ADS1294CZXGR datasheet(91/114 Pages) TI2

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NORMALIZED

ACTUAL

29 A

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

40.2 kW

2.21 k

BASELINE

40.2 kW

IN1P

RESP_MODP

RESP_MODN

IN1N

ADS129xR

ADS1294, ADS1294R, ADS1296, ADS1296R, ADS1298, ADS1298R

www.ti.com

SBAS459K – JANUARY 2010 – REVISED AUGUST 2015

Figure 99 shows a respiration test circuit. Figure 100 and Figure 101 plot noise on channel 1 for the ADS129xR

as baseline impedance, gain, and phase are swept. The x-axis is the baseline impedance, normalized to a 29-

μA

modulation current (see Equation 11).

Figure 99. Respiration-Noise Test Circuit

BW = 150 Hz, respiration modulation clock = 32 kHz

BW = 150 Hz, respiration modulation clock = 64 kHz

Figure 100. Channel-1 Noise vs Impedance for 32-kHz

Figure 101. Channel-1 Noise vs Impedance for 64-kHz

Modulation Clock and Phase

where

•

RACTUAL is the baseline body impedance.

•

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

modulation circuit.

(11)

For example, assume that:

•

Modulation frequency = 32 kHz

•

RACTUAL = 3 kΩ

•

IACTUAL = 50 μA

•

RNORMALIZED = (3 kΩ × 50 μA) / 29 μA = 5.1 kΩ

Referring to Figure 100 and Figure 101, gain = 4 and phase = 112.5° yield the best performance at 6.4

μVPP.

Low-pass filtering this signal with a high-order, 2-Hz cutoff reduces the noise to less than 600 nVPP. The

impedance resolution is 600 nVPP / 29 μA = 20 mΩ. When the modulation frequency is 32 kHz, use gains of 3

and 4, and a phase of 112.5° and 135° for best performance. When the modulation frequency is 64 kHz, use

gains of 2 and 3 and phase of 135° and 157° for best performance.

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ADS1294CZXGR Datasheet(PDF) 91 Page - Texas Instruments

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