TDC-GP21 datasheet(62/91 Pages) AMSCO

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TDC-GP21

acam messelectronic gmbh - Friedrich-List-Str.4 - 76297 Stutensee - Germany - www.acam.de

4-17

4.3

Analog Input Section

Compared to TDC-GP2, the TDC-GP21 has an additional analog input section which can be

used alternatively to the pure digital inputs. Especially the design of ultrasonic flow and

heat meters is greatly simplified by this new option. The external circuit of the ultrasonic

part is reduced to just two resistors and capacitors additional to the piezo transducers.

The ultrasonic signals will be packages of sinusoidal oscillations with several 100 mV

amplitude. The signals are coupled to the inputs by means of a high pass filter as the

comparator can not handle GND as threshold. The threshold of the comparator is set to

1/3 Vcc. An analog multiplexer selects the input according to the active measurement

direction. The comparator is chopper stabilized to guarantee a low offset voltage in the

range of < 2 mV. This is mandatory for a good measurement quality. The input offset

voltage of the comparator is frequently corrected by an internal chopper circuit. If

temperature or supply voltage changes over time, the offset voltage is automatically

adapted and holds at < 2 mV.

All elements are controlled by the TDC-GP21 control unit. They are powered only during

the measurement to keep down the power consumption.

A measurement sequence, triggered by command Start_TOF_Restart looks like the

following procedure, starting with the up flow measurement:



The 4 MHz oscillator is switched on. The chip waits for the programmed delay to

give enough time for the oscillator to reach the full amplitude.



The comparator, the reference voltage and the analog switches get powered.



The capacitor of the transmitting path (STOP1) is connected to GND.



The fire down buffer (FIRE_DOWN) is connected to GND.



The capacitor of the receiving path (STOP2) is charged up to Vref. The TDC waits

the delay set in TW2.



The analog switch selects STOP2 input as input to the comparator.



FIRE_UP is selected as TDC START signal.



The set number of pulses is sent through the fire up buffer, pin FIRE_UP.



The analog signal at STOP2 passes the comparator converted to a digital signal

that is connected to the STOP input of the TDC unit.



When the delay of the stop masking unit (DELVAL) expired the TDC is ready to

measure. It can measure up to 3 stops.

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TDC-GP21 Datasheet(PDF) 62 Page - ams AG

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