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Evaluating ADS1298

Giulio D'Amato
Prodigy 100 points

Other Parts Discussed in Thread: ADS1298, LM78L, ADS1198

Hi everybody, I'm a new member and I'm really excited about this E2E community.

I'm currently trying to develop a new project involving ADS1298, but unfortunately I'm a bit into troubles ... I probably damaged my IC, but i can't understand the reason why, so I'm asking you some questions, hoping to find out my mistakes.

This is my breadboard configuration.

1. AVDD=DVDD=3.3v

2. AVSS=DGND=0v

3. !PWDN=!RESET=CLKSEL=3.3v (short to DVDD) (What is digital input impedence? Should I use pull-up resistors?)

4. START=!CS=DAISY_IN=0v (short to DGND)

5. TESTP_PACE_OUT1=TESTN_PACE_OUT2=3.3v (short to AVDD)

6. VREFN=0v (short to AVSS)

7. I used electrolytic capacitors for suggested capacitance >0.1uF

8. SPI signals are routed to a microcontroller powered up with same supply.

After some seconds i power up the circuit, the 3.3 voltage regulator LM78L33 becomes hot and VDDs become shorted to GNDs.

I have another IC but I'd like to hear from you before wasting another one.

Thank you in advance,

gdam

  • 0
    TI__Guru**** 182601 points

    Hi Giulio!  Welcome to the e2e Forum!

    The ADS1298 connections above sound correct, however I would disconnect the TESTx_PACE_OUTx signals from AVdd until you have everything up and running.  The use of pull up/down resistors would limit the current draw if anything should go wrong, so consider adding 10K resistors on points 3 and 4.  I would also take the /CS input to any available GPIO on your controller, at least during the initial debug phase of your project.  If the SPI bus gets corrupted for some reason, you can reset it by toggling the /CS input.  Having that pin tied to GND will force you to power cycle the setup if there is any problems with the communication making it very difficult to debug.  Taking VREFN directly to ground is fine. 

    What is the current draw of your micro controller?  If memory serves me, the LM78L series of regulators can only provide 100mA of current

  • 0 in reply to Tom Hendrick
    Prodigy 100 points

    Hi Tom, thank you for your quick and accurate reply!

    I'll keep in mind your suggestions and conform my rough design to them ... actually, it hasn't been a great idea to hardwire digital inputs.

    I hope to come back really soon with good news,

    Giulio.

  • 0 in reply to Giulio D'Amato
    TI__Guru**** 182601 points

    Great!

    In case you missed it, there is also an FAQ page for the ADS1298 and ADS1198, you can find that here:

    http://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/frequently-asked-ads1298-and-ads1198-questions.aspx

  • 0 in reply to Tom Hendrick
    Prodigy 100 points

    Hi everybody, I'm here again to ask for some help from the community. It has been a long time since my last message, but this project has slipped in the drawer for a while :)

    I eventually resolved my problem on January, it seems that an unwanted connection (due to the hand soldering job) was the cause of the burnt IC. Now I have a fully working microcontroller test firmware and a MATLAB acquisition/plot script.

    By now I acquire the test signal on all the eight channels and show them onto a real time plot in MATLAB, everything seems ok. I want to make another step further, acquiring ECG signal from ECG simulator. 

    I am a bit confused, you have to forgive me. 

    I want to do 12 lead ECG, and I know I need 10 wires.

    • 6 Wires are from Chest Leads, and go to IN1P to IN6P, with WCT on IN1N to IN6N.
    • 1 Wire is for RLD (I know I need external circuitry, from what i remember it is on datasheet, right?)

    I have 3 remaining leads, LL, RA, LA ... How should I connect them?

    I'm sorry if this question is trivial, but I think this may be a right place to ask.

    Thank you for your help,

    Giulio.

  • 0 in reply to Giulio D'Amato
    Prodigy 100 points

    Ok, my bad :) Answer to my question is on SBAU171, page 45!

  • 0 in reply to Giulio D'Amato
    TI__Guru**** 182601 points

    Great!  Let us know if there is anything else you need, we'd be happy to help out.

  • 0 in reply to Tom Hendrick
    Prodigy 100 points

    Dear Tom, I have some new questions, hope to hear from you about these topics.

    • How should I connect INxP pins (from ADS1298) to lead wires? (Is there a reference or suggested design? From what i understand, on ADS1298ECG-FE there are 2 Resistors, 2 Capacitors and 1 diode)
    • What is the suggested/preferred configuration for RLD? 
    • From what I understand it is common practice to internally connect RLD amp to WCT, is it true? 
    • What are the best/suggested component values for the external op-amp inverter? (Alternatively, how should I set the loop gain?) The non inverting input of the external opamp should be connected to RLD_REF, if I'm not in mistake.
    • Also, from  ADS1298ECG-FE diagram, I can't see RLD op-amp amplifier, but a parallel RC between RLDINV and RLDOUT, why? It seems that we are using internal RLD amplifier rather than an external circuit. (See page 68 from datasheet)
    • From what i understand, on ADS1298ECG-FE there's a ECG_SHD_DRV to drive the cable shielding ... how should I replicate the function of that non inverting buffer circuit? 

    If you have a reference textbook or document to get a deep understand of these arguments, please let me know. 

    Thank you,
    Giulio.