• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 46 subscribers
  • Views 563 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1274 reference and differential input range

George Metaxas
Intellectual 330 points
Other Parts Discussed in Thread: ADS1274, THS4524

I am designing a data acquisition system where I need to capture differential analog signals that range from 0 to +-3.3 V (plus/minus) and having a common-mode voltage of +1.65 V. Resolution must be 24 bits and sampling rate must be at least 10-20 kS/s.

At first, under a quite long search, I chose ADS1274 24-bit ADC with 4 channels.I thought that having a reference voltage of 3.3 V would be enough.

However, on page 4 of the datasheet, it is stated that the maximum value of the reference voltage is 3.1 V. (fCLK will definitely be less than 27 MHz.)

Would it be safe to connect a 3.3 V voltage reference across the VREFP and VREFN pins of ADS1274 or should I consider another ADC? Can anyone give me a suggestion for this?

  • 0
    TI__Mastermind 23235 points

    The datasheet specs are the limits that are allowed by the device; exceeding these voltages can cause damage to the device.

    Since the ADS1274 input must be buffered, you could attenuate the signal to allow for a reference selection that is within the specs of the device.

  • 0 in reply to Greg Hupp
    Intellectual 330 points

    Thank you for your answer, Greg; I had the same thought today before I read your reply...

    I have updated my design, with an ADC reference voltage of +3.0 V and with a differential input gain of 3/3.3 = 0.909.

    In practice, this means that, referring to Figure 89 on page 39 of the ADS1274 datasheet, I replaced the 1 kOhm feedback resistor in both signal paths (positive and negative) with a smaller one (909 Ohms).
    Now I should be OK, isn't it?

  • 0 in reply to George Metaxas
    TI__Mastermind 23235 points

    You should be able to adjust the gain/attenuation as needed using the feedback/input resistors as long as you don't cause instability in the circuit.  For the small change in value you are talking about, it doesn't seem like this should be a risk - just be aware of the possibility.

  • 0 in reply to Greg Hupp
    Intellectual 330 points

    OK Greg, here I go again with a new approach based on your suggestions. Please review it and tell me if it is OK or not.

    I use a +2.5 V reference for the ADS1274 ADC. The common-mode input voltage of the ADC is 2.5 V, too.

    With a reference voltage of +2.5 V in the ADC, the full scale range of each input is +-2.5 V (or 5 V peak-to-peak).
    Since the systems to be measured have a differential range of +- 3.3 V (or 6.6 V peak-to-peak), the gain of the input stage must be 2.5/3.3 = 0.7575...

    Using the THS4524 quad fully-differential amplifier, I choose the biasing resistors of each stage to have a ratio of 0.75 (feedback resistor to signal-connecting resistor).

    Another thing is that the signals to be measured have a large output impedance; this means that the biasing resistors of the amplifier must not be of the order of 1 kOhm, as in the datasheet, but larger. I have chosen values of 100 kOhm and 75 kOhm, or 200 kOhm and 150 kOhm. Is it safe to use such large resistor values with this part?