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TINA/Spice/ADS1298: ADS1298 without INA as the first stage

Rhuan Correia
Prodigy 60 points
Part Number: ADS1298
Other Parts Discussed in Thread: TINA-TI, INA128, ,

Tool/software: TINA-TI or Spice Models

Hi.

I'm developing an ECG with a analog first stage. I mean, i'm using defibrillation protection circuitry, INA128 with G=6 and an anti-aliasing filter placed on 153 Hz.

1) Is it necessary to use the INA? I take several problems of understanding how can I amplify the differencial signal to generate the I, II and III derivations.

2) Now for ADS1298, for derivation I, should I put the single-ended output at the IN1P and the ground at the IN2N? And the same with II, that is, using IN2P and IN2N, generating III by software?

3) Is it applicable to use fully differential amplifiers? I never heard about this possibility but I know that the CMRR isn't desirable at all.

Best regards from Brazil. Thanks!

  • 0
    TI__Mastermind 38745 points

    Hello Rhuan,

    1. Generally it is unnecessary to use an INA with the ADS1298. The noise of that device is low enough to measure ECG and the input impedance is very high. Take a look at the ADS1298ECGFE-PDK schematic at the end of the User's Guide to see how that is set up. That is a good reference for how the electrodes can typically connect to the inputs of the ADS1298.
    2. Refer the the schematic. It will show you the recommended lead connections.
    3. The PGA on the ADS1298 is a fully differential amplifier and has no CMRR itself. This is actually ok because the ADC after the PGA will have very good CMRR. In addition, the device integrates a Right Leg drive amplifier to form a common-mode rejection feedback look with the patient.

    Regards,

    Brian Pisani

  • 0 in reply to Brian Pisani
    Prodigy 60 points

    Hi, thanks!

    About you second answer, I didn't undestand at all; I saw the schematic and I'm believe that I can use ground as the negative reference of ADS1298 differential inputs, that is, with all the grounds of system tied together. But, the body's ''zero'' has a bigger potential than the main ground. 

    For my college's project I need to use the INA and put a test point at the end of amplification stage, also to put out a test signal to the board.

    Thanks for your support!

  • 0 in reply to Rhuan Correia
    TI__Mastermind 38745 points
    Hello Rhuan,

    The body's "zero" should actually be generated by the ADS1298. The RLD amplifier on the ADS1298 is intended to apply a signal to the RL electrode which is connected to the patient. This will set the patient's bias voltage to (vdd+vss)/2. That way, you can measure the leads by simply measuring the voltages between the electrodes differentially, rather than applying a single-ended voltage to the inputs like you are proposing.

    The ADS1298 also allows you to apply an external test signal to the ADC inputs through the TESTP and TESTN inputs. Can you use this instead of putting a test signal on the ECG signal path?

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 60 points
    Hi Brian.

    I got it. Thanks again!

    I have two last questions,

    1) Since I will use the bias signal / mid supply negatively (INxN), I must also connect it to the INA's reference pin.

    Besides, I'm using the GND as INA's ref. This implies that I also must use the GND (INxN) and the single-ended voltage (INxP)?

    2) Considering that I'm using INA pre amp and bias voltage, I can't use the internal gain of PGA as well. I believe that 0.45 V to 2.85 V is a perfectly acceptable range for a 3.3 V supply and VREF = 2.4 V (internal). Using 1.2V I can improve the resolution. Am I right?

    Rhuan
  • 0 in reply to Rhuan Correia
    TI__Mastermind 38745 points
    Hey Rhuan,

    1) If you are using a single supply, you can't connect ground to INxN. The output range of the PGA is limited to within 300 mV of the supply rails. You should instead use mid-supply as the INA's ref.

    2) Most of the noise from the ADS1298 comes from the PGA, so decreasing the reference voltage will not improve the resolution. I am still unclear why you are using an INA instead of using the internal PGAs exclusively. Do they not provide low enough noise?

    Regards,
    Brian