Hello Divya, Please excuse the delay in my response as I was out of office for several days in the past two weeks. The ADS1299 current sources have only a typical ±20% tolerance. Therefore, calibration would be required in order to achieve any reasonably accurate impedance results. Without calibration, I would suggest that they only be used as a method of monitoring gradual change in impedance over time, or simply to monitor ON/OFF status at DC. This article below discusses some techniques which can be used to improve the accuracy of the impedance measurements, especially when using the AC lead-off frequency options. I will highlight a few key points below: Current sources must be calibrated at DC to determine absolute magnitude. The same current sources are used for AC LOFF (alternating INxP and INxN connections). Mid-supply common-mode voltage must always be maintained (VCM = (INxP + INxN)/2). This is to ensure the PGA is operating linearly and the voltages on each input do not force the output to swing too close to AVDD or AVSS. I suspect this can be an issue at higher LOFF current magnitudes. At AC, the data must be adjusted to account for the gain of the digital filter (see datasheet Figure 28). This becomes most important for fDR/4 LOFF frequency. Link to Article: Signal Chain Basics #149: How to accurately measure electrode impedance for lead-off detection in ECG systems - Planet Analog Regards, Ryan
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