ADS1293: GAIN issue and design question - Analog and Digital Filtration - 2 LEAD Holter to pass IEC 60601-2-47

Hello Pontus,

Thank you for your post. Would you mind sharing the complete register settings and the ADS1293 portion of the schematic showing the relevant input and RLD circuits?

Regards,

Ryan

Hello Ryan, 

Thank you for your quick reply. 

Here is the relevant part of our schematic with some notes:

1. On the input, down to the left are TVS diodes for ESD protection.

2. On the input, there are series resistors of 100 Ohm and on the new design (not measured above) are additional 100k resistors in series on each lead.

3. You can see the RLD and the connection to the Inputs here.

I have provided a simplified block diagram below:

And this is the part of the software which I have currently set to put the output in the middle of the 24 bit measure (tested changing the magic number to 0x800000 as well):

This is the full register settings:

And then for starting conversion I call:

ads1293_write_reg(TI_ADS1293_CONFIG_REG, 0x01);

Please let me know if I should try anything differently.

All the best,

Pontus

Thanks, Pontus. Are all resistor values 1% tolerance? Can you try reducing this to 0.1% and minimize the capacitor tolerance as well? 

Could you try removing the capacitors and retest?

Regards,

Ryan

The resistor values are indeed 1%, we are using these: RC0402FR-07100RL for R22, R23 and R24. Is the tolerance important on the RLDOUT (R26, R33 and R38) and CMOUT (R14, R15) as well?

For the minimized resistor tolerance, is it enough to change C5, C35 and C36?

Unfortunately, we are a bit limited on tools and space to remove the caps. Are the any other suggestions to try? 

What are your thoughts on the register settings and the configuration in general?

Hello Pontus,

The resistor tolerance to the RLD and CM circuits are less critical as those signals become common to the patient. The importance of low tolerance is primarily on the input signal path where common-mode noise can become differential noise due to component mismatch.

I will come back to review the register settings early next week.

Regards,

Ryan

Great thank you for your input regarding the tolerances for the resistors and capacitances. 

I've been running a few additional experiments using the fluke Pro Sim 2 and I am quite surprised to see the same results there. There is a gain difference but I'm not sure if it's the same across a multitude of amplitudes I have not tried that yet.

For the ads 1293 there is it constant gain of 3.5. I am supplying it with VDD of 3.3 volt. Do I need to scale the converted values from the ads before I can use them?

Currently, this is what I do following DRDY:

    // 0. Get data

    uint8_t rx_buff[7];

    ads1293_read_ecg(rx_buff);

    // 1. Normalize data

    int32_t data_i = (int32_t)(((rx_buff[1] << 16) | (rx_buff[2] << 8) | (rx_buff[3])) - 0x798000);

    int32_t data_ii = (int32_t)(((rx_buff[4] << 16) | (rx_buff[5] << 8) | (rx_buff[6])) - 0x798000);

    // 2. Filters

    float lead_it = dsp_HPF_I(data_i);

    float lead_iit = dsp_HPF_II(data_ii);

    // 3. Bring to 16bit range

    int16_t lead_i = (int16_t)(lead_it / 8);

    int16_t lead_ii = (int16_t)(lead_iit / 8);

No scaling the data with respect to gain, Vref or VDD. Is this correct?

Hej Ryan, 

Thank you for your reply.

So if I understand you and the datasheet correctly I can rewrite it as this:

With

What do I set ADC max to? I run it in low resolution so I interpret the datasheet to be a resolution of 17.8 bits, although the ADC is 24 bit, which is correct:

Depending on ADC max, I could write the following:

    float V_ref = 2.4f;
    float gain = 3.5f;
    uint32_t ADC_max = 16777215; // 24-bit max value

    // Calculate lead_i
    float lead_i = (2 * V_ref / gain) * (((float)ADC_max / (float)data_i) - 0.5f);
    
    // Calculate lead_ii
    float lead_ii = (2 * V_ref / gain) * (((float)ADC_max / (float)data_ii) - 0.5f);

***EDIT 2024-10-16: My reply got cut short and was missing all equations.****

Great, thank you Ryan.

I will test this setup tomorrow.

Do you have any input on the register settings otherwise?

All the best,

Pontus