UG-1262
Rev. B | Page 153 of 312
VREF_2.5V
AMP 0
LPTIA0
R
LOAD
R
TIA
LP DUAL
OUTPUT
DAC
VBIAS0
SE0
RE0
CE0
MUX
ADC
VZERO0
AIN4_LPF0
CAP_POT0
RC0_0
RC0_1
SW1
SW0
SW5
SW7
SW6
SW2
SW8
SW4
SW10
SW3
RL[2:0]
GAIN[4:0]
RF[2:0]
SW9
SW11
10kΩ
10kΩ
RC00
SW12
SW13
LPREF
LPBUF
SW14
VZERO1
SW15
RE0
CHANNEL 0
SW[0:15] CORRESPONDING TO LPTIASW[0:15]
RECOMMENDED SETTINGS FOR TYPICAL USE CASE:
GAS SENSOR NORMAL DC MEASUREMENT
LPTIASWx = 0x302C,
CLOSE SW2 TO SHORT OUTPUT OF POTENTIOSTAT AMPLIFIER TO CE0
CLOSE SW3 TO APPLY RC FILTER TO SENSOR, EXTERNAL CAPACITOR CONNECTED BETWEEN CAP_POT0 AND CE0
CLOSE SW5 TO CONNECT C
TIA
, WHICH IS AN EXTERNAL CAPACITOR CONNECTED BETWEEN RC0_0 AND RC0_1
CLEAR SW7 TO SHORT UPPER SWITCH TO CONNECT R
TIA
TO RF RESISTOR.
RF RESISTOR WITH EXTERNAL CAPACITOR CONNECTED BETWEEN AIN_LPF0 AND GND ACTS AS A LOW-PASS FILTER
1667
5-
02
7
+
–
+
–
Figure 38. Switches for Low Power Potentiostat Amplifier and Low Power TIA to Measure DC Current from SE0 Node Using Low Power TIA
(3-Wire Electrochemical Sensor Configuration)
PULSE TEST (CHRONOAMPEROMETRY)
The pulse test involves disturbing the normal bias voltage of an electrochemical sensor and monitoring its output current response. The
pulse test is usually used to check the responsiveness of the sensor. In the case of an electrochemical gas sensor, the pulse test checks that
the passage of charge between electrodes through the internal electrolyte during oxidation and reduction is operating properly. Typically,
the current increases sharply and quickly with the step in the sensor bias voltage. If the current step response is slow, there can be an
issue with the sensor electrolyte.
Implementing Pulse Test Using Low Power TIA
The potentiostat and the ADC are assumed to be initially configured using the steps in the Measuring a DC Current Output section.
Current measurement before, during, and after the step is also described in this section.
The V
BIAS
DAC level is stepped by 5 mV to 10 mV higher than it typically is to stimulate the sensor. A typical step duration is 100 ms.
To configure the switches in potentiostat mode for the pulse test, set LPTIASWx, Bits[11:0] = 0x014 to use the low power TIA, write to
the LPDACDAT0 register to change the VBIAS0 output voltage, and write to the LPDACDAT1 register to change the VBIAS1 output voltage.
Implementing Pulse Test Using High Speed TIA
In this test, the user has the option of using the high speed TIA channel to measure the current of the sensor. In this case, the SE0 or SE1 pin is
routed to the high speed TIA instead of the low power TIA. Figure 40 shows the signal path.
The following code connects the SE0 pin to the high speed TIA and disconnects the SE0 pin from the low power TIA0:
pADI_AFE->LPTIASW0 &= 0xF000; // Mask SW12 to 15 control bits
pADI_AFE->LPTIASW0 = 0x94; // Disconnect SE from LPTIA -ve input and connect to HSTIA
// Configure SW0 to SW11 for HSTIA PULSE/RAMP test setting
pADI_AFE->LPDACCON0 |= 0x20; // Configure LPDAC0 switches for Diagnostic mode
pADI_AFE->LPDACSW0 = 0x32; // Disconnect the VBIAS0 and VZERO0 from external caps
pADI_AFE->SWCON &=
(!BITM_AFE_SWCON_SWSOURCESEL); // Step 1: to write to T-Switch control register