UG-1262
Rev. B | Page 157 of 312
Exiting Cyclic Voltammetry Mode
When exiting voltammetry mode to return to biasing the sensor normally or to resume taking dc measurements, take care to minimize
the sensor settling time, as follows:
1.
Before adjusting the switches for a normal dc bias, reconfigure the low power DAC outputs to their required dc levels. Write to the
LPDACDAT0 register or the LPDACDAT1 register.
2.
Disconnect the high speed TIA circuitry from the sensor. Open the T10 and T11 switches of the switch matrix by clearing
TSWFULLCON, Bits[10:9] = 0b00. Disconnect the high speed TIA gain resistors by setting DE0RESCON and DE1RESCON to
0xFF. Optionally, to save power, power down the high speed TIA by clearing AFECON, Bit 11 = 0.
3.
Configure the low power DAC switches for normal dc measurements. Write LPDACSWx = 0. Clear LPDACCONx, Bit 5 = 0 for
normal switch operation around the low power DACs. Set up LPDACCONx, Bits[4:3] to set V
BIAS
and V
ZERO
to 12-bit or 6-bit mode.
4.
After configuring the low power DAC, set up the low power TIA. Set the low power TIA switches for normal operation. Optionally,
the SW0 and SW1 switches around the low power TIA can be closed for a short duration to allow the low power TIA to charge up
the sensor, allowing the SE0 node of the sensor to settle faster to the VZEROx output bias voltage setting. See Figure 16 to locate
SW0 and SW1 in the low power TIA circuitry.
AC IMPEDANCE MEASUREMENT WHILE MAINTAINING DC BIAS TO THE SENSOR
The following sections detail an example configuration setup for an ac impedance measurement of <80 kHz. The impedance
measurement technique is a ratiometric measurement where an impedance measurement is completed on a known, fixed external R
CAL
separately to the measurement of the impedance of the sensor.
In this example configuration, the impedance measurement is taken via the SE0 electrode using Electrochemical Sensor Channel 0. In
Figure 41, Figure 42, and Figure 43, an ac signal of amplitude ±10 mV p-p is coupled onto a dc sensor biased to 0 V (V
BIAS
− V
ZERO
= 0).
However, the dc sensor common-mode voltage is 1.1 V. The ac signal amplitude can be increased to 15 mV.
The high speed DAC full-scale output with the attenuator on is approximately ±607 mV/40 = ±15.1 mV p-p. The voltage to the ADC is
calculated as ±15.1 mV/R
LOAD
× R
TIA
. R
LOAD02
is fixed at 100 Ω, which gives a current of approximately 150 μA across R
TIA
. The testing
featured in this reference manual is designed for an ADC voltage of ±750 mV. As such, set R
TIA
= 5 kΩ.
The impedance measurement is performed in five steps, detailed in the following sections. The following steps assume a sensor with a 0 V bias
requirement between the reference electrode and working electrode of a 3-electrode electrochemical sensor.
Step 1: Initialize
for Impedance Measurement
The electrochemical sensor remains biased via the low power potentiostat loop. To configure the ADC and high speed DAC operating
mode, perform the following steps:
1.
Configure the ADC and DAC circuits for low power mode to minimize current consumption by clearing PMBW, Bit 0 = 0.
2.
Set Bit 20, Bit 15, Bit 14, and Bits[11:5] of the AFECON register to 1 to enable high speed DAC and ADC references, the high speed
DAC excitation amplifier and buffer, and DFT hardware accelerator. The waveform generator also must be enabled. Bit 21 is set
when using a sensor with a dc bias voltage >0 V.
3.
Enable chop mode on the ADC input buffer when measuring signals <80 kHz. ADCBUFCON, Bits[3:0] = 0x4 enable the ADC
front-end buffer and PGA chop. When measuring signals >80 kHz (as in high power mode), disable chopping on the ADC input
buffer. ADCBUFCON, Bits[3:0] = 0xF disable ADC input chopping.
To set up the ADC, configure and calibrate the ADC. Ideally, calibrate the ADC as a current input (high speed TIA) with the desired R
TIA
and ADC PGA gain settings. See the ADC Calibration section for further details. Configure the ADC output data to go to the DFT block
and configure the number of samples used by the DFT block in the DFTCON register.
To set up the high speed DAC, first turn on the high speed DAC. Use the waveform generator to generate a sine wave of the desired
frequency and amplitude by appropriately configuring the following registers:
HSDACCON. Configure the gain settings.
PMBW, Bits[3:2]. Use this register to configure the reconstruction filter settings.
WGCON. Main waveform control register.
WGFCW. Configures the frequency of the ac sine wave. If necessary, adjust the WGPHASE, WGOFFSET, and
WGAMPLITUDE registers.
DACDCBUFCON. Select low power DAC0 or low power DAC1 as the dc level of the common-mode voltage excitation amplifiers.