9.2.3.2 Detailed Design Procedure

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.

9.2.3.3 Application Curves

Input RMS voltage (V)

20

30

40

50

60

70

80

90

100

100P

1m

10m

100m

D001

50 dB Gain

33 dB Gain

Figure 9-10. Hi-Z, High-Gain Front-End Circuit SNR

vs Input

Frequency (Hz)

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

100

1k

10k

100k

1M

10M

D005

Gain setting = 33 dB

Gain setting = 50 dB

Figure 9-11. Hi-Z, High-Gain Front-End Circuit Gain

vs Frequency

OPA607, OPA2607

SBOS981I – OCTOBER 2019 – REVISED FEBRUARY 2021

www.ti.com

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