ADPD6000 datasheet(20/89 Pages) AD

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Data Sheet

ADPD6000

THEORY OF OPERATION

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Right Leg Driver

The use of a driven reference benefits overall performance by

improving common-mode rejection of noise and interference from

external sources, such as power line interference (50 Hz/60 Hz) or

other patient connected instruments. The drive stage also acts to

absorb lead fail currents injected into the ECG electrodes.

The reference electrode drive circuit senses the patient common-

mode voltage and drives an inverted version back to the body,

creating a negative feedback loop around the patient. The RLD am-

plifier uses VCM = AVDD3/2, which centers the electrode voltages in

the middle of the ADC input range. Each electrode input is buffered

and fed to the RLD amplifier through a switch.

The amount of capacitance on the RLD pin affects the RLD amplifi-

er. For best performance, the capacitance on this node must be

less than 2.2 nF. In normal operation, the RLD amplifier output

is applied to the RL electrode via the RLD pin and associated

protection network.

The RLD amplifier can be turned off and the user can drive the RLD

pin by selecting a voltage in the ECG_RLD_OUT_SEL bit field.

BIA SIGNAL CHAIN

The BIA signal chain is designed for body impedance measure-

ment. The signal chain consists of a high frequency precision

excitation loop and a measurement channel, which enables a wide

capability of measurement of the different bioimpedance configura-

tions.

The excitation loop consists of a 12-bit DAC, referred to as the

high speed DAC. This DAC is capable of generating high frequency

excitation signals up to 250 kHz.

The measurement channel features an ADC with input buffers, a

built in antialias filter, and a PGA.

An ultralow leakage, programmable switch matrix connects the sen-

sor to the internal analog excitation and measurement blocks. This

matrix provides an interface for connecting external bioimpedance

and calibration resistors. Figure 32 shows the block diagram of this

BIA signal chain.

Figure 32. BIA Channel Block Diagram (ZCONT Is Contact Impedance and

ZBODY Is Human Body Impedance)

The input multiplexer (mux) of the excitation loop is controlled by

the BIOZ_TSW_x, BIOZ_TRSW_x, BIOZ_DSW_x, BIOZ_DRSW_x.

The input mux for the measurement path is controlled by the

BIOZ_NCHAN_x and BIOZ_PCHAN_x. Table 10 shows the con-

nections of the input mux with the related bit setting. The flexible

design of the BIA channel allows different configurations of the

measurement.

Table 10. TIA Connections in the BIA Channel

Bit Name

Setting

Connection

BIOZ_TSW_x

0001

IMPIP

0010

IMPIN

0100

EXCP

1000

EXCN

BIOZ_TRSW_x

RCALN

BIOZ_RINT_SW_x, Bit 1

RINT_SN1

BIOZ_DSW_x

0001

IMPIP

0010

IMPIN

0100

EXCP

1000

EXCN

BIOZ_DRSW_x

RCALP

BIOZ_RINT_SW_x, Bit 0

RINT_SP1

BIOZ_PCHAN_x and

BIOZ_NCHAN_x

TIA

IMPIP and IMPIN

RCALN and RCALP

RINT

1 RINT_SN and RINT_SP refer to the two terminals of the internal resistor (RINT).

The frequency of the generated sine wave is controlled by BI-

OZ_SINEFCW_x_x, whereas the amplitude of the sine wave is con-

trolled by BIOZ_SINEAMPLITUDE_x. BIOZ_SINE_PHASE_OFF-

SET_x is used to control the sine wave phase.

Equation 2 and Equation 3 show how to set the voltage output

(VOUT) amplitude and sine wave frequency with these bits, respec-

tively, as follows:

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