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UG-1262
Rev. B | Page 159 of 312
LOW POWER
LOOP:
LPTIASWX = 0x302C
NORMAL
OPERATION
D
EXCITATION
AMPLIFIER
LOOP
CE0
R
C
R
E
C
C
SE0
R
W
R
E
C
W
RE0
C = 0.1µF
CE0
RE0
SE0
RC0_0
RC0_1
VBIAS0
C = 0.1µF
RC0_2
RCAL0
RCAL1
R
CAL
= 200Ω
HPTIA
+
–
HPTIACON[1:0] = 01
VZERO0
VZERO0
VZERO1
ADCVBIAS_CAP 1.1V
LPTIASWx[12] = 1
C = 0.1µF
LPTIASWx[2] = 1
Px SWITCHES:
CLOSE PR0
OPEN OTHER
Px SWITCHES
SWCON[7:4] = 0001
PGA
12-BIT
DAC
RCF
DACP
DACN
DIG
WAVE
GEN
P
Nx SWITCHES:
CLOSE NR1
OPEN OTHER
Nx SWITCHES
SWCON[11:8] = 1010
DUAL
OUTPUTS
12-BIT
DAC
VBIAS0
VZERO0
C = 0.1µF
VZERO0
LPTIASWx[13] = 1
LPTIASWx[3] = 1
+
–
LPTIA
V
ZERO
AMP
–
+
VBIAS0
MU
X
16-BIT
ADC
160kSPS
PGA
BUFFER
BUFFER
AAF
DFT
MUXSELP_HPTIA_P
MUXSELP_HPTIA_N
ADCCON[12:0]
SENSOR IS BIASED
P
N
N
Tx SWITCHES:
CLOSE TR0
OPEN OTHER
Tx SWITCHES
SWCON[15:12] = 1000
Dx SWITCHES:
CLOSE DR0
OPEN OTHER
Dx SWITCHES
SWCON[3:0] = 0001
16
67
5
-02
9
Figure 41. Step Three of Impedance Measurement, R
LOAD02
+ R
SENSOR
Measurement
Step 3: Measure R
LOAD02
The counter electrode and reference electrode of the electrochemical sensor are floating during this stage of the measurement sequence.
The main differences between Step 2 and Step 3 are as follows:
The reference electrode is disconnected from the excitation amplifier P node.
The counter electrode is disconnected from the excitation amplifier D node and is connected directly to R
LOAD02
.
The excitation amplifier D node is connected to the working electrode node.
Therefore, the excitation signal is applied to R
LOAD02
and the sensor is floating, as shown in Figure 42. The ac excitation loop D node,
P node, and R
LOAD02
are shorted. The ac excitation loop N node, TIA T node, and R
LOAD02
are shorted. Measure the stimulus via the high
speed TIA. The ADC signal chain and DFT are used to calculate the impedance of the R
LOAD02
resistor.
Configure the Tx, Dx, Nx, and Px switches appropriately, as described in the following example code:
AfeSwitchDPNT(SWID_D7_WE0,SWID_P7_WE0,SWID_N5_SE0RLOAD,SWID_T5_SE0RLOAD|SWID_T9);
// Connect Excitation Amplifier D to the WE Electrode
// Connect Excitation Amplifier P to SE0
// Connect Excitation Amplifier N to SE0 via RLOAD02
// Connect HSTIA to SE0 via RLOAD02. Close T9