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.