To achieve a SNR of greater than 40 dB for signals from 300 µVrms to 30 mV the front-end stage has two gain settings: 6 V/V and 31 V/V. The SW (switch, relay, or analog mux) can be dynamically toggled to ensure maximum sensitively to the receiving signal. The OPAx607-Q1 devices prove to be an attractive solution for this front-end signal chain because of the right balance of low noise and high input impedance. The ultrasonic sensors (for example, piezo crystal) have high output impedance. The OPAx607-Q1 devices have an input bias current of 20 pA (maximum). This small bias current results in reduced distortion and signal loss across the source impedance when compared with a bipolar amplifier with input bias currents in the range of a few hundreds of nano-amperes. The OPAx607-Q1's high-gain front-end is followed by a narrowband band-pass filter that is tuned to a 200-kHz center frequency. The narrowband filter is designed using the OPA837. OPA837 can be used as a variable gain mux / PGA as shown in TIDA-01565. In this application section the OPA837-based band-pass filter was designed using the techniques mentioned in the Filter Design in Thirty Seconds application report.

Figure 9-11 shows the frequency response of circuit in Figure 9-9. As shown in Figure 9-11, the frequency response is a high-Q factor band-pass filter centered around 200 kHz. Designing such a high-Q band-pass filter helps eliminate white band noise along with other interferences present in the circuitry, resulting in a high SNR signal chain. The OPAx607-Q1's front-end combined with the OPA837-based band-pass filter help to achieve a total gain of 33 dB (44 V/V) or 50 dB (316 V/V) based on the SW (switch) position.