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VCA2615: VCA2615

Viktor Grytsenko
Expert 1095 points
Part Number: VCA2615

Hello all,

I use VCA2615 in ultrasonic application.
On the page 18 of Data Sheet
Table 5. shows codes for Feedback Resistor Settings.

I have come across that setting any code other than
'1111' (Open) causes a significant increase in noise.

For example, for code = '1111', input termination Rl=0, amplifier gain = 52dB
(about 16dB for LNA with a single ended input plus 36dB for VCA)
output noise voltage
Uout,rms=1.2mV

For code = '0000'
Uout,rms=3.59mV
For code = '0001'
Uout,rms=3.57mV
..................
For code = '1101'
Uout,rms=2.44mV
For code = '1110'
Uout,rms=2.14mV


Please answer two questions:
1) Why is this happening?
2) What is FEEDBACK RESISTOR for code '1111' (Open) ?

Thank you,

Best regards,
Viktor.pulsarvictor2003@gmail.com

  • 0
    TI__Genius 12025 points

    Hi,

    Thanks for using VCA2615 device.

    We will look into your question very soon.

    Thank you!

  • 0 in reply to Chen Kung
    TI__Genius 12025 points

    Hi,

    for your questions:
    1) Why is this happening?

    Please refer to the VCA2615 datasheet Figure 61 page 19,

    Let's assume you are using active feedback termination,

     this shows LNP Gain as 20dB (vs. your setting is ~16dB)

    also it shows 8 different kinds of LNP Noise levels,

    you can see the curve (it shows Noise Figure vs. source Impedance = Feedback Resistor values)

    which is very similar to your test Curve results as well (showing in mV).

    2) What is FEEDBACK RESISTOR for code '1111' (Open) ?

    Please also refer to the datasheet page 17,

    it mentions in the open position, the input resistance of the LNP is about 100k ohm.

    Thank you!