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TDC1000: Noise on Received Echo Signal

kerem yasar
Intellectual 560 points
Part Number: TDC1000

Hi everybody,

I have my custom measurement board utilizing TDC1000. I am making a gas flow measurement. On the COMPIN pin, I am observing the following signal ;


I have measured the frequency of this noise to be around 5Mhz. I have worked to detect the source of this noise and it is not coming from any exterior region on the board. I believe that this noise is originating from TDC itself. I have probed the LNAOUT and PGAIN pin of TDC and observed the same noise with more huge amplitude on these pins. This noise is reduced to around 50mV after it passed bandpass filter between PGAOUT and COMPIN.

I am using a booster board to drive the transducer with 30V. I have connected the same booster board to TDC1000-GASEVM and observed the signal. On GASEVM I do not observe this 5Mhz noise ;

Please do not get wrong with the voltage division on scope screen as it is500mV here and was 100mV in the previous picture.. I have also observed the signal on GASEVM  with 100mV resolution also and there was no 5Mhz noise..

Here is the circuit schematic of my filters;

It is not seen here but I have 200ohm series resistance on TX lines and 1200 ohms series with RX lines.

I do not think that there is a problem with my booster side. If it were so, I would observe the same 5Mhz noise when I connected my booster to GASEVM... The signal on GASEVM is pretty clear.

Can anybody please tell me why I am observing this noise.. How can I eliminate it ?

Any help/comment is highly appreciated.. Many thanks

  • 0
    Intellectual 560 points

    I think I have found the reason. I am using 2.7V regulator to power TDC1000. TDC1000 datasheet states that at least 2.7V should be applied to VCC pins and thats why I was using 2.7V regulator for battery issues. Yet, 2.7V regultor is outputting something around 2.65V .. The reason was a low supply issue. It would still be good to know why LNA outputs 5Mhz noise when supply is slightly lower(5mV) than recommended operating conditions ?