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usage of AMC1100 isolation amplifier

vidyashree nayak
Prodigy 30 points
Other Parts Discussed in Thread: AMC1100, AMC1301, AMC1200

I am using  AMC1100 isolation amplifier.I have few doubts.

As mentioned in datasheet Differential input resistance is 28kOhm , but practicaally when I worked on this , I am observing aprox. 1.35MOhm resistance .So , can you please tell the reason for  this ?

  • 0
    TI__Guru**** 181831 points
    Hi vidyashree,

    Welcome to the E2E Forum! The differential input resistance of the AMC1100 is based on the internal sampling capacitor and the speed of the internal clock that controls the sampling switches. If you refer to Figure 31 on page 13 of the datasheet, you will find an equivalent input circuit and the formula used to calculate the resistance.
  • 0 in reply to Tom Hendrick
    Prodigy 30 points

    Thank you for the reply!! ,
    I went through the datasheet . Can we use AMC1100 in dc input voltage applications? Actually I am using DC input from 0 to 2.5 v, and trying to maintain the differential input to be 250 mV . We designed  input circuitry by taking differential input resistance to be 28kOhm into consideration, so that voltage across equivalent resistance( parallel resistance of 15kOhm and 28kOhm =approx 10kOhm ) maintained at (approx)250 mV . But whole design went wrong since, we are getting differential input resistance as 1.35Mohm. So , what can be the problem we are facing here, does it have something to do with Nominal gain ? 
    Thanks!!

  • 0 in reply to vidyashree nayak
    TI__Guru**** 181831 points
    Hi vidyashree,

    Yes, you can use the AMC1100 to measure voltage, please see Figure 32. With large value shunt resistors, you will have some gain error. If you can provide the circuit you implemented along with the test results we might be able to provide a little more insight for you.

    You might also want to look into the AMC1301 - that device has a different front end. The resistance is not based on switched cap inputs like it is on the AMC1100 and AMC1200.
  • 0 in reply to Tom Hendrick
    Prodigy 30 points

    Hi Tom Hendrick, 

    Thanks for solving doubts.

    I understood gain error concept of AMC1100 . I observed one more statement in datasheet page number 13 which states, "Internal capacitors are continuously charged and discharged with a typical frequency of 10 MHz."  and in page number 6 that, "Small-signal bandwidth 60 kHz -100 kHz " . I wanted to know whether these parameters related  to each other ? If yes I request you to provide more information about this.

    Thanks and Regards,

    Vidyashree

  • 0 in reply to vidyashree nayak
    TI__Guru**** 181831 points
    Hi Vidyashree,

    The small signal bandwidth of the AMC1100 is based on the active low-pass filter on the analog output side of the device, not the switched caps on the input side.