SLASEC8B February 2017 – January 2019 PGA460-Q1
The PGA460-Q1 device has the option to decouple the transducer from the transformer and the rest of the driving circuitry during the echo detection stage of the record interval. The transducer is less exposed to noise generated by the driving circuit during this process and is less loaded, meaning the detected echo is capable of producing a higher voltage swing to be detected by the PGA460-Q1 device. For this function, the DECPL pin on the PGA460-Q1 device is used that drives the gate (or base) of an external transistor, QDECPL. During the burst and decay stages of the record interval the DECPL pin is high (3.3 V or 5 V depending on the IOREG level) and enables the external transistor which then connects the transformer driving circuit to GND and couples it to the transducer. The time-decoupling function is selected when the DECPL_TEMP_SEL bit in the EEPROM is set to 0.
When the burst stage is complete, a timer is started which times to the value defined by the DECPL_T bit in the EEPROM. When this time has elapsed, the state of the DECPL pin becomes low (GND) which means that the external transistor, QDECPL, is disabled which disconnects the transformer secondary coil from the transducer. Figure 39 shows the circuit implementation.