Reference Manual ADuCM356
USE CASE CONFIGURATIONS
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ment technique is a ratiometric measurement where an impedance
measurement is completed on a known, fixed external R
CAL
sepa-
rately 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 ADuCM356 for Impedance
Measurement
The electrochemical sensor remains biased via the low-power po-
tentiostat loop. To configure the ADC and high-speed DAC operat-
ing 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:
â–º HSDACCON. Configure the gain settings.
â–º PMBW, Bits[3:2]. Use this register to configure the reconstruction
filter settings.
â–º WGCON. Main waveform control register.
â–º WGFCW. Configures the frequency of the AC sine wave. If
necessary, adjust the WGPHASE, WGOFFSET, and WGAMPLI-
TUDE registers.
â–º DACDCBUFCON. Select low-power DAC0 or low-power DAC1
as the DC level of the common-mode voltage excitation amplifi-
ers.
After turning on the high-speed DAC, calibrate the high-speed
DAC output if necessary (optional). The high-speed DAC can be
calibrated to remove the offset error by setting the output code to
0x800, as follows:
â–º Connect the excitation amplifier to R
CAL
.
â–º Measure the differential voltage across R
CAL
by selecting the
ADC inputs as the N node and P node of the excitation amplifier:
ADCCON, Bits[12:0] = 0x1424. There are four offset calibration
registers, DACOFFSET, DACOFFSETATTEN, DACOFFSETHP,
and DACOFFSETATTENHP. The relevant register depends on
the excitation amplifier gain setting and whether the device is in
low or high-power mode. See Table 127.
To set up the potentiostat circuit for impedance measurement,
ensure that the low-power DACs are on, with the VBIAS0 and
VZERO0 outputs set to give the same voltage on the RE0 and SE0
pins. By default, leave the electrochemical sensor fully biased by
the low-power potentiostat loop. Using a potentiostat circuit for im-
pedance measurement uses the same settings as hibernate mode
and for measuring a DC current output (LPTIASW0 = 0x302C).
The following code is an example of how to configure the switches
in the low-power loop:
AfeLpTiaSwitchCfg (channel,
SWMODE_NORM); // Low Power
Loop Normal switch settings (0x302C)
To set up the high-speed TIA for impedance measurement, perform
the following steps:
1. Select the R
TIA
values, power setting, and bias voltage source
for the high-speed TIA. The HSRTIACON register configures
the high-speed TIA parallel capacitor and the main R
TIA
resistor.
The HSTIACON register configures the power mode for the
high-speed TIA. Clear HSTIACON, Bits[6:2] = 00000 for impe-
dance measurements ≤80 kHz. Bits[1:0] are 00.
2. Select the high-speed TIA as the ADC input, as modeled in the
following code.
AfeAdcChan(MUXSELP_HSTIA_P,
MUXSELN_HSTIA_N); //Select
HSTIA output as ADC input versus HSTIA_N to
the ADC
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