Using a TDC1000-7200 Eval kit in Mode 2 to develop a clamp-on style flowmeter using the recommended 1MHz StemInc piezo transducers. The clamp-on aspect means we're getting large signal losses, so the received signal we're interested is on the order of 0.5 mV pp from the piezo. We've tried implementing an amplifier/filter after the PGA to bring up the level as suggested in 8.3.5 of the TDC1000 datasheet, but we're getting a rather noisy signal and thus unacceptable timing variations of around 5ns. Further investigation shows that the output of the PGA (both LNA & PGA enabled, 21 dB gain in PGA) is already very noisy when PGA gain turned up high, so our amplification of the PGA's output is just amplifying noise. This happens even with no input signal.
Does anyone have some insight on whether measuring this low level of signal is practical with this chip? Any ideas of how to best approach amplifying/filtering or quieting the signal, or maybe even a circuit? Note we've already done some obvious filtering and cleaned up Vcc and Vref; this improved things a little, but nowhere near enough. Increasing the transmit voltage is impractical in our application. Thanks!
