AD8310;low frequency input


In the low frequency application of AD8310, the data table says that a simple RC network should be added at the input end to form a low pass filter. I have the following two questions:

1. What is the low frequency here?

2. How to add the RC network to form a low pass filter? Can you give a brief diagram?

Thank you for your time!

  • 0
    •  Analog Employees 
    on Dec 19, 2020 2:28 AM

    Hi again,

    Log RF amps are often very broadband devices, in this case 440 MHz. For very low frequency applications, its often best to kill the sensitivity up at the high undesired frequencies where there is no information, only noise. Hence the need for a low-pass filter.

    See picture below for one example of how to do so. The corner frequency will be approx. 1/(2*pi*(Rs*2)*Cp. Choose the corner frequency perhaps 4x the highest input frequency, and choose Rs typically ~100 Ohms, then solve for Cp.

  • Hi bruce,

    May I ask you a few more questions?1. Why do signal inputs with different frequencies but the same amplitude produce output of different sizes?I want the same circuit with the same output, even at different frequencies.2. I want to apply in the frequency range of tens to thousands of Hertz, but my circuit diagram can only work above a few KHz. What should I do?The following three figures show the output at different frequencies (output refers to the output after passing through the low-pass filter):

    Thank you for your time!

  • +1
    •  Analog Employees 
    on Dec 23, 2020 3:07 AM in reply to rookie1

    Hi rookie1,

    For AD8310 low frequency use, there are 3 places that need bigger capacitors:

    1) input coupling capacitors.

    2) DC offset cancellation capacitor.

    3) output filter capacitor.

    For 1), the schematics above already shows large capacitors here.

    For 2), a capacitor on OFLT pin is needed. See datasheet page 18, "LOWERING THE HIGH-PASS CORNER FREQUENCY OF THE OFFSET COMPENSATION LOOP".

    For 3), an external capacitor on BFIN will help. This can be in addition to the external RC filter on the output of the detector, shown on the LTspice schematic above. See datasheet page 18, "OUTPUT FILTERING" for help calculating the capacitor value based on the minimum input frequency.

    Larger capacitors means longer detector settling time, so be sure to adjust the LTspice transient analysis time accordingly.     -Bruce

  • Hi bruce,

    I used AD8310 as a detector, and converted the detected voltage to the desired voltage range through the invert phase scale converter circuit. However, when I connected the output of the detector to the invert phase scale converter circuit, undesirable results would appear (As shown in Figure 1 and 2).When inserting the voltage follower between them, I can get the result I want (As shown in Figure 3 and 4). However, after the simulation, I find that the output of the invert scale converter circuit is different from what I want. What should I do?The next level is AD8338, and the circuit is shown in Figure 5.

    FIG. 1

    FIG. 2

    FIG. 3FIG. 4

    FIG. 5

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