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TDC-1000 TDC-7100 Evaluation Board Receive path

Bob Cantwell
Prodigy 85 points
Other Parts Discussed in Thread: TDC1000

I am testing  the TDC1000 with the LNA in resistive coupling mode and using just the receiver i.e. not using a single transducer with the transmitter also connected the transducer.   I have removed R42 from my evaluation board and replaced C43 with a 900 resistor.

Since the LNA non-inverting input is biased to VCOM sometime after trigger, I see the following:

Where probe 1 is the trigger, probe 3 is the output from BUFF_PGA, and probe 4 is on the transducer.

My question is it required to capacitor couple the transducer and bias the input to Vdd/2 to get this to work properly? 

I have my TDC1000 eval board set up as shown below:

  • 0
    TI__Expert 4725 points
    Bob,
    Which EVM are you using? What frequency is your transducer?
    Regards,
  • 0 in reply to Bob Cantwell
    Prodigy 85 points
    I am using a 300 kHz transducer
  • 0
    Prodigy 85 points
    Just to add more info to my question, it seems that the non-inverting input of the LNA is held at ground, sometime after trigger is applied, the non-inverting input is driven up to VCOM. The LNA drives its output to make the inverting input match the non inverting input - basic op-amp functionality. Since the inverting input is driven up to Vdd/2 my transducer sees a ground to Vdd/2 pulse as shown in the scope picture. This false pulse causes the transducer to ring. How can I prevent this "false" pulse?
  • 0 in reply to Bob Cantwell
    TI__Expert 4725 points
    Bob,
    You're correct about the LNA input driving to VCOM and effectively exciting your transducer as soon as the receive chain is turned on.

    I notice you're in "Short TOF". This setting is usually isn't used in air/gas settings as the TOF measurements are much longer than what can be measured in the Short TOF time window (1024 x T0).

    Also the TDC1000-TDC7200 EVM is targeted at liquid level, concentration and flow applications utilizing 1-2Mhz transducers. For gas or air applications utilizing 100k-500kHz transducers we recommend the TDC1000-GASEVM as it provides the high voltage excitation required for gas applications.

    If you give me more details about your application and how you want to use the TDC1000 I can give you more guidance on configuring it for your application.
    Best regards,