TDC-GP21: Time-to-Digital Converter with Integrated Analog Frontend

TDC-GP21 Ultrasonic Flow Converter Chip with integrated analog front-endTDC-GP21 is next generation’s upgrade for TDC-GP2. It offers pin-to-pin and functional compatibility, several feature improvements and an extended functionality.

Analog elements like a chopper stabilized comparator and analog switches simplify external circuits of ultrasonic flow meters in a significant manner. In parallel, measurement quality is improved and operating current is reduced. The fire pulse generator is extended, a new low-power 32 kHz oscillator driver is implemented and the temperature measuring unit is further improved. All in all, the TDC-GP21 is perfectly suited to design ultra-compact and low-cost ultrasonic flow meters and heat meters.



  • 2-channel TDC with 22ps resolution
  • Analog frontend for ultrasonic flow meters
  • Chopper-stabalized low-offset comparator for zero crossing detection
  • Precise windowing for hit selection
  • Temperature measuring unit for PT1000 sensors
  • QFN32 package


  • High prescision
  • Low power
  • Few additional external component needed

Circuit Features

Measurement Mode 1

  • 2 channels with typ. 90 ps resolution
  • 1-channel double resolution with typ. 45 ps
  • Range 3.5 ns (0 ns) to 2.5 µs
  • 20 ns pulse-pair resolution, 4-fold multihit
  • Up to 1 million measurements per second in measure mode 1

Measurement Mode 2

  • 1 channel with typ. 90 ps resolution
  • Double resolution mode with 45 ps, Quad resolution mode with 22 ps resolution
  • Measurement range 500 ns to 4 ms
  • 2 x CLKHS pulse-pair resolution with 3-fold multihit capability
  • Each of the 3 events can be assigned to an adjustable measuring window with 10 ns resolution

Temperature Measurement Unit

  • 2 or 4 sensors
  • PT100/PT500/PT1000 or higher
  • Schmitt trigger integrated
  • Very high resolution: up to 17.5 Bit eff. (0.004 °C resolution for platinum sensors)
  • Ultra low current (0.08 µA when measuring every 30 seconds)

Special Functions

  • Fire pulse generator, up to 127 pulses
  • Trigger to rising and/or falling edge
  • Precise stop enable by windowing
  • Low-power 32 kHz oscillator (500 nA)
  • Clock calibration unit
  • 7×32 Bit EEPROM

Analog Input Circuit

  • Chopper-stabilized low-offset comparator
  • 2 analog switches for input selection
  • External circuit is reduced to 2 resistors and 2 capacitors


Block Diagram of the TDC-GP2x Ultrasonic Flow Converters


typ. 90 ps, 45 ps in double resolution mode, 22 ps in quad resolution mode (only Measure Mode 2)


Max data rate: up to ½ million samples per second
Power supply voltage: 2.5 V to 3.6 V
Operating temp: -40°C to 125°C
Quiescent current: < 0.1µA
INL: < 0.1 LSB
DNL: < 0.8 LSB







TDC-GP21 Evaluation System

The TDC-GP21 Evaluation System provides everything you need to successfully evaluate and develop applications using the TDC-GP21 Time-to-Digital Converter. You can begin with the ScioSense (previously acam) TDC-GP21 Time-to-Digital Converter or look for a prototype development platform. The TDC-GP21 Evaluation System can be used to evaluate ultrasonic time-of-flight applications or as an evaluation kit for the TDC-GP21 time interval measurements.



Measurement Range 1

  • PC supported system, USB communication via PICOPROG interface module (included in delivery)
  • SPI Interface connector
  • No additional power supply. The power supply is provided by PICOPROG
  • Comfortable Windows based measurement and evaluation software
  • Compact in size
  • Different power options, selectable by jumper
  • Three reference clock sources for alternate clock options
  • Pt500 / Pt1000 temperature measurement with on board reference
  • Internal / external comparator for temperature measurement
  • Easy connection of external microcontroller boards
  • Data collection to ASCII text files


GP21 Evaluation Software

The PC based system comes with a Windows-based software. It contains six main sheets which allows the user to access all the functionality of the TDC-GP21.

Setup / Settings TabGraphic Tab 


Measurement Settings Tab


Standard TDC-Measurement Tab


Ultrasonic Time-of-Flight Measurement Tab


Temperature Measurement Tab


Graphic Tab Tab



The TDC-GP21 is Perfectly Suited for Low-Cost Ultrasonic Heatmeter Designs.

Thanks to the implemented functionality, including analog section, precision temperature measurement, fire pulse generation, windowing and clock calibration it is sufficient to add a simple microprocessor (without A/D converter). The extremely low current consumption guarantees the necessary long battery lifetime in such applications.

The measurement is fairly automated by the TDC-GP21. The microprocessor just sends a start command. The TDC then fires the transducers and measures the time of flight. It calibrates the results and provides them to the microprocessor.


a. Time measurement

32.768kHz: typ. 1.0 µA
4 MHz: < 1nA in power down, 130 µA when active (@3.0V)
TDC: 4 mA during active time of the high-speed unit

With 2 measurements per second (forth and back flow) the total consumption is < 2 µA.

b. Temperature measurement

A full measurement over all four ports takes less than 2 µAs current. Typically the temperature is measured once in 30 seconds. The average current then is about 0.08 µA. This is about 50 times less than with solutions without a TDC.

c. Quiescent current

Thanks to the current optimized technology the quiescent current is less than 100 nA.

d. Total system current

The complete current for the measuring unit ( incl. Transducer) can be reduced to 2.2 µA.

With a low-power µP (e.g. MSP430 series from TI) the average current consumption of the total device might be in the range of 10 to 15 µA.

It is possible to operate the system from a lithium-thionylchloride ½ AA cell for 10 years without changing the battery. At 6 years runtime it maybe even possible to work with a low-cost 3V CR2450 coin cell battery.

Documentation & Downloads

TDC-GP21: Time-to-Digital Converter with Integrated Analog Frontend

Ordering Numbers

Type Part number Package RoHS compl. Shipping package
TDC-GP22 MNR 1949 QFN32 Yes Tray
TDC-GP22 MNR 1950 QFN32 Yes Tape & Reel (5k)
GP22-EVA-KIT MNR 1951 System Yes Box
GP22-DEMO-KIT MNR 1979 System Yes Box
TDC-GP21 MNR 1839 QFN32 Yes Tray
TDC-GP21 MNR 1720 QFN32 Yes Tape & Reel (3K/5K)
GP21-EVA-KIT MNR 1781 System Yes Box
TDC-GP2 MNR 1058 QFN32 Yes Tray
TDC-GP2 MNR 1480 QFN32 Yes Tape & Reel (2k)
TDC-GP1 MNR 278 TQFP44 Yes Tray
TDC-GPX MNR 975 TQFP100 Yes Tray
TDC-GPX-FG MNR 1089 TFBGA120 Yes Tray