to an external AFE for use in all applications listed below:

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification
  • Liquid and gas flow sensing (highest accuracy)
  • LIDAR time of flight (long distance)

TDC7201-ZAX-EVMwww.ti.com/.../tdc7201-zax-evm

Includes the TDC7200 on an MSP430 boosterpack. Designed to be compatible with the MSP-EXP430F5529LP launch pad (must be purchased separately). Must be connected to an external AFE for use in all applications listed below:

  • Liquid level/proximity sensing
  • Liquid concentration/contamination/identification
  • Liquid and gas flow sensing (highest accuracy)
  • LIDAR time of flight (short and long distance)

How do I set up my EVM and mount my ultrasonic transducer?

Each EVM page linked above includes a comprehensive user's guide for setup of the EVM hardware. All of the TDC1000 EVMs also include a transducer to use for testing. For best results in the evaluation process, it is important to properly mount the transducer to ensure good coupling to the medium the sound will travel through. For liquid level and concentration sensing, please refer to the following application note that describes the ideal mounting method for your transducer:

How to Select and Mount Transducers in Ultrasonic Sensing for Level Sensing and Fluid ID: http://www.ti.com/lit/an/snaa266a/snaa266a.pdf

Frequently Asked Questions

What is the minimum and/or maximum distance that can be measured with the TDC1000?

Both the minimum and maximum distance able to be measured by the TDC1000 depend on the specific application. The maximum distance is determined by the speed of sound in the medium it is traveling through, and to a lesser extent the frequency of the sound produced by the transducer (higher frequencies attenuate faster, and will be harder to detect at long distances). The maximum listen period for the TDC1000 is set at 8ms. To determine the maximum distance under ideal conditions use the following formula: 

Max distance = speed of sound in medium (m/s) * 0.008 seconds / 2

This formula accounts for the round trip time in the case of a single transducer being used as both transmitter and receiver. As an example, the speed of sound in water is ~1480m/s, so the maximum distance measurable by the TDC1000 in water is ~5.9m one way.

The minimum distance measurable is dependent on the transducer (its frequency and ringdown characteristics) and the number of TX pulses selected by the user. The frequency of the transducer will determine the period of the TX pulses output by the TDC1000, and therefore the length of time that the TDC1000 is transmitting. Since ultrasonic transducers are resonant devices, the transducer will continue to output sound for a short time after the TX pulses have stopped (called ringdown). While the transducer is ringing, it will be mask any incoming echo signals and the TDC1000 will not be able to register a return echo to generate a STOP signal. The theoretical minimum distance can be calculated with the following formula:

Min time (theoretical) = (number of TX pulses (at least 3)) * (1 / transducer frequency) + ringdown time

In practice, the minimum time is further limited by the blanking time of the TDC1000. For the smallest possible time of flight measurement, the TDC1000 must be in Short TOF mode. The TDC1000 will not start receiving until the Short TOF Blanking Period has ended. The minimum blanking period determines the minimum time of flight, and is set using the following formula:

Min blanking period = 8 * (1 / system clock frequency)

The maximum system clock frequency is 16Mhz, which equates to a minimum blanking time of 500ns.

The effective minimum time that can be measured will be the higher of the two times calculated above. Finally the minimum distance can be calculated using the following formula along with the min time determined above:

Min distance = speed of sound in medium (m/s) * min time / 2.

 What settings should I use for my system?

The specific settings needed for a given system are entirely dependent on the components in the system, and the measurement parameters (medium, expected distance, flow rate, etc.). Since it is impossible to provide settings for all possible systems, this section will provide a flow for how to configure the TDC1000 for TOF measurement: which settings to change, and how they affect the measurement sequence. For the most detailed description of all available settings, reference the TDC1000 datasheet.

1. Select TOF measurement mode (TOF_MEAS_MODE) depending on application and transducer configuration:

  • Mode 0: Fluid level and identification. Transducer transmits then receives its own signal.
  • Mode 1: Flow sensing. Transducer 1 transmits and transducer 2 receives. Requires manual channel switching
  • Mode 2: Flow sensing (preferred). Transducer 1 transmits and transducer 2 receives. Allows automatic channel switching.

2. Select TX frequency depending on input clock and transducer frequency:

Set the TX_FREQ_DIV to divide the input clock to the correct frequency for the transducer. Ex: 8Mhz input clock with 1Mhz transducer. Set TX_FREQ_DIV to divide by 8.

3. Select NUM_TX depending on range needed, and saturation point of transducer:

Larger NUM_TX numbers will increase the excitation and sound pressure level (SPL) of the transducer output up to a point. Eventually the transducer will become saturated and will not produce more SPL even when provided with more transmit pulses. Care must be taken with short distance measurements as well, because a longer transmit period could mask the return echo. This setting requires experimentation with the exact transducer being used in the system.

4. Choose short or standard TOF mode:

Short TOF mode is useful for short distance measurements, where the receive echo would return before the common mode settling and autozero periods have completed. It moves these functions to start before the transmit pulses have begun:

Standard TOF mode allows for longer distance measurements with a wider range of listen period lengths. It also allows for a blanking period if the blanking mode is set.

5. Choose the blanking time, TIMING_REG, autozero period length, and echo timeout length to suit your desired measurement range:

All settings are based off of the period of the input clock (T0). For a wider range of values, the input clock can be divided by 2 (T0 period increased 2x) by setting the CLOCKIN_DIV setting. Reference the above timing diagrams when choosing the settings listed below.

  • Blanking time:
    • in short TOF mode set SHRT_TOF_BLNK_PRD long enough that it masks the ringdown time of the transducer, but short enough that it does not mask the return echo for the shortest possible distance measured
    • in standard TOF mode with blanking, set TIMING_REG in the same manner as above, but keep in mind that the AUTOZERO_PERIOD and the common mode settling time also add to the effective blanking time of the mode
  • Autozero period:
    • in short TOF mode set AUTOZERO to a low setting to minimize total measurement time, or to a longer time if the total measurement time is not critical
    • in standard TOF mode AUTOZERO can be used as an additional blanking period adjustment
  • TIMING_REG:
    • if TIMING_REG is set to 30 or lower or if the FORCE_SHORT_TOF bit is set to 1, this enables the short TOF mode. Otherwise the device is in standard TOF mode
    • in standard TOF mode without blanking the TIMING_REG determines the Echo Listen time period along with the TOF_TIMEOUT_CTRL
    • in standard TOF mode with blanking the TIMING_REG determines the blanking period length
  • Echo timeout:
    • in all modes, the TOF_TIMEOUT_CTRL setting determines the Echo Listen time period

6. Set the receive path gain and echo quality threshold:

  • PGA CTRL: the receive path PGA can be enabled or disabled
  • PGA_GAIN: select the gain for the input signal from 0dB to 21dB. This can be used to increase the level of the input signal in order to bring it within the range of available echo quality thresholds
  • ECHO_QUAL_THLD: selec the echo quality threshold so that the return echo triggers a STOP signal, but received noise does not. This is measured in -mV from the common mode (VDD/2) at the COMPIN pin.

7. Select the receive mode and the number of stops:

  • RECEIVE_MODE:
    • Single echo mode generates a stop for each time the signal crosses the echo quality threshold. See figure above.
    • Multiple echo mode generates a stop for each echo pulse envelope. See figure below.

  • NUM_RX: determines how many STOP signals will be generated by the TDC1000

Example: 

  • Fluid level application
  • Input clock = 8Mhz (T0 = 0.125us)
  • TX Freq = 1Mhz
  • Medium = water (speed of sound 1480m/s)
  • Minimum distance = 2cm (~27us round trip in water)
  • Maximum distance = 20cm (~270us round trip in water)
  • NUM_TX = 5

  1. Set Mode 0 for fluid level sensing
  2. TX_FREQ_DIV = divide by 8
  3. NUM_TX was selected as 5 based on distances and transducer
  4. Short TOF mode selected. Can be forced with FORCE_SHORT_TOF set to 1
  5. Set CLOCKIN_DIV to divide by 2 for longer receive time (T0 = 0.25us)
  6. Set blanking time. Ringdown will be estimated as 2x the transmit period. Transmit period is NUM_TX * 1/TX freq = 5us. Transmit + ringdown = 15us. SHRT_TOF_BLNK_PRD = 64*T0 = 16us. (less than minimum distance, greater than transmit + ringdown)
  7. Set AUTOZERO_PERIOD to minimum for shortest measurement cycle 64*T0 = 16us.
  8. TIMING_REG is not used. Leave at 0 to keep short TOF mode.
  9. Set TOF_TIMEOUT_CTRL to max. 1024*T0 = 256us. Transmit (5us) + blanking (16us) + echo listen (256us) = 277us (greater than maximum distance)
  10. Set PGA_CTRL to enabled, and PGA_GAIN to 9dB
  11. Set ECHO_QUAL_THLD to -125dB
  12. Set receive mode to multiple echo for level sensing
  13. Set NUM_RX to 1 for a single expected return echo

 

How do I change the bandpass filter on the EVM? 

The TDC1000-TDC7200EVM is equipped with a simple bandpass filter between the PGAOUT and COMPIN pins as shown in the schematic from the user's guide:

This filter is configured for a roughly 1 Mhz center frequency. To change the filter to suit transducers of a different frequency, reference the "Receiver Filters" section of the datasheet (section 8.3.5) for example equations to calculate new component values. Additionally, attached below is a TINA simulation of the existing filter topology from the EVM that can be used to test the transfer characteristics of new filter components.

 Why are my temperature readings incorrect on the GUI?

This situation is caused by erroneous short pulses generated when measuring the resistance of the RTD. The mechanism and workaround for this behavior is described in the following application note:

Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic Sensing: www.ti.com/.../snaa230.pdf

When measuring the temperature value with the TDC1000-TDC7200 GUI ensure that the TEMP_MODE on the TDC1000 page is set to "REF_RTD1_RTD2", and that TEMP_RTD_MODE on the TEMPERATURE page is set to "RTD1 and RTD2" regardless of whether one or two RTDs are being measured at that time. The GUI automatically implements the workaround to provide the correct reading for RTD1.

 

Useful E2E Posts


Short distance measurements in air (Level from the top): e2e.ti.com/.../498792

How to perform liquid level sensing from the bottom of a tank: e2e.ti.com/.../467713

TDC1000 Overview Webinar: e2e.ti.com/.../504584

Fluid Identification using TDC1000 and time of flight measurements: e2e.ti.com/.../494576

Basic level sensing FAQ: e2e.ti.com/.../418605

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Bharat Aravamudhan

Sensor Signal Conditioning Applications

1 Reply

  • This is a FAQ on the TDC1000

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    Bharat Aravamudhan

    Sensor Signal Conditioning Applications

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