Multimeter IC selection

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Aug 1, 2015.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    1. Trying to make a multimeter with ac-sc voltage & current measurement with auto-ranging.
    2. Looked at several IC which directly give measured output. one found is ICL7106.
    3. I have SPI based TFT on which data is to be displayed. So have to read BCD data & display.
    4. Also is there any other IC, which have all the features of ICL, but gives data in other digital format.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    An icl7106 (for 7-segment LCD) or icl7107 (for 7-segment LED) are rather complex chips with specialized outputs. Multiplexed to activate four digits while seven more pins activate 1 digit (as the 4 multiplexing pins sequentially activate. This will be a difficult task acquire the mess with a Microcontroller and then reformat it into an SPI serial datastream.

    If you have a Microcontroller, then just use the A/D converter on a good Microcontroller or use a good ADC chip and you will already have a digital value in logical sequence instead of a nonsensical 7-segment display value. Look for 11-bit or more ADC chip.

    Note - there is no accessible BCD on the icl7106 / icl7107. These chips directly drive 7-segment displays.
     
    Last edited: Aug 1, 2015
  3. DickCappels

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    Aug 21, 2008
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  4. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    I will go with ICL7135. :)
     
  5. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    @DickCappels , I was looking at datasheet of MCP3425.
    Page 10 , equation 4.1, says that voltage can range between -vref to +vref i.e +-2.048V.

    But 4.5.2, says that voltage at each pin cannot fall below vss-0.3V.

    I will use it in voltmeter application. The gnd of MCP3425 will be different & voltage applied at mcp3425 differential pins will be different. ?
    Will it suit my application. (i am planning to replace ICL7135)
     
  6. DickCappels

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    Aug 21, 2008
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    4.1 May look confusing because it addresses the differential input voltage range. The absolute differential input voltage is equal to Vin+ - Vin-, and both must in the range specified in 4.5.2. It might be more clear to go by the words that precede the equation (emphasis added by me):

    "The device performs conversions using its internal
    reference voltage (VREF = 2.048V). Therefore, the
    absolute value of the differential input voltage (VIN),
    with PGA setting is included, needs to be less than the

    internal reference voltage. The device will output
    saturated output codes (all 0s or all 1s except sign bit)
    if the absolute value of the input voltage (VIN), with
    PGA setting is included, is greater than the internal


    Whether it meets your requirements depends on what you expect in terms of accuracy and linearity. While you are stuck with the internal voltage reference, you can always adjust the input attenuator(s) if you need better absolute accuracy.

     
  7. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    The part I don't understand is:

    1. MCP will have its separate ground from 9V battery. & I will apply voltage at two probes as in voltmeter(red & black). Now this voltage let say 1V at differnetial channel will not have relation to ground of MCP.

    So will it work? How can I tell the that those voltage limits are filled for example vin > vss-0.3.
    When in my case there is no connection between vin_diff & ground of MCP
     
  8. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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  9. DickCappels

    Moderator

    Aug 21, 2008
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    You must keep the voltage within the limits spelled out by the datasheet or risk your project not working. Can you connect the negative input of the battery to Vin-?

    The advice in datasheets are there for a reason. Read the datasheet carefully and take heed.
     
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