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Noise in VCA2615

Avishai Henley
Prodigy 60 points
Other Parts Discussed in Thread: VCA2615

Hi,

I am using the VCA2615 as an Rx amplifier in my system. The input signal level is few microvolts and it is a differential signal.

Apparently, is seems as if the noise level (even in the EVB) is around few 00s of microvolts. This makse it difficult for us to interpret the signal we are looking for.

Signal frequency: ~1MHz

In order to amplify the signal, the following path is used:

input_signal --> filter--> LNA_A --> VCA_A --> filter --> LNA_B --> VCA_B --> ADC

The filter is a single pole LPF, set at around 1MHz.

Please advise on the following:

1. What is the CMRR and PSRR of the VCA2615?

2. Is there a problem with cascading the 2 channels of the VCA2615?

3. Is there an amplifier that is designed for 10uV input signal level, which will fit my design more than the VCA2615?

Thanks,

 Avishai

  • 0
    TI__Guru* 93105 points

    Avishai,

    The VCA2615 is handled by TI's Medical/ Hi-Rel group. They have an E2E forum and should be able to answer your specific VCA2615 questions.

    I am not very familiar with the VCA2615, but when I look over the section it speaks about the low-noise preamplifier (LNP) and indicates it is a very low noise, wide bandwidth preamplifier. The noise performance listed in Table 1 shows the LNP noise to be a very low, 0.8 NV/rt Hz (G = 20 dB). That might be improved upon with a low-noise, wide bandwidth operational amplifier, but all the versatility of the PGA is then lost. Chances are building up the discrete function would not necessarily result in improved noise performance and noise rejection when compared to the VCA.

    It is easy to introduce noise into a system that rivals the very low signal levels such as those at microvolt levels. Very conscientious efforts have to made regarding layout, shielding, power supply decoupling, etc. Much care and forethought must be exercised in order to keep introduced from corrupting the very low level signal being measured.

    Do repost your inquiry with the VCA2615 product group via their E2E forum. They should be able to assist you in applying the amplifier for best performance.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Mastermind 18285 points

    Thanks Thomas for jumping in here.

    Avishai,

    My name is Amy and I am with the Medical Applications Team.

    1. Based on our understanding, our customers do not require a PSRR or CMRR measurement for ultrasound, which is why it is not in our datasheet.

    2. and 3.  Cascading the 2 channels should not be a problem and should work with your design.

  • 0 in reply to Amy Schnoor
    Prodigy 60 points

    Hi, Amy,

    Thanks for your fast response

    Please note my original email where I mensioned that the problem is that the circuit I use (actually your EVB with some patches) seems to have internal noise in the magnitude of 00s uV where I need to measure signal as high as ~8uV.

    I woul like to understand if I have to ass a pre amp or use a different VGA in case you say that cascading the channels is a working soolution.

    Regards,

     Avishai

  • 0 in reply to Avishai Henley
    TI__Mastermind 18285 points

    Our VCA2615 can be used as a preamp. Your 8uV should be 0.8nV*sqrt(BW) right?

  • 0 in reply to Amy Schnoor
    Prodigy 60 points

    Hi,

    My signal is ~10-20 uVp-p in 1MHz.

    Regards,

    Avishai

  • 0 in reply to Avishai Henley
    TI__Mastermind 18285 points

    What patches have you put on the board?