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ADC for 4 channel voltmeter application & suggestions

Genius 3300 points
Other Parts Discussed in Thread: ADS1248, PGA280, ADS1220

I have to make a 4 channel voltmeter. This voltage is to be read by MCU & display on graphical lcd.
Three with +-20V range (differnetial) & one with 0-3.3V(singl ended).
Need to take 10sps of each.


I was looking at ADS1248 which has 8 channel SPI interface.

Some of problems i think are:
1. Usually in voltmeter/multimeter the black probe is not shorted with ground of its 9V battery.
So if I use ADS1248 then its ground will be differnet then voltage at differnetial pins.
Then I don't know how Vcm range equation is satisfied here.
Can I apply voltage at differnetial inputs of adc which have no direct relation with ground of adc?

2. Even if I short black probe with gnd for measurement purpose & provide +-2.5V to adc for negative voltage measurement,
then it will short one channel of all channels, which I want to avoid. i want to keep all channel isolated from each other.

3. Also planning to place LTC1966, a rms to dc converter in front of each channel so that i can measure exact dc value even if ac voltage is applied.

4. Currenly I am searching for some switch or mux (single pole 4 pole), so that I can switch between channels for isolation in case its not possible to isolate each channel otherwise. Also in this case i will have to place only one LTC1966.

5. ADC interface preferable is I2C.

Edit: 6. Can provide system upto +-3.3V only. This is one rule. For higher voltage measurement i have to  step down the signal.

Any suggestions how to do it.

  • Vindhyachal,


    I'm not sure I know enough to comment on making voltmeter, but I do have a couple of inputs to your questions.

    The ADS1248 input needs to have an established common-mode voltage. You cannot apply a differential input that does not have a relation to ground. All of our ADCs will need this input common-mode voltage established.

    One way to expand your input range is to use PGA280 as a buffer. This also allows for establishing the input common mode voltage to the ADC. For this device, I think you'll still need to establish the input common-mode voltage and you'll need a high voltage supply for the gain amplifier.

    For the ADC, we don't have any 24-bit ADCs that used I2C communications. There are a few 16-bit ADCs that have a I2C communications, but none have four differential channels. If you're interested in them, let me know and I can list a few that might work.

    At this point, I don't have any recommendations for the mux, I can look into it and see if we've ever made any recommendations.


    Joseph Wu
  • Hi Joseph,

    1. I have decided that i am going to make gnd common of adc & gnd of voltmeter common. (because I didn't find any solution :( )
    2. Any how, I thought I could use ADS1248 easily since I can power it with +-2.5V supply(bipolar), & measure +-20V.
    3. Only problem is it has SPI & i am using raspberry pi. SPI port on it is already occupied with LCD which I cant change. Si i am left with I2C & uart.
    I have also tried to do bit-banding spi on it, but no luck on it still.

    4. So i am looking for some ADC with i2c interface. I need 14 bit or higher adc.
  • Vindhyachal,


    One other possibility is using two ADS1115s. These devices are 16 bit and have two differential channels each. They also are I2C, with multiple address options which are pin selectable.

    This will still require the establishing a dc common mode input, but the input goes to either rail, even with the different gain (or reference magnitude) options.


    Joseph Wu
  • Hi Joseph,

    1. Can you give some link to app note or example where dc common mode is esatblished.

    2. Meanwhile I am looking for some SPI to i2c bridge from silabs also.
  • Vindhyachal,


    1. The ADS1220 datasheet has a write-up about the PGA common-mode requirements starting on page 21. This will be similar to the ADS1248 and explains why the input common mode needs to be within the specified certain range. I'd read through that section and see if it helps. However, I don't have an application note on how to establish the dc common-mode input. In most cases, sensors and measurement will operate within the supply rails of the ADC.

    2. I'm not sure if TI makes any SPI/I2C bridges. I wasn't able to find any through a search of the website. However, if you're interested, you could post to the Interface I2C Forum to see if there is anything available.


    Joseph Wu