ADS1291IRSMT datasheet(63/83 Pages) TI2

63 / 83 page

NORMALIZED

ACTUAL ´

ACTUAL

ADS1291, ADS1292, ADS1292R

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SBAS502C – DECEMBER 2011 – REVISED APRIL 2020

Product Folder Links: ADS1291 ADS1292 ADS1292R

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Typical Application (continued)

9.2.1 Design Requirements

This design requires the measurement of respiration and ECG signals on the right arm (RA) and left arm (LA)

with very low noise. Standard requirements are respiration impedance values ranging from 2 kΩ to 15 kΩ,

modulation clock frequencies of 32 kHz or 64 kHz, and noise levels of less than 10 µV.

9.2.2 Detailed Design Procedure

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).

where:

RACTUAL is the baseline body impedance

IACTUAL is the modulation current, as calculated by (VREFP – AVSS) divided by the impedance of the

modulation circuit

(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.

ADS1291IRSMT Datasheet(PDF) 63 Page - Texas Instruments