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ADS130E08: Maximum and minimum analog input voltage

Weslley Duarte
Prodigy 20 points
Part Number: ADS130E08

Hello,

I'm developing a device that needs to read data from -4V to 4V (or 2.828Vrms).

According to the following table on the datasheet that is possible.

But when reading other sections of the datasheet I ended up getting confused with these values.

For example, at this table, AVDD - AVSS have a maximum value of 5.5V, but if I use the 4V - (-4V) I'll get 8V.

My schematic for the ADC:

- VSSA: -4V

- VDDA: +4V

- VDD: +3.3V

My schematic for one of the INPUTS:

By the way, the circuit is working fine, I'm only worried that I'm infringing the acceptable values for the device.

Thanks in advance.

Weslley.

  • 0
    TI__Guru 62916 points

    Hi Weslley,

    Thank you for your post.

    The Absolute Maximum Ratings refer to the voltage applied to the pins relative to AVSS. For the ADS130E08, each input pin can accept an absolute voltage of AVSS - 300 mV to AVDD + 300 mV without damaging the device. However, this is different than the voltage which the ADC can convert to a digital value.

    The ADC conversions are limited by the reference voltage, VREF = VREFP - VREFN, where VREFN = AVSS. The ADC will compare the differential analog input voltage (INxP-INxN) to VREF to make its conversion. This differential input voltage can range from -VREF/Gain to +VREF/Gain.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hello, Ryan.

    Thanks for the reply.

    Ok, they are different things. But, with VREF = AVSS and the minimal value of AVSS being -2.5V. How I'll be able to read a signal ranging from -4V to +4V.

    Best regards.

    Weslley

  • 0 in reply to Weslley Duarte
    TI__Guru 62916 points

    Hi Weslley - VREFN = AVSS, not VREF. VREF refers to the differential reference voltage (VREFP - VREFN). In order to measure +/- 4 V input signals, connect VREFN = -2.5 V and VREFP = +1.5 V to create a 4-V reference.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hello, Ryan.

    I did what you suggested. My setup now it's as follows:

    - VDDA is 2.5V;

    - VSSA is -2.5V;

    - VREFN is connected to VSSA, so -2.5V;

    - VREFP was left floating (measured 1.5V with the voltmeter);

    - The PD_REFBUF bit is set to 1;

    - The VREF_4V bit is set to 1;

    But, when I acquire data from the ADC, it gets "cut" around 2.5V and -2.5V.

    Regards,

    Weslley

  • 0 in reply to Weslley Duarte
    TI__Guru 62916 points

    Hi Weslley - I would also check the input common-mode voltage on each pin. Since you are trying to reach the maximum differential signal it is important for the common-mode to be close to mid-supply on both input pins. The exact allowable range can be calculated using equation 2 in the Input Common-Mode Range section. Can you please validate what the input signal looks like on INxP and INxN on an oscilloscope? 

    Also, can you provide details about your input signal setup? Assuming this is coming from a function generator, I would also confirm that each signal output (INxP and INxN) is set to high-Z source impedance and that the ground connection is shared with the circuit board.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hello, Ryan.

    My input setup is a function generator set to 2Vrms Offset 0V 60Hz, connected to the board as follows.

    I measured the signal coming in the circuit from the function generator and it's fine.

    I remember reading on the datasheet that it is only possible to use VREF_4V with a 5V analog supply. What does that means?

    I'm using a bipolar analog supply of +/-2.5V. Do I need to operate on Unipolar analog supply +5V?

    Regards,

    Weslley

  • +1 in reply to Weslley Duarte
    TI__Guru 62916 points

    Hello Weslley,

    Thanks for sharing these details. 

    It looks like your signal generator is outputting a single-ended waveform, meaning that only the red (+) terminal carries the AC signal to IN8P and the black terminal is the function generator ground (i.e. 0 V). Can you please confirm if that's how you're using it? There are a couple concerns to point out:

    1. The black terminal of the function generator does not have a direct DC path to connect to the ADC circuit ground. This is important in order to establish the correct common-mode voltage on the red (+) terminal. Unless the ADC circuit is battery powered, it will eventually connect to the same AC main supply as the function generator (i.e. through a wallwort or PC power adapter, etc.). In that case, the ground of the function generator will naturally be close to the ground of the ADC anyway, but it's still a good practice to make an extra direct connection between the two grounds.
    2. If only the red (+) terminal carries the signal, you will only use half of the ADC full-scale range at most. On the scope, it appears IN8P swings from +/- 2.9 V (5.8 Vpp). This violates the Absolute Maximum Ratings as each input pin must stay within the AVDD and AVSS limits (+/- 2.5 V). Beyond this, the PGA on each channel will clip (see image below).
    3. Furthermore, if IN8N is fixed to 0 V, then the differential voltage (IN8P - IN8N) will only be +/- 2.5 V differential maximum, so only (2.5/4) = 62.5% of the ADC codes will be used.

    Originally, you mentioned you needed to measure 2.828 Vrms (4 Vp). In order to achieve this, both INxP and INxN need to swing between +/- 2 Vp. This keeps each input within the supplies while achieving +/- 4 Vp differentially. See image below:

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hi, Ryan.

    Thank you so much for all the help again.

    I think I got it now ... in order to use the full range, I will need to work with differential inputs. Right?

    If that is the case, and I understood correctly, the topic may be closed.

    Best regards.

    Weslley

  • 0 in reply to Weslley Duarte
    TI__Guru 62916 points

    Hi Weslley - Yes - measuring differentially will allow you to use the full-scale range and measure a +/- 4-V signal with +/- 2.5-V supplies. I'm glad this resolved your issue. Feel free to open a new thread if you have any additional questions.

    Regards,

    Ryan