Front end for Analog to Digital Conversion ADC

Discussion in 'Analog & Mixed-Signal Design' started by scoobydoo, Jul 25, 2016.

  1. scoobydoo

    Thread Starter New Member

    Nov 25, 2013
    I'm wondering how I would sample an analog input with range say 0-24VDC with an ADC that does only 0-2.4VDC? My first thought is an opto-isolator with transistor or darlington output but it doesn't seem right or complete anyway.

    My other thought is that I will be somehow compressing the analog signal so I might lose effective resolution especially if the signal is changing quickly. So I might want to up my bits of resolution to to compensate.

    Is there a name for the type of circuit I want to make? Thanks.
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    Unless you need to isolate the grounds of the two circuits you definitely don't want an opto-isolator.
    All you need is a simple 10:1 potential divider - e.g. an 18k resistor from your source to the ADC and a 2k resistor across the ADC input.
    You do not lose any resolution this way and it is as accurate as the resistors are.
  3. scoobydoo

    Thread Starter New Member

    Nov 25, 2013
    Thanks. I guess I was thinking something more complex along the lines of dynamic voltage dividing because the analog input could vary depending on the device being measured. Typically this would be 10V or 5V but 24V could be possible. But the max input value would be quantized (ie 17.65V would not make sense), so I just need to set up some sort of array with jumpers to do the voltage divider for each input. I will probably use opto-isolators afterwards as I do want to isolate the grounds because the sensor and adc might be on different power supplies located some distance apart.

    Any recommendation for an opto-isolator?
  4. crutschow


    Mar 14, 2008
    You could go to some sort of auto-ranging circuit that will change the signal attenuation based upon the voltage, similar to what an autoranging digital voltmeter does.

    You could also use non-linear compression but that would be less accurate.

    If you need isolation between the sensor and ADC then you might consider an isolation op amp. It's difficult to get high analog accuracy with an opto.
    (How many bits is your ADC?)
    cmartinez likes this.
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    If the input signal is an analog signal and you want to capture all of the intermediate values, then an optoisolator is *not* the way to go. Optos, even ones specifically designed to transfer analog signals, and not very linear and will introduce both absolute and relative errors. If you really want galvanic isolation, there are other ways.

    BUT - if what you want is just to measure the peak amplitude of the signal, like measuring the output of a power supply or a digital (on/off) signal, then an opto might be good enough.

    Two important questions
    1. what is the nature of the signals you want to capture?
    2. What is the accuracy or precision or number of digits or number of decimal places you need?

  6. scoobydoo

    Thread Starter New Member

    Nov 25, 2013
    The nature of the signals will vary depending upon what type of sensor is connected. This has to be a generic solution as I'm developing an interface for a datalogger platform.

    Its been suggested that I might want to use a digital isolator such as the the ones provided by silicon labs:

    I couldn't clip the picture because its pdf but the diagram on page 7 of this application note seems to be close to what I want. The only difference is that all my inputs will be DC so I will have to have an LDO regulator that can handle upto 24VDC: documents/technicaldocs/an614.pdf