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Data Sheet

AD5940

Rev. 0 | Page 29 of 130

RELATIONSHIP BETWEEN THE 12-BIT AND 6-BIT

OUTPUTS

The 12-bit and 6-bit outputs are mostly independent. However,

the selected 12-bit value does have a loading effect on the 6-bit

output that must be compensated for in user code, particularly

when the 12-bit output level is greater than the 6-bit output.

When the 12-bit output is less than the 6-bit output,

12-Bit DAC Output Voltage = 0.2 V + (LPDACDAT0,

Bits[11:0] × 12-BIT_LSB_DAC)

6-Bit DAC Output Voltage = 0.2 V + (LPDACDAT0,

Bits[17:12] × 6-BIT_LSB_DAC) – 12-BIT_LSB_DAC)

When the 12-bit output is ≥ the 6-bit output,

12-Bit DAC Output Voltage = 0.2 V + (LPDACDAT0,

Bits[11:0] × 12-BIT_LSB_DAC)

6-Bit DAC Output Voltage = 0.2 V + (LPDACDAT0,

Bits[17:12] × 6-BIT_LSB_DAC)

Therefore, in user code, it is recommended to add the

following:

12BITCODE = LPDACDAT0 [11:0];

6BITCODE = LPDACDAT0 [17:12];

if (12BITCODE < (6BITCODE *64))

LPDACDAT [11:0] = (12BITCODE – 1);

This code ensures that the 12-bit output voltage is equal to the

6-bit output voltage when LPDACDAT0, Bits[11:0] = 64 ×

LPDACDAT0, Bits[17:12].

LOW POWER DAC USE CASES

Electrochemical Amperometric Measurement

In an electrochemical measurement, the 12-bit output sets the

voltage on the reference electrode pin via the potentiostat circuit

shown in Figure 17. The voltage on the CE0 pin and RE0 pin is

referred to as VBIAS0. The 6-bit output sets the bias voltage on the

LPTIA_P node; this output sets the voltage on the sense

electrode pin, SE0. This voltage is referred to as VZERO0. The bias

voltage on the sensor effectively becomes the difference

between the 12-bit output and the 6-bit output.

DUAL

OUTPUT

DAC

RE0

RTIA

VBIAS

VZERO

SE0

CE0

SENSOR

LPTIA

–

Figure 17. Electrochemical Standard Configuration

Electrochemical Impedance Spectroscopy

In many electrochemical applications, there is significant value

in carrying out a diagnostic measurement. A typical diagnostic

technique is to carry out an impedance measurement on the

sensor. For some sensor types, the dc bias on the sensor must be

maintained during the impedance measurement. The AD5940

facilitates this dc bias. To perform this measurement, set

LPDACCON0, Bit 5 = 1. VZERO0 voltage is set to the input of the

high speed TIA and the high speed DAC generates an ac signal.

The level of the ac signal is set via the VBIAS0 voltage output of

the low power DAC, and the voltage on SE0 is maintained by

VZERO0 voltage. The high speed DAC dc buffers must also be

enabled by setting AFECON, Bit 21.

Low Power DAC in 4-Wire Isolated Impedance

Measurements

For 4-wire isolated impedance measurements, such as body

impedance measurements, a high frequency sinusoidal wave-

form is applied to the sensor via the high speed DAC. A common-

mode voltage is set across the sensor using the low power DAC

6-bit output voltage, VZERO, and the low power TIA. This config-

uration sets the common-mode voltage between AIN2 and AIN3

(see Figure 18). To enable this common-mode voltage setup,

SWMUX, Bit 3, must be set to 1. The VBIAS0 voltage output of

the low power DAC also sets the common-mode voltage for the

high speed DAC excitation buffer.

AD5940 Datasheet(PDF) 29 Page - Analog Devices

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