ADS1291_14 Datasheet, PDF (59/75 Pages) Texas Instruments – Power, 2-Channel, 24-Bit Analog Front-End for Biopotential Measurements

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ADS1291
ADS1292
ADS1292R
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).
R₂
40.2 kW
R_BASELINE = 2.21 kW
R₂
40.2 kW
ADS1292R
IN1P
RESP_MODP
RESP_MODN
IN1N
Figure 57. Respiration Noise Test Circuit
10
Data Rate = 125Hz
9 _{Respiration Modulation Clock = 32kHz}
8
7
6
5
4
PGA=3, PHASE = 112.5
3
PGA = 4, PHASE = 112.5
2
PGA = 3, PHASE = 135
PGA = 4, PHASE = 135
1
2.2
5.2
8.2
11.2
14.2
Normalized Baseline Respiration Impedance (kΩ) _G058
Figure 58. Channel 1 Noise versus Impedance for
32-kHz Modulation Clock and Phase
(BW = 32 Hz, Respiration Modulation Clock = 32
kHz)
15
14 Data Rate = 125Hz
13 ^{Respiration Modulation Clock = 64kHz}
12
11
10
9
8
7
6
5
PGA=2, PHASE = 135
4
PGA = 3, PHASE = 135
3
PGA = 2, PHASE = 157.5
2
PGA = 3, PHASE = 157.5
1
2.2
7.2
12.2
15
Normalized Baseline Respiration Impedance (kΩ) _G059
Figure 59. Channel 1 Noise versus Impedance for 64-kHz Modulation Clock and Phase
(BW = 32 Hz, Respiration Modulation Clock = 64 kHz)
R = _NORMALIZED
R_ACTUAL ´ I_ACTUAL
30 mA
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.
Copyright © 2011–2012, Texas Instruments Incorporated
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59
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