Figure 69 shows a respiration test circuit. Figure 70 and Figure 71 plot noise on channel 1 for the ADS1292R as baseline impedance, gain, and phase are swept. The x-axis is the baseline impedance, normalized to a 30-µA modulation current (as shown in Equation 10).
For example, if modulation frequency = 32 kHz, RACTUAL = 3 kΩ, IACTUAL = 50 µA, and RNORMALIZED = (3 kΩ × 50 µA) / 29 µA = 5.1 kΩ.
Referring to Figure 70 and Figure 71, it can be noted that gain = 4 and phase = 112.5° yield the best performance at 4.6 µVPP. Low-pass filtering this signal with a high-order 2-Hz cutoff can reduce the noise to less than 1200 nVPP. The impedance resolution is 1200 nVPP / 30 µA = 40 mΩ.
When the modulation frequency is 32 kHz, gains of 3 and 4 and phase of 112.5° and 135° are recommended.
When the modulation frequency is 64 kHz, gains of 2 and 3 and phase of 135° and 157° are recommended for best performance.