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Hardware Reference Manual
UG-1262
Rev. B | Page 161 of 312
LOW POWER
LOOP:
LPTIASWx =
0x302C
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
LPTIASW[12] = 1
C = 0.1µF
LPTIASW[2] = 0
Px SWITCHES:
CLOSE PR0
OPEN OTHER
P SWITCHES
SWCON[7:4] = 0001
PGA
12-BIT
DAC
RCF
DACP
DACN
VZERO0
VBIAS0
DIG
WAVE
GEN
P
Nx SWITCHES:
CLOSE N5
OPEN OTHER
N SWITCHES
SWCON[11:8] = 0101
DUAL
OUTPUTS
12-BIT
DAC
VBIAS0
VZERO0
C = 0.1µF
VZERO0
LPTIASW[13] = 1
+
–
LPTIA
V
ZERO
R
LOAD02
AMP
–
+
VBIAS0
MU
X
ADCCON[12:0]
SENSOR IS FLOATING
P
N
N
Tx SWITCHES:
CLOSE T5, T9
OPEN OTHER T
SWITCHES
SWCON[15:12] = 0101
SWCON[17] = 1
Dx SWITCHES:
CLOSE D5
OPEN OTHER
D SWITCHES
SWCON[3:0] = 0101
LPTIASW[3] = 0
LPTIASW[8] = 1
ULPTIASW[13] = 1
V
ZERO
10mV p-p
V
BIAS
16675
-031
16-BIT
ADC
160kSPS
PGA
BUFFER
BUFFER
AAF
DFT
MUXSELP_HPTIA_P
MUXSELP_HPTIA_N
Figure 43. R
CAL
Measurement
The R
CAL
measurement is used to calibrate the R
SENSOR
+ R
LOAD
measurement in Step 3 by using ratiometric measurements. As shown in
Figure 43, the electrochemical sensor is biased via the low power potentiostat and low power TIA amplifiers for Step 4. The waveform
generator and the high speed DAC generate a 10 mV amplitude sine waveform.
The ac excitation loop D node, P node, and RCAL0 pin are shorted by setting the Dx switch and Px switch to close the DR0 and PR0 switches
(SWCON, Bits[7:0] = 0x11). Other Dx switches and Px switches are open. The ac excitation loop N node, high speed TIA T node, and
RCAL1 pin are shorted by setting the Nx switches and Tx switches to close the NR1 and TR1 switches (SWCON, Bits[15:8] = 0x85).
Close T9 to select R
TIA02
(SWCON Bit 17 = 1). Other Nx switches and Tx switches are open. The stimulus is added to the 200 Ω R
CAL
and
the high speed TIA. The ADC signal chain and DFT block are used to calculate the impedance of R
CAL
.
Set the low power loop switches to allow the excitation amplifier and the high speed TIA to set the dc bias voltage level of the sensor. The
following example code demonstrates how to configure the switches in the low power loop:
AfeLpTiaSwitchCfg (channel, SWMODE_AC); //Low Power Loop for AC impedance switch settings (0x3180)
Step 5: Calculate the Impedance of Electrochemical Sensor Sensing Electrode Node
Calculate the magnitude and phase of Step 2, Step 3, and Step 4. The following code example and equations show how to calculate the
magnitude and phase of each step:
// Get RMS result of each step based on DFT REAL and IMAG outputs
DFT_Mag[i] = DFTREAL* DFTIMAG* DFTIMAG;
DFT_Mag[i] = sqrt(DFT_Mag[i]);
// Use atan2 function to get phase result of each step based on DFT REAL and IMAG outputs
Phase[i] = atan2(DFTIMAG, DFTREAL); // returns value between -pi to +pi (radians) of
ATAN2(IMAG/Real)
In Step 4,
( )
( )
(
)
CAL
CAL
V s
I s
Z
magnitude
(10)