UG-1262
Rev. B | Page 109 of 312
Ensure CLKSEL, Bits[1:0] selects a 32 MHz clock source. For example, an internal high speed oscillator is selected if CLKSEL,
Bits[1:0] = 00. Ensure that the system clock divide ratio is 1 (CLKCON0, Bits[5:0] = 0 or 1).
If the internal high speed oscillator is selected as the system clock source, ensure the 32 MHz option is selected. Clear HPOSCCON,
Bit 2 = 0.
Hibernate Mode
When configuring the high speed DAC for hibernate mode, take note of the following requirements and features:
enters hibernate mode, the clocks to the high speed DAC circuits are clock gated to save power.
When the
is in active mode but the high speed DAC is not required, disable the high speed DAC circuits to save power.
To do this, clear Bit 20, Bit 14, Bit 10, Bit 9, and Bit 6 in the AFECON register. Leave Bit 5 set if the ADC is in use, because Bit 5
controls the high power reference source.
RECOMMENDED CONFIGURATION IN HIBERNATE MODE
To minimize leakage on the switches connected to the excitation amplifier P and N nodes and to minimize leakage on the high speed
TIA, tie the switches to the internal 1.8 V LDO regulator generated voltage, as follows:
Close the PL and PL2 switches. PSWFULLCON, Bits[14:13] = 11.
Close the NL and NL2 switches. NSWFULLCON, Bits[11:10] = 11.
In hibernate mode, assume that only the dc bias voltage from the low power amplifiers is required for the sensor.
HIGH SPEED DAC FILTER OPTIONS
The high speed DAC has a configurable reconstruction filter on its output stage. It is important that this filter be configured
appropriately depending on the output signal frequency of the DAC. PMBW, Bits[3:2] configure the 3 dB cutoff frequency of the filters.
Ensure the cutoff frequency is higher than the required DAC output frequency. The output filter cutoff frequency details are as follows:
PMBW, Bits[3:2] = 01 for optimal performance if the DAC output signal frequency is <80 kHz.
PMBW, Bits[3:2] = 10 for optimal performance if the DAC output signal frequency is <100 kHz.
PMBW, Bits[3:2] = 11 for optimal performance if the DAC output signal frequency is <250 kHz.
HIGH SPEED DAC OUTPUT ATTENUATION OPTIONS
Scaling options for the high speed DAC output exist to modify the output signal amplitude to the sensor. The output of the 12-bit DAC
string before any attenuation or gain is approximately ±300 mV. At the DAC output, there is a 1 or 0.2 gain stage that is controlled by
HSDACCON, Bit 0. At the PGA stage, there is a 2 or 0.25 gain option that is controlled by HSDACCON, Bit 12.
It is recommended to only use the maximum and minimum overall gain options with the lowest DAC code of 0x200 and maximum
DAC code of 0xE00. The characterized DAC output ranges are as follows:
HSDACCON, Bit 12 = 0 and HSDACCON, Bit 0 = 0. The overall gain is 2. This setting gives a full-scale voltage of approximately
±607 mV to the sensor for HSDACDAT Code 0x200 to Code 0xE00.
HSDACCON, Bit 12 = 1 and HSDACCON, Bit 0 = 1. This setting gives a full-scale voltage of approximately ±15.1 mV to the sensor
for HSDACDAT Code 0x200 to Code 0xE00.
COUPLING AN AC SIGNAL FROM HIGH SPEED DAC ONTO THE DC LEVEL SET BY LOW POWER DAC
The
contains two independent low power potentiostat channels that configure two separate electrochemical sensors. In
normal operation, the bias voltage of the sensor between the reference electrode and working electrode is set directly by the low power
DAC outputs, VBIASx and VZEROx. See Figure 15 for the setup.
In normal operation, the high speed DAC circuits are not used. However, to connect an ac signal onto the counter electrode, the
potentiostat amplifier must be disconnected from the sensor and the whole signal must be applied from the high speed DAC excitation
amplifier output. The bias voltage setting of the sensor must also be completed by the high speed TIA, rather than the low power TIA.
The ac signal generated by the high speed DAC is coupled onto the dc voltage level set by the low power DAC for the channel under test.
The DACDCBUFCON register, Bit 1 selects LPDAC0 or LPDAC1 as the dc level voltage source that couples to the high speed DAC.
The DAC dc buffers shown in Figure 24 are enabled by setting AFECON, Bit 21 = 1. This setting feeds the sensor dc bias voltage to the
excitation amplifier. For the appropriate LPDACx channel, set LPTIASWx, Bits[11:0] = 0x180 to set the low power TIA and potentiostat
switches for ac impedance measurement mode.