AD5940 datasheet(30/143 Pages) AD

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Data Sheet

AD5940/AD5941

LOW POWER DAC

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Rev. D | 30 of 143

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).

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 VBIAS0

pin by configuring the SW12 switch (see Figure 22). An external

filtering capacitor can be connected to the VBIAS0 pin.

The 6-bit output sets the voltage to the positive low power TIA inter-

nal 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 22). 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.

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.

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 18.

The resistor matching between the 12-bit and 6-bit DACs means 64

LSB12 (VBIAS0) is equal to one LSB6 (VZERO0).

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

12‐BIT_DAC_LSB= 2.2 V212 − 1=537.2 µV (3)

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 18). 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

(4)

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 LPDAC-

DAT0, Bits[17:12].

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.

The following code demonstrates how to correctly set the LPDAC-

DAT0 value:

SEQ_WR(REG_AFE_LPDACDAT0, 0x1234);

SEQ_WAIT(10); // Wait 10 clocks for LPDACDAT0

to update

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.

LOW POWER DAC SWITCH OPTIONS

There are a number of switch options available that allow the

user to configure the low power DAC for various modes of opera-

tion. These switches facilitate different use cases, such as electro-

chemical impedance spectroscopy. Figure 22 shows the available

switches, labeled SW0 to SW4. These switches are controlled

either automatically via Bit 5 in the LPDACCON0 register, or individ-

ually via the LPDACSW0 register

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 LPDAC-

CON0, 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 electro-

chemical 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.

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].

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