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Inputs to ADS6445EVM and Altera FPGA interfacing

Steven Schultz1
Prodigy 20 points

Other Parts Discussed in Thread: THS4509, ADS6445, ADS4145, ADS6145

Hello,

I'm looking for an ADC that will pair with my Altera dev board through the HSMC port. I'm considering the ADS6445EVM or any other EVM as long as the ADC has a 14 bit resolution, 500k sps (all of the EVM ADCs are well over this rate), and more than 2 channels. My main concern is the input to the ADS6445EVM. I know the ADS6445EVM has a wideband differential op-amp (THS4509) that would allow a wider range of signals to be used with the ADC. What I want to input is a (+/-)3V analog differential signal (a <100 Hz sine wave). After looking through the documentation for the THS4509, I'm still unsure if this would be suitable. 

I'd appreciate some ideas/feedback on if using the THS4509 chip is suitable for the signal I need to convert or alternate routes besides the EVM ADCs.

Thanks!

  • 0
    TI__Mastermind 29220 points

    Hi,

    You will need some kind of signal conditioning circuit to shape your signal to the requirements for the data converter input.  The majority of our  customer applications do not include an input bandwidth down to DC or 0Hz, so the default configuration of our EVM is usually transformer coupled.  If the input circuit must include a bandwidth down to DC, then a transformer is not an option and an amplifier circuit is the usual choice.    in your case, a signal of < 100Hz is pretty much a DC signal compared to the bandwidth of the usual transformer coupled input. 

    The THS4509 that is included on the ADS6445 EVM (on one of the four channels, not on each of the four channels) would be as good a choice as any as far as i know.  Note that the default circuit around that amp on the EVM is still AC coupled with series AC coupling caps, so you would probably still have to do some soldering to the EVM to set the amp up for the way you want it.  There is a different E2E forum here for the high speed amplifier group that can help with questions about the design of the amp circuit. 

    The goal of the signal conditioning you need to do is to scale your signal to be less than the data converter's full scale range of 2V peak to peak differential, and to have the level of the signal shifted to the desired common mode voltage of 1.5V.  If a signal is AC coupled, then the shifting to the VCM voltage is easy with just connecting the VCM voltage to the common point of the termination resistors or to the center tap of the transformer coupling.  But if DC coupled and using an amplifier, the level shifting can be done with a common mode input to the amp if the amp has such an input - which the THS4509 does.

    in your case, your signal is larger than the data converter full scale range, so you need attenuation instead of amplification.  Attenuating the signal is easy enough with a resistor divider, but the level shifting is still an issue so I think the amp path on the EVM is still the easiest option with some soldering of components around the amp circuit on the EVM.

    If you want to try the high speed amplifier forum for specifics on the THS4509 or advice on a potentially better option, I could move this post over to their forum or you could repost there. 

    For the choice of data converter EVM, do you want a device with a serialized LVDS output on one or two LVDS pairs like the ADS6445 EVM, or would you find it easier to create your FPGA firmware around a simpler parallel bus of dual-data-rate LVDS pairs such as the ADS6145 or ADS4145 EVM?  The ADS4145 EVM also has the THS4509 amp.

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    Prodigy 20 points

    Thanks so much for the response, Richard. I'm assuming there is no way to get 1 THS4509 per input channel on the EVM boards? I absolutely need at least 2 channels.

    As for the LVDS, I'd go with parallel bus of DDR LVDS pairs. If you don't mind, could you move this thread to a forum for a better option of doing this, since I now better understand the limitations of the EVM boards and theTHS4509.

    -Steven