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ADS4245: interfacing with 1:4 input transformer at low frequency

Oleg Aryutkin
Prodigy 10 points
Part Number: ADS4245

Hello to forum members and Texas Instruments staff.

I want to use the ADS4245 in application where my sampling frequency is 36 Mhz and my signal is baseband, 30 Mhz bandwidth. (I am sampling "I" and "Q" signals and reconstructing them digitally, hence the under-Nyquist sampling frequency). 

I need to have 50 ohm, isolated input, and want to have voltage gain in my input stage - using 1:4 voltage transformer, so I will have 1:16 impedance transform, and for 50 ohm input I need to terminate the ADC with ~800 ohm "source resistance". 

The datasheet says that by using the "high source resistance' the circuit will suffer from inability to smooth out glitches caused by sample-and-hold capacitor switching inside the ADC chip, and I will have "degraded performance".

Quote from the datasheet: "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)."

My question is - is this relevant for my relatively low sampling frequency? Maybe it's even more relevant since my the sample-and-hold switching frequency is close to my signal bandwidth?

Best regards,

Oleg. 

  • 0
    TI__Guru* 91947 points

    Oleg,

    If you plan on using 1:4 transformer, where is the 1:16 impedance coming into play?

    Regards,

    Jim

  • 0 in reply to jim s
    TI__Guru* 91947 points

    Oleg,

    1. When splitting termination to two 400 Ohm resistors connected from VCM to INP and INM,  there will be a common mode voltage drop across the resistors. This can be calculated using the ‘analog common mode current’ number given in the electrical spec table of datasheet.

       1.5uA/MSPS*400*36=21.6mV. Since the datasheet recommends a drop within 50mV from VCM, this looks fine.

    2. We know from experiments that sampling glitch is better absorbed by smaller termination resistors. But that may not be true for all sampling rates i.e. there can be a sampling rate for which you can use higher termination resistance and still get good performance. This is something that will need to be tested.

    Regards,

    Jim