AD8639 as AC Preamplifier for Thermopile?

Hi All,

Please refer to Datasheet: " 16 V Auto-Zero, Rail-to-Rail Output Operational Amplifiers AD8638/AD8639."

Fig 52 on page 15 of the datasheet is AC amplifier using AD8639.

As I know Thermopiles output DC in micro volts. How does the AC Amplifier circuit in fig 52 apply?

May be I need to understand something not obvious to me. Welcome detail elaboration.



Subject made a Question
[edited by: laxtronics@ at 4:32 AM (GMT 0) on 1 Jun 2020]
  • 0
    •  Analog Employees 
    on Jun 4, 2020 1:51 PM

    Hi Laxman,

    If you take a closer look, the first amplifier on that configuration has a gain of 1000. This would greatly amplify any DC errors on the inputs of the amplifier, including the low offset voltage of the AD8638 from 3uV to almost 3mV. The 10uF capacitor would filter out these DC errors.

    The next thing to look out for is the bias setting resistor. Since in the first amplifier stage, the inputs are amplified at a gain of 1000, it is only apparent that we adjust the common mode voltage so that the input voltage and output voltage range of the AD8638 is not violated.



  • Hi Donnie and Laxman,

    On the one hand, the question was that the sensor generates a constant voltage while there is a capacitor between the stages of the amplifier. Why does this work? You didn't give an explanation for this

    On the other hand, the description clearly states that this amplifier works with alternating voltage, therefore it must work with sensors-generators of alternating signal



  • Hi Donnie and Kirill

    Your answer and explanation has helped in provoking thoughts for reading the mind of the publisher of this circuit application.

    What enters my mind is:

    The Title of the paragraph is “INFRARED  SENSORS.” There is mention of Thermopile as sensor.

    Thermopile output is very low level DC in range of few micro volts when infrared captured from the object (over Thermopile); particularly in application of measuring human body temperature (Non Contact /contact less). The temperature range of interest is within 30 to 40 Deg Celsius and measurement duration lasts for few seconds.

    I think the circuit i.e. AC amplifier as presented helps to pass on this change to the following circuit while it blocks the interference of amplified DC offset of the first amplifier and also its very low frequency noise. Yeah! the following circuit has to be intelligent to capture this change and measure/display the temperature.

    Since detailed explanation of this circuit, from application point of view, is not there (nor I could find it elsewhere,) I am inclined to think/interpreting the circuit this way.

    Article in detail from application expert designer should be welcome.

    Thanks and look forward for interesting electronics circuit blogs.