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THS4511: Schematic review of drive circuit using THS4511

vishak udayakumar
Prodigy 120 points
Part Number: THS4511
Other Parts Discussed in Thread: ADS4222

Hi,

I am using THS4511 to drive ADS4222. My input signal swings from 0.9V to 1.8V. Since the input ranges near DC to 20 MHz, i decided to go for DC coupled operation with gain 0 dB. Also the source can't drive > 1 K ohm.

Considering this I decided to use 1K ohm for Rf and Rg for the amplifier feedback path. Since, the requirement of ADS4222 is that the input signal at input P and N should swing symmetrically around 0.95V common mode, I had to compress the range of input signal to the amplifier using a resistor divider configuration. Also Vcm output of the adc is used as vcm of amplifier.

Split supply of +3V(+Vs) and -2V (-Vs) is used for the operation.

I simulated the amplifer circuit in TINA and it gives me expected results. KIndly review my schematic and point out anything I may have missed regarding the proper operation of the circuit.Amp_Sch.pdf

Thanks and regards,

Vishak

  • 0
    TI__Expert 7875 points
    Hi Vishak,

    One thing I noticed on your circuit is that you will want to balance the input impedances on both inputs to the fully differential amplifier. On the VIN- side you have 1K to ground. Ideally the VIN+ side should also look like 1K to ground. Right now VIN+ will see 1k in series with a parallel 2k and 1k plus source resistance.

    Is your input source low impedance or does it have a fixed output impedance value. With that knowledge, I can suggest a resistor value for R256 that will balance the inputs.

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 120 points

    Hi Jacob,

        My source is log out of HMC948LP3E logarithmic detector.  hmc948.pdf. From the datasheet of the part, it seems the detector output is being driven by a buffer. This means the source impedance would be low.

    Regards,

    Vishak

  • 0 in reply to vishak udayakumar
    TI__Expert 7875 points
    Hi Vishak,

    In that case I would change R256 so that it is roughly 667 ohms to balance the input impedances. In the current configuration you are likely getting a differential output offset. I would also re-check your signal gain after the changes to make sure you still have the desired gain.

    Regards,