ADS1294 datasheet(75/88 Pages) TI

75 / 88 page

40.2kW

=2.21kW

BASELINE

40.2kW

IN1P

RESP_MODP

RESP_MODN

IN1N

ADS1294R/6R/8R

9.5

8.5

7.5

6.5

NormalizedBaselineRespirationImpedance( )

3690

4845

2214

15258

8076

9155

Phase=112.5,PGA=4

Phase=112.5,PGA=3

Phase=135,PGA=4

Phase=135,PGA=3

NormalizedBaselineRespirationImpedance( )

3690

4845

2214

15258

8076

9155

Phase=135,PGA=3

Phase=135,PGA=2

Phase=157,PGA=3

Phase=157,PGA=2

NORMALIZED

ACTUAL

29 A

ADS1294, ADS1294R

ADS1296, ADS1296R

ADS1298, ADS1298R

www.ti.com

SBAS459H

– JANUARY 2010 – REVISED OCTOBER 2011

Figure 70 shows a respiration test circuit. Figure 71 and Figure 72 plot noise on channel 1 for the

ADS1294R/6R/8R as baseline impedance, gain, and phase are swept. The x-axis is the baseline impedance,

normalized to a 29

µA modulation current (as shown in Equation 8).

Figure 70. Respiration Noise Test Circuit

Figure 71. Channel 1 Noise versus Impedance for

32kHz Modulation Clock and Phase

(BW = 150Hz, Respiration Modulation Clock =

32kHz)

Figure 72. Channel 1 Noise versus Impedance for 64kHz Modulation Clock and Phase

(BW = 150Hz, Respiration Modulation Clock = 64kHz)

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.

(8)

For example, if modulation frequency = 32kHz, RACTUAL = 3kΩ, IACTUAL = 50µA, and RNORMALIZED = (3kΩ ×

µA)/29µA = 5.1kΩ.

Referring to Figure 71 and Figure 72, it can be noted that gain = 4 and phase = 112.5

° yield the best

performance at 6.4

µVPP. Low-pass filtering this signal with a high-order 2Hz cutoff can reduce the noise to less

than 600nVPP. The impedance resolution is 600nVPP/29µA = 20mΩ.

When the modulation frequency is 32kHz, gains of 3 and 4 and phase of 112.5

° and 135° are recommended.

When the modulation frequency is 64kHz, gains of 2 and 3 and phase of 135

° and 157° are recommended for

best performance.

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