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ADS4245: ADS 4245 with 1:4 transformer

Yinon Kares1
Prodigy 40 points
Part Number: ADS4245

Hi,

I work with ADS4245 and want to use 1:4 transformer between my receiver and the ADC.

that for acheiving a voltage gain.

there is a problem to use that ratio (insteed of 1:1) ?

i see some comment in the data sheet but i dont sure about their meaning/impact on ADC performance.

best regards,

Yinon

  • 0
    TI__Guru* 91947 points

    Yinon,

    You can use a 1:4 part but make sure the board impedance is correct on both sides of the transformer. Also make sure the analog inputs do not exceed the limits of the data sheet. You may lose some performance as you will not be able to use two transformers back to back and achieve the gain you need. Per the data sheet, "Connecting two identical RF transformers back-to-back helps minimize this mismatch; good performance is obtained for high-frequency input signals".

    Regards,

    Jim

  • 0 in reply to jim s
    TI__Guru* 91947 points

    Also per the data sheet:

    With a 1:4 transformer, the source impedance is 200 Ω. The higher source impedance is unable to

    absorb the sampling glitches effectively and can lead to degradation in performance (compared to using 1:1

    transformers).

    Regards,

    Jim

  • 0 in reply to jim s
    Prodigy 40 points

    Hi jim,

    I don’t sure about the meaning of glitch in that context, and how it depend on source impedance.

    Can you enlarge about that ?

    Thanks,

    Yinon

  • 0 in reply to Yinon Kares1
    TI__Guru* 91947 points

    Yinon,

    There are internal sampling switches used on the analog input. When the switch is activated a small amount of energy will feedback to the input pins. This is the glitch. Series resistors are used to dampen this energy.

    If you use a 4:1 transformer, one would normally use a 50 Ohm single-ended trace to the input of the transformer and use 200 Ohm differential output traces from the transformer. This way the impedance is matched from end to end of the circuit.

    Regards,

    Jim 

  • 0 in reply to jim s
    Prodigy 40 points

    Hi,

    so, why in 200 ohm case there is a problem to absorb that glitch ?

    "The higher source impedance is unable to

    absorb the sampling glitches effectively and can lead to degradation in performance"

     

    thanks,

    Yinon

  • 0 in reply to Yinon Kares1
    TI__Guru* 91947 points

    Yinon,

    I am checking with design regarding this.

    Regards,

    Jim

  • 0 in reply to jim s
    TI__Guru* 91947 points

    Yinon,

    This is what I received:

    "50ohms is empirically found (by experiments)- it seems that glitch is better absorbed by low termination resistance compared to high".

     

    Regards,

     

    Jim