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Sigma Delta Modulator output to USB interfacing for reading the output as digital values or as an analog signal

nischal venugopal
Intellectual 360 points
Other Parts Discussed in Thread: ADS1205, AMC1210, AMC1210MB-EVM

hi all,

i am currently working with a sigma delta converter and i am in need of your help. the sigma delta converter is used for current sensing of a motor phase. The sigma delta converter's output is quite unclear to me. normally, it would be a 1-bit data stream. however, the datasheet mentions that the output from the modulator is unsigned 15 bits. I am using a HCPL-786J Sigma delta modulator. I found a way around this by choosing a HCPL-872; which, readily converts the output of 786j's output to 3-wire SPI output, ready to be interfaced with a microcontroller. MY question to you guys is..... Is there a way to directly interface this spi data to USB so that i can display the value of the measured current?? if so, how do i go about breaking down the data and (or) displayying it??? or you guys can give me a completely different approach without the HCPL-0872 chip.

What i know about the conversion from the Sigma delta modulator:

The current sensing from the modulator gives out 111111111111111 if the voltage across the shunt resistor is +320mV and 000000000000000 if -320mV where the first bit the sign bit.

P.S. I was also wondering if i could use TI's ZNP mini kit to read the output from the HCPL-0872 chip. Since, i am familiar with this development kit..... it would be great to know if i could just use the SPI to USB function of the kit and use a serial oscilloscope to represent the signal in analog domain.

Any help is appreciated.

Thanking you,

Nischal

  • 0
    TI__Guru* 92715 points
    Nischal,


    I'm not really familiar with Avago's line, but it looks like the HCPL-786J is just the modulator. As you mention, it puts out a one bit data stream with a 1's density proportional to the analog input that it's measuring.

    The HCPL-0872 is basically a digital filter, it periodically accumulates the output of the data stream. This accumulation makes the two devices combined an ADC, instead of just a simple modulator. That's why the data output comes out as a 15 bit output word, instead of a bitstream.

    I wrote a basic outline of how this works in two blogs last year. If you're interested, you can read about them here:

    e2e.ti.com/.../delta-sigma-adc-basics-understanding-the-delta-sigma-modulator

    e2e.ti.com/.../delta-sigma-adc-basics-how-the-digital-filter-works

    When it comes to your output, are you looking to record just the 1-bit data stream? I'm not sure what's out there to do that. I would note that we do have some similar evaluation modules. There is the AMC1210MB-EVM, which combines an ADS1205 with an AMC1210. The evaluation module has a USB connection and software with a GUI. However, I'm not sure it does what you want it to, and it was designed on Windows XP. I think that some have gotten it to work on Windows 7, but you'd have to search the Precision Data Converters Form for information about it.

    If you have questions about the ADS1205 or similar devices, I'd go ahead and post the question in the Precision Data Converter Forum here:

    e2e.ti.com/.../

    Also, I'm not sure about using the ZNP mini kit to extract the data. I've never used the kit, and I'm certainly not an expert in that plaform. If you want to learn more about that, I would post a question on the Zigbee forum here:

    e2e.ti.com/.../


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 360 points

    Hi Joseph,

    Thank you for your reply. I have a good understanding of the module as to how it works, however i am stuck for an idea as to how do i go about for representing the output of 786j. to answer your quesion, the output of the 786j- the sigma delta modulator, is in 1-bit stream. i would just like to represent them as decimal values on a display. the display can either be LCD or the PC. i chose the latter, and hence the HCPL-0872 chip so that i get a spi interface to the data at the 786j's end. with this spi interface i thought i could use a microcontroller and transmit it to the PC's Serial port, where i could log the data or process them.

    Now to put it simply, i would like to ask you, if you could suggest me a way to represent these 1 bit stream as  decimal values. Is the digital filter necessary for doing this? or can i directly use this 1-bit stream and convert it into decimal format for representation ?

    By the way, i went through your article. It is a well written article. :)

    Thanking you,

    Nischal

  • 0 in reply to nischal venugopal
    TI__Guru* 92715 points
    Nischal,


    I think that you'll either have to use the HCPL-0872 or some other digital filter to get a representation of the output. Note that the digital filter could be as simple as a counter that is periodically reset. As long as you have a repeated accumulation, it should work.

    Unfortunately, I don't have a good solution to display the output. I'm sure there are plenty of good options, but I'm more familiar with the workings of data converters than with the interface.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 360 points
    Joseph,

    Thank you. I found a way to get the data from the output of HCPL-0872. I am going to use a SPI-USB interface from Microchip. The Module from Microchip is called MCP2210 Breakout Module. They even have a windows application for reading the data and representing them in Hex format or Binary format, also, it supports data logging. I guess I'll just use the data logging to get the data and process them according to my needs.


    Nischal
  • 0 in reply to nischal venugopal
    Intellectual 360 points

    Hello Joseph,

    I have finally got my board and also the HCPL-0872 chip. I am yet again stuck with a problem. i am using the MCP2210 SPI-USB Bridge for reading the data from the HCPL-0872 chip using SPI interface. I am not able to get any data out of the SDAT line form the HCPL-0872 even though the CLK and CS signals are there. The conversion should happen after the CS goes low. However, after the CS goes low, there is a clock signal and the SDAT goes high, but no data comes out. when i rig up my oscilloscope to check the CLK signal and the SDAT signal, the SDAT signal is jsut "not there". Any insights on this problem?? Please, find attached my screenshots form the oscilloscope. HCPL-0872.pdf

    Thanking you,

    Nischal Venugopal

  • 0 in reply to nischal venugopal
    TI__Guru* 92715 points
    Nischal,


    I'm not sure what the issue could be. Again, I'm not familiar enough with the HCPL-0872 or the MCP2210 to offer much help. However, when I've had problems communicating with a device, the oscilloscope is a very useful tool (as you've used). Even better, if you have a logic analyzer, that helps to capture more of the communication.

    I'd look for some sort of setup issue in the HCPL-0872 to be the issue.

    If this turns out to be a communication issue, last year, I wrote a blog about debugging digital communication. Look through this blog for some ideas.

    e2e.ti.com/.../help-i-can-t-talk-to-my-data-converter-what-s-wrong


    Joseph Wu