ADPD4100 datasheet(47/101 Pages) AD

47 / 101 page

Data Sheet

ADPD4100/ADPD4101

Rev. 0 | Page 47 of 101

Improving SNR with Integrator Chopping in ECG

Measurements

Integrator chopping can also improve SNR in ECG measurements

with the sleep float mode by eliminating low frequency noise

content. The procedure to enable integrator chop mode is the

same as explained previously in the Improving SNR Using

Integrator Chopping section. However, ECG measurements

with integrator chopping require additional considerations. The

sign of the dc offset voltages in Figure 52 can be positive or

negative. When the sign of the net dc offset voltage is negative,

integrator chop mode can result in clipping of the ECG signal.

To prevent clipping of the ECG signal, lit data must be used.

Figure 23 shows the datapath. In Figure 23, lit and dark values

can be optionally written to FIFO. This option allows the use of

negative signal values by writing both lit and dark values to the

FIFO, and the user can perform a signed subtract in external

processing of the data to calculate the signal value. Therefore,

integrator chopping can be used regardless of the sign of net dc

offset voltage.

Lead Off Detection

To perform a lead off detection measurement, the ADPD4100/

ADPD4101 measure the impedance of the electrode skin

contacts to determine whether one or more of the electrodes are

not making contact with the skin. Lead off measurement can be

performed in two different ways based on the number of

electrodes used.

Three-Electrode Lead Off Measurement

The three-electrode configuration requires a third electrode

connected to an unused VCx pin to provide a stimulus to the

body. The RC network of the ECG measurement is bypassed by

wiring the electrodes directly to a separate set of inputs through

25 kΩ resistors. The response from the stimulus is measured

from this separate set of inputs. Three-electrode lead off

measurement is capable of determining which electrode is loose

or has lost the contact with the skin. Figure 54 shows the circuit

for the three-electrode lead off detection measurement. RBODY is

the resistance of the body.

CH2

CH1

IN1

IN2

IN4

470pF

VC2

ADPD4100/

ADPD4101

500kΩ

25kΩ

IN3

25kΩ

RBODY

47pF

Figure 54. Circuit Used for Three-Electrode Lead Off Detection Measurement

ECG and three-electrode lead off detection are measured as

follows:

1. ECG is measured in Time Slot A as defined in the ECG

Measurement with the ADPD4100/ADPD4101 section.

2. Lead off detection of the ECG electrodes is taken in

Time Slot B by making simultaneous single-ended

impedance measurements of ECG Electrode 1 (E1) and

Electrode 2 (E2) into Channel 1 and Channel 2, respectively.

When both ECG electrodes, E1 and E2, are making contact

with the skin during the measurement, an ECG signal is visible.

The impedance measurements of the E1 and E2 electrodes have

some readout indicating that contact is being made with the

skin and current is flowing into the ADPD4100/ADPD4101

through the body of low impedance when the stimulus is

applied. When either ECG electrode stops making contact with

the skin or is loose, there is no ECG signal in the acquired trace.

When contact between both electrodes and the skin is restored,

the ECG signal appears immediately. Because two inputs and

two channels are allocated to detect leadoff condition for two

electrodes, this measurement method can determine if one

electrode loses contact with skin or both electrodes lose contact.

This measurement can also detect which one of the electrodes

loses contact with skin if only one electrode loses contact.

Figure 55 illustrates a representation of ADC output changes in

different cases for leadoff condition. In Figure 55, before

Time tA, both ECG electrodes make contact with the skin. At

Time tA, E1 is disconnected from the skin. The time between tA

and tB shows the case where only E1 is disconnected from the

skin. At Time tB, E1 starts to make contact with skin and output

of the two channels go to their initial levels. At Time tC, only E2

is disconnected from the skin and it stays disconnected until

Time tD. At Time tE, E2 starts making contact with the skin

again. At Time tE, both E1 and E2 are disconnected from the

skin, and they stay disconnected until Time tF. At Time tF, both

E1 and E2 start making contact with the skin. Therefore, lead

off condition is detected in all electrode connection cases, and

three-electrode lead off measurement detects and distinguishes

all the different cases. The level of actual ADC outputs

associated with Channel 1 and Channel 2 may differ because

the type and placement of the electrodes may be different in

each case, and RBODY differs from person to person, which

affects the amount of current that each channel receives.

TIME

CHANNEL 2

CHANNEL 1

Figure 55. Graph of Three-Electrode Lead Off Measurement

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