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AD8330 full-scale input spec definition in datasheet

Category: Hardware
Product Number: AD8330 - Variable Gain Amplifier

Hey ADI Team,

I have a question regarding the AD8330 VGA: 

Based on the datasheet page 3, the full-scale input of the input interface is specified as MIN: +/-1.4V, Typ +/-2V. Then on page 21 of the datasheet in the right top corner is a basic connections diagram (Figure 57) which specifies: INPUT 0V to +/-2V MAX.
I would like to understand what the maximum input voltage is and I understand that the maximum value also depends on what the gain (Vdbs) setting is. But the datasheet mentions a "MIN = +/-1.4V", "TYP = +/-2V" and MAX = "+/-2V" on various pages.

Question 1: what is intended with the 'minimum value of +/-1.4V' for the input voltage, when page 21 specifies a MAX value of +/-2V?

Question 2: I'm currently running this device at supply voltage of 5.0V. Do any of the performance parameters change if I would lower this voltage to 3.3V, is there any adverse impact? My goal is to have the maximum performance and gain possible. For example, VMAG is specified from 0V to 5V (based on 5V supply) and VMAG shows a proportional graph between gain multiplication factor and VMAG. If the supply is limited to 3.3V, is my assumption correct that VMAG is limited to 3.3V as well and that the graph in figure 5 will basically be cut-off at 3.3V (or around a multiplication factor of 5.5 to 6)? Is noise performance impacted by changing the supply from 5V down to 3.3V?


Question 3: I selected this device based on the convenient wide gain of 0-50dB which can be 20-70dB by adjusting Vmag, but I'm concerned with the output referred noise performance (fig 19) which shows that output noise increases dramatically when increasing Vmag, up to nearly 5000nV/√Hz. In the addition, the AD8330 datasheet is referring to this device as a "Low Cost VGA". Previously I used the AD8331 which has a gain range that is not as wide and hence I had to cascade two of these amps. The AD8331 datasheet points to much improved output referred noise when compared to the AD8330, which is important for our application since we are driving a 16-bit ADC and need as much resolution as possible. Is there perhaps another option, no matter how expensive, that offers wide gain and low(er) noise than either AD8330 and AD8331?

Application:
We have a photodiode in an optical network with transimpedance amplifier (based on the ADA4817) which is driving the VGA. We need to ability for our VGA to have at least 50-55dB gain with as little noise as possible since we are driving 16 bit ADC's and we have observed significant noise in the digitized signal when increasing gain. 

Thank you so much for your feedback!

Best,

Peter

Top Replies

    •  Analog Employees 
    May 23, 2023 in reply to pjroos +1 verified

    Hi  

    Thank you for your patience in waiting.

    Here are the answers for each question.

    1. The 1.4V value is the (full-scale input) minimum value of the largest signal amplitude that can be applied before…

  • And question 4: I'm driving the AD8330 single ended, so I'm assuming that the max input swing is half that of the differential input swing?

  • Hi  

    Thank you for considering ADI device on your application. We will get back to you as soon as we have confirmed our assumptions by evaluating the device on the evaluation board.

    Can you share your intended input signal values for your application? (Example: input frequency, expected input voltage). That would help us verify the device thoroughly.

    Thanks and regards,

    JE

  • Hi JE,

    Thanks for getting back to me.
    The frequency of the signal provided to the input of the AD8330 will range from ~10KHz up to 37.5MHz and the intended magnitude is as high as the AD8330 allows, single ended, and also down to uV range. Basically we have a photodiode which feeds into a transimpedance amplifier (ADA4817) which translates 0-2.8mA from the photo diode to a single ended signal which should maximize the input range of the AD8330. The output of the AD8330 will be a differential signal of 2Vpp max, going into a digitizer. 

    Hope to hear from you soon.

    Regards,

    Peter

  • Hi JE,

    Is there any feedback on the datasheet spec for the full-scale input interface per question below?

    ------------------------------------
    Based on the datasheet page 3, the full-scale input of the input interface is specified as MIN: +/-1.4V, Typ +/-2V. Then on page 21 of the datasheet in the right top corner is a basic connections diagram (Figure 57) which specifies: INPUT 0V to +/-2V MAX.
    I would like to understand what the maximum input voltage is and I understand that the maximum value also depends on what the gain (Vdbs) setting is. But the datasheet mentions a "MIN = +/-1.4V", "TYP = +/-2V" and MAX = "+/-2V" on various pages.
    ------------------------------------

    Regards,

    Peter

  • Hi  

    Thank you for your patience in waiting.

    Here are the answers for each question.

    1. The 1.4V value is the (full-scale input) minimum value of the largest signal amplitude that can be applied before the signal is clipped. However, this is case to case basis per unit. On my evaluated unit, the maximum value it is clipped is same as the typical value which is 2V. The 0 to 2V data on other pages are the full-scale range.

    2. Yes, you assumption is correct. The multiplication factor is same for both 5V and 3.3V. The range of the multiplication factor is decreased due to decreased supply voltage. The performance of the unit in terms of gain and calculation is same for both 5V and 3.3V.

    3. Upon checking the list of possible parts, the AD8330 has the highest gain with the specification that you've provided. With the issue of the noise, we can improve it by adding a low noise precision buffer at the end of the VGA, if it is fit to your application.

    4. Yes, you are correct.

    Hope this helped. You can contact us again for any further concerns.

    Regards,

    JE