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Original question:

ADS1158: Circuit review request

ADS1158: ADS1158; Request for Circuit verification

Vidhyanandhan Sathishkumar
Prodigy 20 points
Part Number: ADS1158
Other Parts Discussed in Thread: REF5025, , OPA365, LM1117, REF5040

I've designed an circuit for a bio-medical application (university project). The adc input will be +or- 1V. Please verify if the circuit has any errors.

Microcontroller to be used- raspberry pi 4

* dvdd from micro controller

* avss,avdd from 3.7 v li-ion batteries connected to 2.5v voltage regulators,(+2.5,-2.5)

* Vref +2.048,-2.048 using ref5025.

* 1V differential input

schematic https://drive.google.com/file/d/1KlNXJuF30Fy11g3TR3n-QaEXTD4h00b_/view?usp=sharing

  • 0
    TI__Mastermind 42031 points

    Hi Vidhyanandhan,

    A couple of notes about your schematic:

    1. I would add a resistor between Pin 4 and Pin 1 on U5 (OPA365), otherwise this diff in/ diff out stage might not operate properly. This is shown in Figure 55 in the ADS1158's datasheet. I assume you could also use these amps in a buffer configuration if you do not need gain
    2. It was not very clear to me what you are trying to do with the LM1117's, so I will leave it to you to make sure that circuit operates properly. As long as the output is a clean +/-2.5 V supply, the ADS1158 should be okay
    3. Many other users have had difficulty tuning the drive circuit for the crystal that you have shown in your schematic - you can search e2e for many posts about this topic. If you are able to use a clock oscillator in your system, that would be a better choice. It looks like you have provisions for this via JP1 and the CLK header
    4. I don't think the VREF circuit you have shown there will give very good performance. I would recommend using the REF5040 instead, tying the GND pin on the REF5040 and REFN on the ADS1158 to AVSS (-2.5 V) and supplying VIN on the REF5040 with 2.5 V. You would then tie the output of the REF5040 directly to REFP on the ADS1158. This should give you a better path to +/-2.048V VREF voltage

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Thank you for the suggestions!

    1. As i don't need gain I've removed the resistors and shorted them as suggested.

    What is the purpose of that 2.2nF capacitor across ADCINP and ADCINN ? (C1 in Schematic, as in fig. 55 in datasheet).

    2. I'll be connecting two different batteries to those LM1117's. So, I thought of connecting +2.5 of one LM1117 to the ground of other and having that point as reference, I'd be able to power the ADC with +/-2.5V.

    3. Yeah,I saw those forums. In many cases the crystal did not start up, that was the reason I made provisions for connecting an external clock in case of crystal failure.

    4. I've changed ref5025 and the circuit for inverting the 2.048V with ref5040. As my input will be between +/- 1V and the analog supply is +/-2.5V, guess having Vrefn as -2.5V and Vrefp as 1.596V will be fine. 

    If there are any other issues with the circuit please do ping me, and if possible suggest me some way for connecting the clock :).

    Please refer to the corrected schematic.

    Thank you.

  • 0 in reply to Vidhyanandhan Sathishkumar
    TI__Mastermind 42031 points

    Hi Vidhyanandhan,

    The cap, along with the adjacent resistors, creates a lowpass filter to help filter out noise.

    The ADC just needs a differential reference voltage, it does not matter what the absolute voltage is on each pin (assuming they are within the recommended operating limits). So you will be measuring your differential signal (+/-1V) against the ADS1158's full-scale range (+/-1.06*4.096).

    Note that you would not be able to actually measure a 4.096V input in this configuration given your +/-2.5 supply range, since the absolute voltage on the analog inputs is limited to AVDD+0.1V. But this should not matter for your application.

    -Bryan