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Original question:

ADS8920BEVM-PDK: ADS8920BEVM-PDK: single ended to differential input for bipolar signals

ADS8920BEVM-PDK: Building a system for ultrasonic measurement using ultrasonic signals

Dejana Vukovic
Intellectual 845 points
Part Number: ADS8920BEVM-PDK
Other Parts Discussed in Thread: TMDSEVM6657, OPA625

Hello everyone, 

I have already wrote on the forum several times about several things regarding my project. However, I would like to get a general opinion wheather or not I am reasoning several things correctly. I am working on my master thesis and I would really appreciate the help.

I am using the following equipment:

TMDSEVM6657

DAC80504EVM

ADS8920BEVM-PDK

Benthowave Hydrophone with following datasheet:

BII-7186Datasheet SO#1109764.pdf

And three 300 kHz ultrasonic transducers:

https://uk.farnell.com/multicomp/mcusd13a300b09rs/transceiver-300khz-13mm-metal/dp/2362690 .

I would like to use three ultrasonic sensors, which should transmit three different signals. Those signals should be received with hydrophone. The signals shall be modulated chirps of 300 kHz. I would like to use the DSP to generate the signals and then to process the received signals with matched filtering in order to compute distances between the ultrasonic signals and hydrophone.

DAC in matter gives output unipolar signals and my hydrophone has built in 30kHz HP filter, therefore I would need to have something to convert unipolar to bipolar signals. I have been advised on the e2e how to design that circuit. Therefore, if output of DAC is 0-2.5V, then the output of the U2B board will be -10/10V. I have designed the circuit using this User Guide https://www.ti.com/tool/TIPD125.

The hydrophone output voltage is 5Vpp single ended. And the ADC has differential inputs. Therefore a circuit for single ended to differential conversion is needed. I also got a suggestion for it from e2e. Using this http://e2e.ti.com/support/data-converters/f/73/p/895475/3310907#3310907 . circuit I would convert bipolar single ended inputs to unipolar differential output. But the output of the ADC would be then again bipolar.

Therefore if I transmit bipolar signals(by sending unipolar signals to DAC, then converting them to bipolar using custom PCB and then driving the sensors with these signals), my hydrophone will have the output voltage of -2.5/2.5V, and the single ended to differential board will give on the output unipolar signals with amplitude 0/5V and -5/0V. The converted signal  that I would have as input of DSP will then have the amplitude -5/5V.

Here is a small image of the explained system:

 Did I understand all of this well?

My initial idea was to have three LUT in the DSP for the signals that I want to transmit. I use those signals to send them on DAC outputs, but also to match them with received signal using matched filtering. However, if I need to send unipolar signal to DAC and if received signal is bipolar, then that would mean that I should have 6 signals stored as LUT. Unipolar and bipolar version.  Do you think that this is correct?

Do you know how can I check if the c6657 will have enough memory space for 6 signals of duration 5ms sampled with 1MHz?

Also, I think that DAC in matter cannot really generate three different signals at the same time. It can update the values of outputs simultaneously via SPI, but with the same value. Therefore, in order to generate three different signals on three outputs I should have some time delay between them?

Please let me know what you think. I am working on all of this without any menthoral support and I having lot of problems with this. E2E is basically my only source help.

Kind regards,

Dejana

  • 0
    TI__Mastermind 47930 points

    Hi Dejana,

    As you have described, your hydrophone will produce a 5Vpp signal at full scale.  However, I looked at the datasheet and it appears that you can power this from a single 9V DC source.  In this case, the output will have a common mode DC voltage, probably around 1/2 supply voltage, or 4.5V.  For a full scale signal, you would then have a unipolar output from 2V to 7V, or 5Vpp plus the 4.5V common voltage.  In any case, depending on how the front end signal conditioning circuitry is scaled, this will result in a signed binary code, negative full scale corresponding to 2V output from the hydrophone, positive full scale corresponding to 7V, and a zero code for 4.5V.

    If you can confirm with the hydrophone manufacturer on the exact output voltage produced when operating from a single 9V supply, I can modify the single ended to differential circuit to accommodate this.

    You can always convert the signed binary code out of the ADC to a unipolar code, i.e., 0 to 65535 decimal for negative full scale to positive full scale simply by adding 32768 to the result.

    Regarding the capabilities of the DSP, I suggest posting these questions separately on the DSP forum.  The applications engineers that support those devices will be able to give the most accurate information.

    Regards,
    Keith Nicholas
    Texas Instruments
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Intellectual 845 points

    Hi Keith,

    Thanks for your answer.

    I will check with hydrophone manufacturer about the output voltage and get back to you.

    The built-in preamp is BII- 1092 and it has 30 kHz highpass filter. -  :  https://www.benthowave.com/products/Specs/BII-1090Datasheet.pdf

    Also, I have been looking into the schematics of ADS8920BEVM and I think that in order to expand the signal path bandwidth, I would need to adjust the parameters of active low pass filters, so that I can send 300 kHz signals. Currently, the bandwidth is 160 kHz (p6 of User Guide). Maybe just changing the capacitors at the amplifier OPA625 feedback to pF range. I already have 18 pF capacitors of corresponding size.

    Kind regards

    Dejana

  • 0 in reply to Dejana Vukovic
    Intellectual 845 points

    Edit: 

    I checked with hydrophone manufacturer about the output voltage, but they were very crude about it. 

    Maximum Output Vo max: (Supply Voltage – 3.4) Vpp

    The output is AC coupled. There is no DC in output waveform.

     

    Cable:

    Max resistance at 20 °C: 223 Ω /km.

    Since they state that the ouput is AC coupled than I don't need to worry about the external source impedance Rs and its matching to the resistance R1 (p7 User guide 2.2.2 Input Common-Mode Jumper Configuration)?

  • 0 in reply to Dejana Vukovic
    TI__Mastermind 47930 points

    Hi Dejana,

    You can reduce the feedback capacitors, C31 and C42, from 1000pF to 470pF to double your bandwidth.

    Regards,
    Keith