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
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:
Dual channel ultrasonic time-of-flight sensing AFE
Single channel ultrasonic time-of-flight sensing AFE
Time to digital converter
Time to digital converter with combined mode for short time measurements
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:
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.
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.
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.
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:
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:
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.
6. Set the receive path gain and echo quality threshold:
7. Select the receive mode and the number of stops:
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: http://www.ti.com/lit/an/snaa230/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.
What transducer should I use?
Why can't I see the tank level on the GUI?
Can I use the EVM with an external microcontroller?
How can I boost the voltage output from the TDC1000?
Useful E2E Posts
Short distance measurements in air (Level from the top): https://e2e.ti.com/support/sensor/ultrasonic/f/991/t/498792
How to perform liquid level sensing from the bottom of a tank: https://e2e.ti.com/support/sensor/ultrasonic/f/991/t/467713
TDC1000 Overview Webinar: https://e2e.ti.com/support/sensor/ultrasonic/f/991/t/504584
Fluid Identification using TDC1000 and time of flight measurements: https://e2e.ti.com/support/sensor/ultrasonic/f/991/t/494576
Basic level sensing FAQ: https://e2e.ti.com/support/sensor/ultrasonic/f/991/t/418605
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