To achieve a SNR of greater than 40 dB for signals from 300 uVrms 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 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 (Ex. piezo crystal) have high output impedance. The OPAx607 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'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'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.