Shielding of AD8237 input leads

Hi,

Please help me correctly shield the + and - input leads of a system on AD8237 close to that on Figure 77, p.26:

http://www.analog.com/static/imported-files/data_sheets/AD8237.pdf

I like to use something like on the Fig. 22, p 6 of:

http://www.analog.com/static/imported-files/application_notes/41727248AN_347.pdf

to eliminate the lead cable leakage resistance.

Thanks,

Michael.

  • 0
    •  Analog Employees 
    on Dec 11, 2014 1:54 AM

    Hi Michael,

    I'm sorry that I missed your question. Unfortunately, because AD8237 measures differential signals, it's not quite as easy to shield the inputs as the op amp circuits in the application note you linked. Stepping back for a second, you could say that there are three types of shielding for cables:

    1) a simple foil (or braid) around the cable that is not connected to anything

    2) a shield that is connected to chassis ground

    3) or a shield that is driven to a specific voltage, often referred to as guard.

    To prevent leakage, you would want the third option, a guard driven to a voltage that tracks the input signal, which bootstraps the leakage resistance. There are two ways to do this for differential signals; please see figures 43 and 44 on AD620’s data sheet

    http://www.analog.com/static/imported-files/data_sheets/AD620.pdf

    Figure 43 is probably the best way to do it, because figure 44 only tracks the average voltage between the two inputs (or rather, the common-mode voltage), therefore there is a compromise between cost (of the circuit and the cable itself) and performance. At high gains when the differential voltage is small, figure 44 can be chosen. The problem, however, is that the AD8237 doesn't have Rg pins to sense the voltage like the AD620 does, so the voltage must be sensed at the inputs with a high input-impedance buffer instead of the method shown in the AD620 datasheet.

    One way to do it is to add a low power RRIO op amp such as the ADA4505-2 configured as a voltage follower from each input to the corresponding cable shield with a small resistor in series with the output for stability. Depending what was your reason for choosing the AD8237, another option would be to use a traditional three-OpAmp inamp that has Rg pins similar to the AD620, like the AD8220, AD8226, or AD8422. For the latter case, you can try our In-Amp Common-Mode Range web tool prototype (linked here) to narrow down your selection to products that fit your configuration and signal range.

    I hope this helps.

    Best regards,

    Scott

  • 0
    •  Analog Employees 
    on Aug 2, 2018 2:46 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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