AD5941 datasheet(32/134 Pages) AD

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AD5940/AD5941

Data Sheet

Rev. C | Page 32 of 134

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/AD5941 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

configuration sets the common-mode voltage between AIN2 and

AIN3 (see Figure 21). 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.

Figure 21. Low Power DACs Used in a 4-Wire Impedance Measurement (HSTIA_P = Positive Output of High Speed TIA)

LOW POWER DAC CIRCUIT REGISTERS

Table 15. Low Power TIA and Low Power DAC Registers Summary

Address

Name

Description

Reset

Access

0x00002128

LPDACCON0

Low power DAC configuration register

0x00000002

R/W

0x00002124

LPDACSW0

Low power DAC switch control register

0x00000000

R/W

0x00002050

LPREFBUFCON

Low power reference configuration register

0x00000000

R/W

0x0000235C

SWMUX

Common-mode switch mux select register

0x00000000

R/W

0x00002120

LPDACDAT0

Low power DAC data output register

0x00000000

R/W

LPDACCON0 Register—LPDACCON0

Address 0x00002128, Reset: 0x00000002, Name: LPDACCON0

Table 16. Bit Descriptions for LPDACCON0 Register

Bits

Bit Name

Settings

Description

Reset

Access

[31:7]

Reserved

Reserved.

0x0

WAVETYPE

Low power DAC data source. This bit determines the DAC waveform type.

0x0

R/W

Direct from LPDACDAT0.

Waveform generator.

GAIN

EXCITATION

BUFFER

HSTIA

–

ADC/

800kHz

DFT = 2048

FIFO

16MHz

OSC

SEQUENCER

AIN2

AIN3

AIN1

CE0

RTIA

10MΩ

RLIMIT

VCM

HSTIA_P

AD5940/AD5941

CISO1

1.1V

UNKNOWN Z

RACCESS1

CISO3

CISO4

CISO2

RACCESS3

RACCESS4

RACCESS2

AIN4/

LPF0

CLPF

HSDAC

WAVEFORM

GENERATOR

LPDAC0

LPTIA

–

RFILTER

CTIA

VZERO

VBIAS

10MΩ

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