ADPD6000 datasheet(32/89 Pages) AD

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Data Sheet

ADPD6000

APPLICATIONS INFORMATION

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Rev. 0 | 32 of 89

BIA PATH

The BIA path in the ADPD6000 can be used to perform 4-wire

impedance measurement on the body. This approach uses a high

precision, ac voltage source to excite a sensor with a known ac

voltage (VAC). To calculate the impedance, measure the current

(I) that flows from the unknown impedance (ZUNKOWN) and the

voltage across the unknown impedance (VZ_UNKNOWN). Calculate

the impedance by using the following equation:

ZUNKNOWN =VZ_UNKNOWNI

(11)

In real-world applications, medical devices must conform to the

IEC 60601 standard. This standard limits the amount of dc and ac

voltage that can be applied to the human body.

In Figure 47, there are discrete isolation capacitors (CISO1, CISO2,

CISO3, and CISO4) that ensure no dc voltage occurs across the body.

RLIMIT limits the current provided to the sensor to conform to the

IEC 60601 standard.

RCONTACT represents the resistances of the electrodes connecting

to the unknown impedance.

Figure 47. BIA Path Diagram

As shown in Figure 47, a 4-wire bioimpedance solution requires a

precision ac voltage source, a high precision current meter, and a

precision differential voltage meter.

The ADPD6000 uses a high speed DAC and waveform generator to

generate the precision ac voltage. The device uses a high speed,

high precision TIA for converting current from the sensor into a

voltage measured by the ADC. The TIA channel measures the

response current.

The ADC converts the current measurement with a 1 MSPS speed.

A DFT is performed on the data. The DFT is implemented on

the ADPD6000. The number of DFT points is configurable up to

8192. The ADPD6000 calculates the real and imaginary parts, and

the host microcontroller calculates the unknown impedance of the

sensor.

There are a number of discrete components needed in the system

to guarantee safety and accuracy.

To conform to IEC 60601 standards, limit the amount of ac current

entering the human body. The maximum allowable ac current is 500

µA at 50 kHz and 600 µA at 60 kHz. When calculating the RLIMIT

resistor value, the maximum output voltage from the ADPD6000 is

0.8 V p-p (0.2828 V rms). Set the maximum allowable ac current

to 80% of maximum, or 400 µA rms. The following equation is the

result of these values:

RLIMIT=0.2828V rms

400μA

=707Ω

(12)

As such, a ~1 kΩ RLIMIT is selected and connected to the EXCP

pin on the ADPD6000. This calculation ignores CISOx because of its

small size.

To conform to IEC 60601 standards, a 10 µA maximum dc current

is allowed to enter the human body. In this application, the dc

current is guaranteed to be zero due to the addition of isolation

capacitors. A value of 0.47 µF is selected for the isolation capaci-

tors because 0.47 µF is a sufficiently large capacitance that is also

available in small packages suitable for wearable electronics.

The ADPD6000 runs the BIA time slot and fills the FIFO with the

DFT real and imaginary results for both the voltage and current

measurements (four data points in total). The host microcontroller

reads the data FIFO and uses the real and imaginary DFT results to

calculate the unknown impedance. Calculate the impedance of the

sensor by using the following equations:

Volage Measurement Magnitude=

r2+i2

(13)

Voltage Measurement Pℎase=tan−1ir (14)

To calculate the impedance, use Ohm’s law by dividing the voltage

magnitude by the current magnitude. Convert the current measure-

ment value into a voltage using RTIA. This gain must be taken

into account. Therefore, the equation to determine the unknown

impedance is as follows:

ZUNKNOWN =Voltage Magnitude

Current Magnitude

×RTIA (15)

MULTIMODAL

Figure 48 shows the basic design reference schematic for PPG,

ECG, and BIA multimodal applications.

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