All references I find about TUSS4440, including EVM schematics, show the single ended input from the transducer. In a bistatic configuration, what would happen if the RX transducer and INN were not referenced to ground? Would the common voltage exceed the limits of the DC caps? Would there be significant differences in performance? The simple fact we name INN pin as it is indicate that it may or may not be connected to ground - is that a valid assumption?
Thanks!
Hello Lenio,
Thanks for posting to the sensing forum and for such a great question! The differences between single ended and differential outputs on a transducer is not something I had really looked into previously mainly because TUSS4440 is a single ended input device, this is why all the schematic and figures you see are represented in this manner. To answer some of your questions:
1. What would happen if the RX transducer and INN were not referenced to ground?
If the Rx transducer was not referenced to ground or connected differentially the output voltage on the transducer would not look correct so it would not function as intended. I have not tried floating INN on the device before mainly because the C_INN capacitor is used to help limit current and is part of the filter network of the analog front end, for this same reason we do not recommend connecting INN directly to ground. INN should be treated as an auxiliary filter/ground pin, not an input.
2. Would the common voltage exceed the limits of the DC caps?
The voltage created by ultrasonic transducers is very small so in a bistatic configuration there is very little concern for the common voltage to exceeding the limit of the DC caps, this is why so much gain is needed in the AFE. This helps get the return echo into a usable level for processing.
3. Would there be significant differences in performance?
I would not expect there to be any differences in performance if the TUSS device could be used in a differential manner. The reason I say this is because I tested a transducer connected in a single ended format as well as a differentially and the waveforms from the two were identical. To me, this indicates that even though the transducer is connected in a single ended manner to the TUSS device, the result at the front end of the device would be similar if the device had the capabilities of handling a differential input. Please note that internally, the AFE of the device is biased to 0.9V which is done to allow for the positive and negative voltage swing of the transducer.
4. The simple fact we name INN pin as it is indicate that it may or may not be connected to ground - is that a valid assumption?
Confusing, but that is not a valid assumption, perhaps the datasheet could have been more clear if we would have named this pin something else, but as metioned above the INN pins should be connected to ground via the C_INN capacitor.
In case there is any interest in the waveforms, the blue signal is the transducer being driven at 5V and the orange signal is the signal that was received (voltage scales for each to the right). This is connected in a single ended manner but connecting the transducer in a differential form yields the same result.
Let me know if there are any other questions!
Best,
Isaac
