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ADS6424 or ADS6425 Single-input configuration

moki fonk
Prodigy 220 points
Other Parts Discussed in Thread: ADS6425, ADS900, ADS800, ADS5294, ADS5263

Hi,

I have look at the datasheet of ADS644 and ADS6425 to check if it is possible to used either in the single-input configuration. But did not find any information on single-input configuration. want to used the single-input configuration with an analog input of 1.5Vpp. Need help with a reference circuit for either of these ADC in the single-input configuration.

 

  • 0
    TI__Mastermind 29220 points

    Hi,

    the ADS64xx family requires the analog input to be fully differential and symmetrical about VCM at all times.  It is not acceptable to bring a single-ended signal into one side of the differential input while tying off the other input pin to VCM. 

    (Older devices such as the ADS800 family or ADS900 family would have a typical input circuit in the datasheet for just such a case.  Newer, faster and higher performance ADCs do not allow this.)

    If one side of the input were tied off to VCM while the other input were to swing *around* VCM then *each* leg of the input would of course have a voltage centered about VCM when looked at individually.  But at any instant of time the VCM of the differential signal would be swinging about the desired VCM voltage.   For example, at the peak of the signal swing the IN- might be at 1.5V while the IN+ might be at 2.0V.   At this instant of time the actual VCM of the differential signal would be at 1.75V - not 1.5V.   The ADC wants to see the instantaneous VCM of the differential signal at 1.5V at all times - in other words a symmetrial balanced signal biased to the VCM voltage.

    You would need a conditioning circuit to take the single ended signal anc convert it to fully differential.  That is why we have input transformers on the EVM.  The lab equipment used (sine wave signal generators) are single ended, so the transformer is used to convert to differential while the inherent AC coupling of the transformer allows the signal to then be biased up to the VCM level. 

    If DC coupling is required (for an input signal that includes a DC component) then transformers could not be used for this signal conditioning, and an amplifier circuit would need to be used instead.

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    Prodigy 220 points

    Hi Richard,

    Thanks, I have look at the ADS529X family of ADC especially the ADS5294. My input signal is AC couple and it implied I will be using transformers. I indeed to used two ADC type ADS5294 which will be 16 channels. If the input signal is 2Vpp which transformer will best suit this application? In case you have reference circuit of the transformers and ADC combination it will be useful to have it. 

    Regards,

    Moki

  • 0 in reply to moki fonk
    TI__Mastermind 29220 points

    Hi,

    Your choice of transformer would depend on the range of frequencies you wish to pass through it, and since many of our ADCs have a full-scale input voltage of 2V peak to peak differential, a 1:1 ratio transformer would suit your application i expect.  You wouldn't need to use the transformer to step up or step down the voltage swing.   

    Most of our EVMs for these ADCs will have the schematics available on the TI web for you to look at.  Our older EVMs used to have the schematics and pates list included as figures in the User Guide for the EVM and even graphics of the layers of the circuit board.  But as the EVMs got more complex we began to make a zip file of this information and put this on the web page with the EVM as the physical design package.  You can look at what was done on almost any of our EVM front end circuits.  A Coilcraft WBC1:1 transformer for example is used often for a bandwidth that runs from under 1MHz to over 300MHz, if that covers your signal of interest. 

    The part number that you changed over to looking at is actually out of our Medical and High Reliability group, so if that remains the device you are interested in then I will move this posting over to that forum.  Even if your application is not medical, the high channel count devices are often popular with ultrasound and so the support for that device would come from that forum.

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    Prodigy 220 points

    Hi Richard,

     The ADS5294 is the device that I am interested in, moved the posting over to the forum.

    Regards,

    Moki

  • 0 in reply to moki fonk
    TI__Genius 14540 points

    Hi,

    As Richard mentioned, many of our EVMs use wide-band back to back transformers on the analog inputs.  The ADS5294 EVM uses two ADT4-1WT+ from mini-circuits situated back to back.  The ADS5263 EVM uses two coilcraft devices, WBC1-1.  There are a number of devices to select from depending on your frequency of operation.

    Thanks

    Christian

  • 0 in reply to Christian Yots
    Prodigy 220 points

    I have look at the ADS5294 EVM User Guide, My frequency of operation will be max 100MHz. I need to use two ADS5294 ADC chip because I want to have 16 channels.  I intend to use the onboard COMS Oscillator for the two AD5294 chip, is it’s possible to use it? As for the transformer, since my input signal will have 2Vpp Max and frequency of 50MHz is there need to used two transformers as in the ADS5294?

    Regards,

    Moki