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SM73201: AMC1311

Waqar Ahmed1
Prodigy 70 points
Part Number: SM73201
Other Parts Discussed in Thread: AMC1311,

Hi,

I am using AMC 1311 and read the shunt value the output of AMC1311 connected directly to SM73201 ADC without Anti aliasing filter. I am trying to read nA with 2.048 reference voltage. I read the value but every time value has been changed with few nA. The are not stable could you please guide me how my value is stable. I am running my Arduino on 1Mz.

  • 0
    TI__Mastermind 46760 points

    Hello Waqar,

    Can you provide a schematic showing the AMC1311 and SM73201?  What is your shunt resistance value?  Do you have any gain between AMC1311 and the SM73201?

    Also, the SM73201 will require an input filter (Figure 34 in the datasheet).  The purpose of this filter is not for anti-alias, but to provide a charge reservoir for the ADC input sampling capacitor.

    Also, you mention 1MHz, is this your SCLK frequency?

    Regards,
    Keith Nicholas
    Precision ADC Applications 

  • 0 in reply to Keith Nicholas
    Prodigy 70 points

    The shunt. value is 2 k and Max current I read 1mA. In uA rang I read fine but when I switch to nA  It flucation and i read different. Value every time. I am not using any filter directly connected with ADC. The gain is 1 yes 1Mhz is Sclk.

  • 0 in reply to Waqar Ahmed1
    TI__Mastermind 46760 points

    Hello Waqar,

    Using a 2kohm shunt resistor with the AMC1311, you will be limited to about 74nArms of noise at 10kHz bandwidth.

    You can add additional filtering, either a passive analog filter, or some type of digital filter (average several readings), to further reduce the bandwidth and the noise.

    Also, I suggest adding a 49.9ohm resistor and 1nF capacitor between each output of the AMC1311 and input of the SM73201, similar to Figure 29 or Figure 30 in the datasheet.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 70 points

    Hi Keith,

    Thanks, I already add digital filter, but still some fluctuation observe on serial port. I will try to add Low pass filter as you suggested.

    regard

    waqar

  • 0 in reply to Waqar Ahmed1
    Prodigy 70 points

    Hi Keith,

    Before adding this filter I try digital filter and I found some smoothness. Actually Why I avoid the hardware filter, I am using fast switch 1st measure the pedestal and then actual reading and then subtract. If I add I afraid its slow and I miss the value. Need your suggestion and frequency on same components.

  • 0 in reply to Waqar Ahmed1
    TI__Mastermind 46760 points

    Hello Waqar,

    To get lowest noise performance, you will need an RC filter between the AMC1311 output and the SM73201 input.  In order to support the max sampling rate of 250ksps, insert a filter as shown in Figure 29 of the datasheet.  A starting point for the resistors will be R=25ohm and C=1nF.  This filter will fully settle when operating at 250ksps.

    These values may need adjusted depending on the response of the AMC1311 output stage.  If the simple RC filter plus AMC1311 does not get to lowest noise, then an additional driver amplifier may be needed.  

    Regards,
    Keith