During a measurement sequence, bias voltages must be applied appropriately to the transmit signal and receive signal. In auto mode, the entire measurement sequence including applying appropriate bias voltages is fully controlled by the ASQ. In register mode, the application must apply the correct bias voltages during a measurement sequence. Figure 21-13 shows the two bias voltage sources, Tx bias and Rx bias. The Tx bias must be applied to the output channel before generating excitation pulses. The Rx bias must be applied to the input channel before capturing the input signal. A typical measurement sequence is as follows [assuming that excitation (Tx) at CH0 and reception (Rx) at CH1]:
Before generating excitation pulses and stop pulses, apply the Tx bias to CH0_IN while CH0_OUT is connected to GND through the SWG0 switch (see Figure 21-14 and Figure 21-12).
Apply Tx bias to CH0_IN while CH0_OUT is connected to GND through SWG0.
Figure 21-14 Before Excitation
Generate excitation pulses and stop pulses through CH0_OUT, and CH0_IN is connected to PGA dummy load to meet the impedance-matching requirement. The pulses are driven to the transducer 0 (TR0) (see Figure 21-15).
Excitation pulses are driven on CH0_OUT, and CH0_IN is connected to the PGA dummy input.
Figure 21-15 Excitation
Wait for almost the complete estimated time of flight (TOF) between the transducer 0 (TR0) and the transducer 1 (TR1). The delay must be less than TOF to capture the beginning of the arriving pulse train.
Before the signal arrives at the transducer 1 (TR1), power on the SDHS ADC so that the SDHS is fully settled before sampling data.
Before the signal arrives at the transducer 1 (TR1), apply the Rx bias to CH1_IN while CH1_OUT is connected to GND through the SWG1 switch (see Figure 21-16 and Figure 21-12).
Apply Rx bias to CH1_IN while CH1_OUT is connect to GND through SWG1.
Figure 21-16 Before Reception
Trigger the SDHS to sample the arrived signal at the transducer 1 (TR1): CH1_IN is connected to PGA input for data capturing. CH1_OUT is connected to GND through the SWG1 switch (see Figure 21-17).
CH1_IN is connected to the PGA input, and CH1_OUT is connected to GND through the SWG1 switch.
Figure 21-17 Reception
The Tx bias voltage and the Rx bias voltage are programmable independently by SAPHBCTL.EXCBIAS and SAPHBCTL.PGABIAS, respectively.
