• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 46 subscribers
  • Views 271 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.

ADS124S08: Using TPSM82912 and LM27762 as powersupply for bipolar adc setup with mcu

Johannes Maier
Prodigy 60 points
Part Number: ADS124S08
Other Parts Discussed in Thread: TPSM82912, LM27762,

Tool/software:

Hello,

I am currently looking into power supplys for the ADS124S08. I am new to hardware design and I feel kind of lost with all the things I have to consider Slight smile

After some investigation this is what I have learned so far:

  • DC/DC converters are efficient but have a ripple voltage output, which can be mitigated with a LDO-regulator.
  • Using LDO-regulators only is inefficient and introduces heat into the system.
  • AVDD and DVDD should be low noise and low ripple, as both can introduce noise into the measurement

My desgin has the following specifications:

  • The design must drive the adc and a mcu. The external voltage I am expecting ranges from 6 to 12 V.
  • Both mcu and adc need a digital voltage supply of 3.3V
  • The analog voltage supply should be bipolar +- 2.5 V.
  • The adc measurement rate is slow but high precision(1 -100 Hz)
  • I do have limited pcb-space, so a small footprint is a must.

I am thinking about using a DC/DC converter such as TPSM82912 for converting from 6 - 12 V to 3.3 V for DVDD of the ADC and for the DVDD of the MCU.

For AVDD/AVSS I want to use the LM27762 as bipolar source for AVDD/AVSS with Vout of +- 2.5 V.

The idea is, that with the TPSM82912, I dont need an extra LDO-Regulator for DVDD and can drive both ,the adc and the mcu.

With the Power Designer I get a footprint of 93 mm2 for the TPSM, which is kind of large, but ok, if I don't need a lot of other parts.

What do you think?

  • Is the TPSM82912 suitable for drivng the DVDD of the ADC and the mcu?
  • What other aspects do I need to consider?

Thank you for your time and answer

Regards

Johannes

  • 0
    TI__Guru 53466 points

    Hi Johannes Maier,

    Why do you need bipolar supplies in your system? This will add cost and complexity, but nowhere in your post did you really justify why this is necessary. Can a single 3.3V supply for both AVDD and DVDD work? That will be the simplest solution

    Also, what is your power source? You said "6-12V" - is that a battery?

    The amount of power dissipated by the LDO (and therefore the heat generated) is linearly related to the voltage drop across the LDO. So if you step down from 6V to 3V, you will generate less heat than if you stepped down from 9V to 3V for example. You can reference an LDO datasheet for more specific information about how to calculate the heat generated by an LDO given the LDO specs, the input and output voltages, and the current consumed by the downstream circuitry. You can also of course post a thread on our power management e2e forum. Note that the ADS124S08 is a relatively low power ADC, so you are already in a good position to create a low power system

    How are you attenuating the analog inputs? You said that the input voltage is 6-12V, but the ADC can only tolerate an input signal up to its supply (AVDD) voltage, which must be <5V.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Hello Bryan,

    thank you for your reply.

    Regarding your questions:

    The power is supplied by an external system. There are different legacy versions of this system, which do supply different voltages ranging from 6 to 12V. The system I am designing needs to handle these voltages. At the moment I can not say how stable this external power supply is, but I am assuming that it will have noise and ripple.

    The ADC must be setup for slow measurements like temperature with a PT100 sensor and also for AC voltage measurements with a range of +-2 V. The measurement rate wil be 1 Hz to 100 Hz, depending on the measurements.

    With a bipolar setup I dont need to add extra components to shift the voltages into the range of the adc.

    I have the idea form this ref design : https://www.ti.com/tool/TIDA-01434, which uses the LM26772.

    Also the setup will be configured for either temperature or voltage measurement. Not both at the same time. But both should be configurable through software.

    The attenuation of the analog inputs will be between 100 mV to +-2 V.

    Thanks for the tip about the heat calculation. I thought about using only Linear Regulators for a simpler design and maybe the generated heat is tolerable.

    About DVDD and AVDD being the same:

    I thought that separating the supplys is considered the best option, since noise or voltage spikes from other components like the MCU degrade the measurement.

    Regards

    Johannes

  • +1 in reply to Johannes Maier
    TI__Guru 53466 points

    Hi Johannes Maier,

    Thanks for providing additional background. The TI reference design you linked to should work well for your power supply design. The LM27762 has built-in LDOs so the analog supplies should be clean

    I would recommend an LDO on DVDD, you don't want a lot of noise/ripple getting into the digital circuitry

    The ADS124S08 is generally intended for DC measurements, so ensure that you understand the integrated filter response if you want to measure AC signals. the sinc filter has low bandwidth and lots of passband droop, so it is not ideal for AC signals. However, the ADS124S08 is really designed for RTD measurements, so that is the right choice for temperature measurements

    I don't think there should be an issue using the same supply for analog and digital, as long as the source is clean i.e. has an LDO. However, since you are using bipolar supplies this is really a non-issue. You can see that the ADS124S08EVM can use the same supply as DVDD, IOVDD, and AVDD, as shown in the image below

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Hello Bryan,

    Thanks for the tip on AC signals, I will look into that.

    For now, I have deciced to start with two linear regulators. The first is converting up to 12 V to 3.3 V. I have calculated a Tjunction of 42 degree C and this might be ok for us.

    The second regulator is the LM27762 regulating 3.3 to +-2.5 V.

    I would considers this my minimal setup and go from there if noise and ripple will be an issue.

    As a last question:

    Would you consider https://www.ti.com/product/TPSM82912#pps a valid option for reducing the noise and ripple in my setup?

    Or asked in another way: when would you consider TPSM82912 as a valid option?

    Regards

    Johannes