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ADS1298R base impedance measurment

Rafal Radziejewski
Intellectual 280 points
Other Parts Discussed in Thread: ADS1298R

Dear TI.

I need some explanation about measuring the impedance in ohms from the result given by ADS1298R which is in Volts. 

I attach a fragment from user guide which supply an equation of measurment of expected DC voltage depending on Rb (Equation 6.). I dont use a simulator, I use my own signal. so I've observed an output of 38500 with Vref = 4V, GAIN=1, Fresp=32kHz, fi=90degrees.

So I would like to calculate hownmany Ohms is my Rb. To calculate it i use equation 6:

DC = 385000 from measurment
DC_V = 0,183582328 V in Volts - (385000/8388607 * 4V)
R_b =  3848,264914 Ohm (using 6. to calculate Rb from DC)

Is this correct calculation? I have doubts because you use GAIN=6 and calculate DC as it was 1 (so the output is 6x times higher than real).

Moreover, as I observe, this formula seems not correct. When you make Rb higher, you also receive DC higher which abuse Ohms law. Can you explain me formula 6?

  • 0
    TI__Guru 66386 points

    Hello Rafal,

    Thank you for your question and welcome to our forum.

    The example shown in section 5.3.2 of our User Guide does not include the cable impedances in series with R_B. I believe these measurements were done with only wires to connect the simulator outputs to the ADS1298R EVM inputs. The results shown are input-referred with PGA gain taken into consideration.

    3.85k Ohms seems to include about 2k worth of cable impedance. Please refer to the previous sections (5.2.1 - 5.2.2) for example calculations which include cable impedances. R78 and R79 are placed on the EVM to model cable impedance, R77 models body impedance, and R76||R77 models the ΔR.

    Best Regards,