Datasheet
R =
NORMALIZED
R I
ACTUAL
´
ACTUAL
30 Am
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2.2 7.2 12.2 15
Normalized Baseline Respiration Impedance (kΩ)
Channel 1 Noise (µV)
PGA=2, PHASE = 135
PGA = 3, PHASE = 135
PGA = 2, PHASE = 157.5
PGA = 3, PHASE = 157.5
Data Rate = 125Hz
Respiration Modulation Clock = 64kHz
G059
1
2
3
4
5
6
7
8
9
10
2.2 5.2 8.2 11.2 14.2
Normalized Baseline Respiration Impedance (kΩ)
Channel 1 Noise (µV)
PGA=3, PHASE = 112.5
PGA = 4, PHASE = 112.5
PGA = 3, PHASE = 135
PGA = 4, PHASE = 135
Data Rate = 125Hz
Respiration Modulation Clock = 32kHz
G058
R
40.2 kW
2
R = 2.21 k
BASELINE
W
R
40.2 kW
2
IN1P
RESP_MODP
RESP_MODN
IN1N
ADS1292R
ADS1291
ADS1292
ADS1292R
www.ti.com
SBAS502B –DECEMBER 2011–REVISED SEPTEMBER 2012
Figure 57 shows a respiration test circuit. Figure 58 and Figure 59 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 11).
Figure 57. Respiration Noise Test Circuit Figure 58. Channel 1 Noise versus Impedance for
32-kHz Modulation Clock and Phase
(BW = 32 Hz, Respiration Modulation Clock = 32
kHz)
Figure 59. Channel 1 Noise versus Impedance for 64-kHz Modulation Clock and Phase
(BW = 32 Hz, Respiration Modulation Clock = 64 kHz)
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. (11)
For example, if modulation frequency = 32 kHz, R
ACTUAL
= 3 kΩ, I
ACTUAL
= 50 µA, and R
NORMALIZED
= (3 kΩ × 50
µA) / 29 µA = 5.1 kΩ.
Referring to Figure 58 and Figure 59, it can be noted that gain = 4 and phase = 112.5° yield the best
performance at 4.6 µV
PP
. Low-pass filtering this signal with a high-order 2-Hz cutoff can reduce the noise to less
than 1200 nV
PP
. The impedance resolution is 1200 nV
PP
/ 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.
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