Data Sheet
AD5940
LOW POWER DAC
The ultra low power DAC is a dual output string DAC that sets
the bias voltage of the sensor. There are two output resolution
formats: 12-bit resolution (VBIAS0) and 6-bit resolution (VZERO0).
If the system clock is 16 MHz, LPDACDAT0 takes 10 clock cycles
to update. If system clock is 32 kHz, LPDACDAT0 takes one clock
cycle to update. Take these values into consideration when
using the sequencer.
In normal operation, the 12-bit output sets the voltage on the
reference electrode and counter electrode pins, RE0 and CE0,
via the potentiostat circuit. This voltage can also be sent to the
The following code demonstrates how to correctly set the
LPDACDAT0 value:
V
BIAS0 pin by configuring the SW12 switch (see Figure 19). An
SEQ_WR(REG_AFE_LPDACDAT0, 0x1234);
external filtering capacitor can be connected to the VBIAS0 pin.
SEQ_WAIT(10); // Wait 10 clocks for LPDADAT0
to update
The 6-bit output sets the voltage to the positive low power TIA
internal node that connects to the ADC mux, LPTIA_P. The
voltage on the sense electrode is equal to this pin. This voltage is
referred to as VZERO0 and can be connected to the VZERO0 pin by
configuring the SW13 switch (see Figure 19). In diagnostic
mode, the VZERO0 output can also be connected to the high
speed TIA by setting Bit 5 in the LPDACCON0 register to 1.
SEQ_SLP();
Optionally, the waveform generator described in the Waveform
Generator section can be used as the DAC codes source for the
low power DAC. When using the waveform generator with the
low power DAC, ensure that the settling time specification of
the low power DAC is not violated. The system clock source
must be the 32 kHz oscillator. This feature is provided for ultra
low power, always on, low frequency measurements, such as
skin impedance measurements where the excitation signal is
approximately 100 Hz and system power consumption needs to
be <100 μA.
The low power DAC reference source is a low power, 2.5 V
reference.
The low power DACs are made up of two 6-bit string DACs.
The main 6-bit string DAC provides the VZERO0 DAC output,
and is made up of 63 resistors. Each resistor is the same value.
LOW POWER DAC SWITCH OPTIONS
The main 6-bit string with the 6-bit subDAC provides the VBIAS0
DAC output. In 12-bit mode, the MSBs select a resistor from the
main string DAC. The top end of this resistor is selected as the
top of the 6-bit subDAC, and the bottom end of the selected
resistor is connected to the bottom of the 6-bit subDAC string,
as shown in Figure 16.
There are a number of switch options available that allow the
user to configure the low power DAC for various modes of
operation. These switches facilitate different use cases, such as
electrochemical impedance spectroscopy. Figure 15 shows the
available switches, labeled SW0 to SW4. These switches are
controlled either automatically via Bit 5 in the LPDACCON0
register, or individually via the LPDACSW0 register
The resistor matching between the 12-bit and 6-bit DACs
means 64 LSB12 (VBIAS0) is equal to one LSB6 (VZERO0).
When LPDACCON0, Bit 5, is cleared, the switches are configured
for normal mode. The SW2 switch and the SW3 switch are
closed and the SW0, SW1, and SW4 switches are open. When
LPDACCON0, Bit 5, is set, the switches are configured for
diagnostic mode. The SW0 switch and the SW4 switch are
closed and the remaining switches are open. This feature is
designed for electrochemical use cases, such as continuous
glucose measurement where, in normal mode, the low power
TIA measures the sense electrode. Then, in diagnostic mode,
the high speed TIA measures the sense electrode. By switching
the VZERO0 voltage output from the low power TIA to the high
speed TIA, the effective bias on the sensor, VBIAS0 − VZERO0, is
unaffected. Using the high speed TIA facilitates high bandwidth
measurements, such as impedance, ramp, and cyclic
voltammetry.
The output voltage range is not rail to rail. Rather, it ranges
from 0.2 V to 2.4 V for the 12-bit output of the low power DAC.
Therefore, the LSB value of the 12-bit output (12-BIT_
DAC_LSB) is
2.2 V
212 −1
12-BIT_DAC_LSB =
= 537.2 µV
The 6-bit output range is from 0.2 V to 2.366 V. This range is
not 0.2 V to 2.4 V because there is a voltage drop across R1 in
the resistor string (see Figure 16). The LSB value of the 6-bit
output (6-BIT_DAC_LSB) is
6-BIT_DAC_LSB = 12-BIT_DAC_LSB × 64 = 34.38 mV
To set the output voltage of the 12-bit DAC, write to
LPDACDAT0, Bits[11:0]. To set the 6-bit DAC output voltage,
write to LPDACDAT0, Bits[17:12].
Use the LPDACSW0 register to control the switches individually.
LPDACSW0, Bit 5, must be set to 1. Then, each switch can be
individually controlled via LPDACSW0, Bits[4:0].
Rev. 0 | Page 27 of 130
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