AD7739 AIN Protection

The AD7739 datasheet states that the device can operate with +/- 2.5 volt inputs.

The Absolute Maximum ratings for the AIN pins are listed as -0.3V to AVDD + 0.3V which would appear to contradict the input voltage range.

I wish to use the device with an external instrumentation amplifier operating with supplies of ca +/- 15V and a ouput voltage range of +/- 2V in normal operation.

How should I protect the input of the AD7739 from overvoltage ouput of the instrumentation amplifier.

  • May help to use the additional stage to match the ADC input range and instrumentation amplifier output.

  • 0
    •  Analog Employees 
    on Jan 10, 2013 4:38 AM

    Hi Lyn,

    Adding diodes such as schottky diodes from the analog input pin to AVDD and AGND will protect the AD7739 against overvoltages.

    Regarding the analog input range of the AD7739, the device can accept an analog input of +/-2.5V. The device operates from a single power supply so the analog input must be biased up. For example, if the voltage on AIN+ swings from 1.25V to 3.75V and the voltage on AIN- is 180 degrees out of phase, then the voltage AIN+ - AIN- = 3.75V-1.25V = 2.5V when AIN+=3.75V and AIN-=+1.25V. When AIN+=1.25V and AIN-=3.75V, the voltage AIN+ - AIN- = 1.25V – 3.75V = -2.5V. So, the ADC sees an analog input of +/-2.5V. However,please ensure the voltage on AIN+ and AIN- is between 0.2V and AVDD – 0.3V as per the spec for the absolute voltage on AIN+/AIN- on table 1 in page 3 of the datasheet.

    About the input to the instrumentation amplifier, what magnitude and frequency are you using?

    We have parts like the AD7190 that features an on-chip PGA with gains of up to 128 and an output data rate of up to 4.8kHz. You might find this helpful in your application.

    Hope this helps.

    Thanks and Best Regards,

    Chris

  • 0
    •  Analog Employees 
    on Jan 10, 2013 7:25 AM

    Hi Lyn,

    I forgot to mention in my previous post that although the example given was for differential inputs it is also applicable for single-ended inputs.

    The AD7739 can be operated with single-ended inputs that could accept +/- 2.5V given that it follows the condition given in the previous post.

    Hope this helps.

    Thanks and Best Regards,

    Chris

  • Dear Chris

    Thank you for your reply. When I printed out the data sheet and studied it more carefully, I realized that the analogue input specification of +/- 2.5 V was for a differential input.

    The signals I am dealing with are very low frequency with a sample rate the order of 1 second or more so the dynamic performance is not really an issue. The signals have a range of +/- 2V so using an Instrumentation amplifier with a gain of one, I end up with a signal of +/- 2V from whatever reference I choose. I can obviously divide signal by a factor of two and use an ADC with an input range of 0 - 2.5 V, but this is not attractive as I will require eight precision dividers.

    My conclusion is that I am better off using an ADC with a +/- 2.5 input such as the ADS1258 or with an input range of 0 - 5V with an offset reference.

    Please let me know if you have any better ideas.

    Yours sincerely

    Lyn Roberts

  • 0
    •  Analog Employees 
    on Jan 17, 2013 1:11 AM

    Hi Lyn,

    May I also suggest that you look at the AD7734 and AD7738 parts which may fit your needs.

    The AD7734 will accept bipolar signals using by it's internal biasing network. The AD7738 is similar in functionality to the previously mentioned part and also includes the MUX OUT pins allowing for only a single amplifier to work for all channels (providing it can settle fast enough).

    If you would like to provide some more information on your application we can help you with selecting some products to suit.

    Regards,

    Rob