4Measurements in Practice
We propose to start with one of the example configurations. The most important parameters to
review and set are:
•Fire pulse frequency (typically 100kHz, 200kHz or 500kHz)
•Number of fire pulses (20 is a good start)
•Number of TOF hits (10 is a good start)
•Noise mask Window should be =1. Then directly after the fire burst, with about 10µs delay, the
chip switches to receive. Typically you see a short oscillation due to the switching between the
send and receive transducer.
•Charge pump voltage and PGA gain of receiving amplitude (to get a receive amplitude of 200 to
400mV)
In general, we recommend to have a look at the receive signal by means of an oscilloscope. This
clearly helps to find the right settings.
If the signal is measured at the PGA output, at test pin J4 (pin 17, COMP_IN) then there is no influence
of the probe to the measured time values. In Figure 3 this point is marked. Another probe should be
connected to US_UP for the trigger signal. When looking at minimum noise then this probe of course
needs to be removed.
Figure 6: Typical send and receive signals
When working with real gas there might be the need for an external amplification with higher gain
than offered by the internal PGA. In this case, please refer to application note SC-000942-AN,
“AS6040 with external amplifier” for details.
