ADPD4200 datasheet(41/93 Pages) AD

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Data Sheet

ADPD4200

APPLICATIONS INFORMATION

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Rev. 0 | 41 of 93

Table 24. Relevant Configuration Registers for ECG Measurement Using Sleep Float Mode

Group

Time Slot A

Bit Field Name

Description

0x010C, Bits[15:8]

MOD_WIDTH_x

Sets width of connect pulse in 1 μs increments, which is the time required to transfer the

charge from the external capacitor. Set to approximately three time constants based on

the time constant created between the external capacitor and the series input resistor (500

Ω or 6500 Ω based on setting of INPUT_R_SELECT_x).

0x010C, Bits[7:0]

MOD_OFFSET_x

Sets start time of first connect pulse in 1 μs increments. Set to INTEG_WIDTH_x + 4.

1 This is the Time Slot A register address. Add 0x020 for the identical register address for each subsequent time slot. For example, Register 0x0100 is the location

for SAMPLE_TYPE_A. For Time Slot B, this register is at Address 0x0120. For Time Slot C, this register is at Address 0x0140. For Time Slot D, this register is at

Address 0x0160, and so on.

Sleep Float Mode with Multiple Charge

Transfers

When the electrode half cell potential mismatch becomes very

large, on the order of hundreds of millivolts or greater, the half

cell dc signal (VH-C, see ECG Measurement with the ADPD4200)

uses a significant amount of the dynamic range available for the

ECG measurement. This half cell potential mismatch is a result of

using electrodes of different materials, for example, stainless steel

for one of the electrodes and titanium for a second electrode. The

maximum amount of charge that the ADPD4200 can accommodate

for a single sample is ~7.5 pC when using the 200 kΩ TIA gain set-

ting. The maximum allowable charge per sample scales inversely

with the gain of the TIA. The presence of a large dc signal creates

excess charge on the sampling capacitor, which can saturate the

input to the ADC. For example, a half cell potential mismatch of

100 mV adds an additional 47 pC of charge to the 470 pF capacitor.

To accommodate the half cell voltage without reducing the size of

the sampling capacitor, the recommendation is to reduce the TIA

gain to 50 kΩ or 100 kΩ, and to transfer the accumulated charge

in multiple short transfers. For example, to transfer 47 pC of charge

from the external capacitor to the internal integrator, set the TIA

gain to 50 kΩ to accommodate 30 pC of charge per sample, and set

the input resistor, RIN, to 6.25 kΩ. Setting RIN = 6.25 kΩ limits the

rate of charge transfer into the TIA with an RC time constant of 2 ×

6.25 kΩ × 470 pF = 5.9 μs.

Next, reduce the amount of time allowed to transfer the charge

by using modulation pulses, which are some fraction of the time

constant such that a smaller percentage of the overall charge

is transferred and integrated per pulse to avoid saturating the

TIA. Multiple transfer cycles are then used to fully discharge the

sampling capacitor. The ADPD4200 automatically sums the results

of the transfer cycles and report the total charge.

The timing for this mode is the same as shown in Figure 39 except

the device is set up for multiple modulation pulses. All registers are

set the same as for sleep float mode except the following:

►

MOD_WIDTH_x and INTEG_WIDTH_x are shorter to accommo-

date shorter pulses.

►

INPUT_R_SELECT_x = 1 to select the 6.25 kΩ input resistor.

►

NUM_REPEAT_x is set to >1.

Lead Off Detection

To perform a lead off detection measurement, the ADPD4200

measures the impedance of the electrode contacts to determine

whether one or more of the electrodes are not making contact

with the skin. This measurement requires a separate electrode

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

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

by wiring the electrodes directly to a separate set of inputs through

50 kΩ resistors. The response from the stimulus is measured from

this separate set of inputs. Figure 40 shows a circuit that can

be used for the lead off detection measurement. RBODY is the

resistance of the body.

Figure 40. Circuit Used for Lead Off Detection Measurement

ECG and three-electrode lead off detection are measured as fol-

lows:

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

Measurement with the ADPD4200 section.

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

B by making simultaneous single-ended impedance measure-

ments of ECG Electrode 1 (E1) and Electrode 2 (E2) into

Channel 1 and Channel 2, respectively, as shown in Figure 40.

Note that the third electrode (E3) is used for biasing the body.

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

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