Datasheet

R
40.2kW
2
R =2.21kW
BASELINE
R
40.2kW
2
IN1P
RESP_MODP
RESP_MODN
IN1N
ADS1294R/6R/8R
10
9.5
9
8.5
8
7.5
7
6.5
6
NormalizedBaselineRespirationImpedance( )W
Channel1Noise( V )m
PP
3690 48452214 152588076 9155
Phase=112.5,PGA=4
Phase=112.5,PGA=3
Phase=135,PGA=4
Phase=135,PGA=3
20
18
16
14
12
10
8
6
NormalizedBaselineRespirationImpedance( )W
Channel1Noise( V )m
PP
3690 48452214 152588076 9155
Phase=135,PGA=3
Phase=135,PGA=2
Phase=157,PGA=3
Phase=157,PGA=2
R =
NORMALIZED
R I´
ACTUAL ACTUAL
29 Am
ADS1294, ADS1294R
ADS1296, ADS1296R
ADS1298, ADS1298R
SBAS459I –JANUARY 2010–REVISED JANUARY 2012 www.ti.com
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 29µA 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:
R
ACTUAL
is the baseline body impedance,
I
ACTUAL
is the modulation current, as calculated by (VREFP – AVSS) divided by the impedance of the
modulation circuit. (8)
For example, if modulation frequency = 32kHz, R
ACTUAL
= 3kΩ, I
ACTUAL
= 50µA, and R
NORMALIZED
= (3kΩ ×
50µA)/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 6.4µV
PP
. Low-pass filtering this signal with a high-order 2Hz cutoff can reduce the noise to less
than 600nV
PP
. The impedance resolution is 600nV
PP
/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.
74 Submit Documentation Feedback Copyright © 2010–2012, Texas Instruments Incorporated
Product Folder Link(s): ADS1294 ADS1294R ADS1296 ADS1296R ADS1298 ADS1298R