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VCA2615: VCA2615 HP filtering

Linas Svilainis
Prodigy 40 points
Part Number: VCA2615

VCA2615 description is confusing:

FUNCTIONAL BLOCK DIAGRAM on page 2 indicates the DC presence of DC decoupling capacitor on VCAin. Meanwhile, page 17p advises : "When  applying  an external signal to the MUX (VCAIN), the signal should be biased to a common-mode voltage in the range of 1.85V to 3.15V." From VCA2615EVM User's Guide Figure1.VCA2615EVM Schematic one can guess that VCAin biasing is not required, since only DC - decoupled transformer is used on VCAin.

Another question is what is the input impedance of VCAin. The only explanation for guess is on page 19: "Amplifier A1, along with transistors Q2 and Q3, forms a voltage follower that buffers the +IN signal ...Amplifier A3,...plays the same role as A1....  A1 and its circuitry form a voltage-to-current converter". That is, input impedance of VCAin is high, i.e. not determined.

HP filtering is extremely important in ultrasonic signals acquisition if same transducer is used for transmission and reception. You have envisaged the HP filtering ability, but it seems that cutoff frequency will depend on gain setting. Page 21 explains: "Transistors ... and Q24 form a variable resistor network that is programmed..", "...The value of this variable resistor ranges.. from 15Ω  to  5000Ω...", "...Low Frequency BW=1/2piRC, where R is the value of the attenuator". It turns out that HP cutoff frequency will vary with gain. Please advise what value such HP filter contains?

It is also not clear which R: group Q10-Q24 or Q11-Q25? Is this "R" a series equivalent or just one branch?

We want to implement the HP filtering between LNPout and VCAin., because quality of HP and distance from the input are important. But it remains unclear whether simple LC filters commutation will do: if proper bias (2.5V) can be expected on LNPout, this will vanish at HP output. Please advise.

It is also unclear what should be done if clipping is not required. Please advise.

It is also unclear whether channel B can be disconnected, if only channel A is required.Pin 36, "Channel B + Supply" and 27, "VDD B Channel LNP" are separate. What if power supply is not connected to those inputs? Please advise.

It is also unclear whether LNA input can be driven deferentially, by using both LNP+ and LNP- inputs, e.g. using central tap transformer as VCAin per VCA2615EVM User's Guide Figure1. Will there be any benefits in if LNP- input is also involved? Please advise.

  • 0
    TI__Mastermind 49186 points
    Hello Linas,
    I apologize for the delay in my response.
    Regarding your questions,
    Let me check with the designers on the DC blocking caps at the input to the VCA on page 2 block diagram.
    As you have mentioned correctly, the input impedance to the VGA is really high in the order of MOhms.
    With the external coupling cap of 22nF, the cutoff frequency will be around 1.5KHz - 500KHz.
    Transistors branch Q10-Q11 will form the series equivalent. Similarly Q12-Q13 gets included as the variable gain is changed.
    Yes LC filters at the LMPout to VCAin should suffice to implement the HP filtering.
    If clipping is not required, set VCLMP to 5V.
    The LNA inputs can be driven differentially with the center tap of the transformer connected to the VCM voltage of 2.5V.
  • 0 in reply to Praveen Aroul
    Prodigy 40 points

    Dear Praveen,

    thank you for your response. Still few answers are missing:

    1. What was the idea behind CFilt? How R will change vs. gain setting? Is it series equivalent or each branch?

    2. LNP+ and LNP- inputs - what happens if active impedance matching is used?

    3. LNP+ and LNP- inputs transformer drive: is it enough to use AC coupling of central tap or it HAS to be biased? In normal connection those inputs are just AC coupled, biasing comes from internal circuitry.

    4. LNP+ and LNP- inputs driven by transformer - is there any advantage of doing so? Compared to transformer + output connected to LNP+ but LNP- input AC coupled as per standard circuit.

    5. How to power down channel B?

    Sincerely,

    Linas Svilanis

  • 0 in reply to Linas Svilainis
    TI__Intellectual 2885 points
    Hello Linas,

    I will be closing this thread due to inactivity, if you have further questions feel free to ask a new question. Thanks
  • 0 in reply to Matt Mendez
    Prodigy 40 points
    My question was not fully answered. I added reply, indicating which part of my question was not answered, but so far 1 month I only got suggestion to close the thread.