ADPD4100 datasheet(37/101 Pages) AD

37 / 101 page

Data Sheet

ADPD4100/ADPD4101

Rev. 0 | Page 37 of 101

Self Capacitance-Based Proximity Measurement

Capacitive proximity measurements can be performed by

measuring self capacitance. As in mutual capacitance-based

proximity measurement, the BPF is bypassed by setting

AFE_PATH_CFG_x to 0E6.

Self capacitance based proximity measurement, however,

requires only one electrode connected to one of the inputs of

ADPD4100/ADPD4101. Capacitance measurement in this case

is performed by creating the voltage difference. To create the

voltage difference, pulse TIA_VREF while the input used is

preconditioned to TIA_VREF, and read the change in the ADC

output when ΔC is induced through the change in the

proximity of the human tissue.

This measurement modality makes use of the capacitance of the

human body to the earth, and ΔC formed due to tissue proximity.

The human body capacitance allows the use of TIA_VREF

pulsing at the input as a voltage difference needed to measure

ΔC. Figure 42 shows a representation of this measurement.

Because TIA_VREF is pulsed at the input, all the extra voltage

at pulsing shows up the same way at the TIA output. Therefore,

TIA has only a positive response compared to the mutual

capacitance-based measurement. The integration sequence

must be centered in such a way that all the dc shift is canceled

and remaining small ac charge due to the change in proximity is

integrated. Figure 43 shows integration sequence timing with

respect to the TIA_VREF pulse to cancel dc shift and integrate

ac charge. The constant part in the TIA output represents the dc

charge, and it must be canceled by the integration sequence so

that only the surge charge accumulation at the positive and the

negative edges of the TIA_VREF pulse are integrated.

However, the baseline measurement without tissue in proximity

is needed to determine true ΔC, which is proportionate to the

change in ADC output read at the proximity event.

Calculate ΔC as follows. For example, when TIA_VREF is

pulsed from 0.9 V to 1.14 V by setting VREF_PULSE_VAL_x to

0, AFE_TRIM_VREF_x to 2, and VREF_PULSE_x to 1, ΔC

then becomes

ΔC = (Δ(ADC Output in LSB) × 0.92 fC/LSB ×

(RINT/2RF)/Number of Pulses)/(2 × (1.14 V − 0.9 V))

Table 22 summarizes the relevant registers for this measurement.

Integrator chop mode can be enabled for this measurement.

CINT

ELECTRODE

TIA

TIA_VREF

INx

INT

RINT

Figure 42. Self Capacitance Measurement

PRECONDITION

SENSOR

PULSE TIA_VREF

–

INTEGRATION

SEQUENCE

START OF TIME SLOT

MOD_OFFSET_x

MOD_WIDTH_x

INTEG_OFFSET_x

INTEG_WIDTH_x

TIA OUTPUT

PERIOD

(AUTOMATICALLY CALCULATED)

–

PRE_WIDTH_x

(DEFAULT 8µs)

Figure 43. Timing Diagram for Self Capacitance-Based Proximity Measurement

Table 22. Relevant Registers for Self Capacitance Based Proximity Measurement

Group

Time Slot A Register Address1

Bit Field Name

Description

Self Capacitance-Based

Proximity Setup

0x0100, Bits[13:12]

SAMPLE_TYPE_x

Leave at the default setting (0) for default

sampling mode.

0x0101, Bits[8:0]

AFE_PATH_CFG_x

Set to 0x0E6 for TIA, integrator, and ADC.

Bypass BPF.

0x0102, Bits[15:0]

INPxx_x

Enable desired inputs.

0x0103, Bits[14:12]

PRECON_x

Set to 0x5 to precondition sensor to TIA_VREF.

0x0104, Bits[5:0]

TIA_GAIN_CHx_x

Select TIA gain.

0x0104, Bits[7:6]

VREF_PULSE_VAL_x

Select 0x0 to pulse TIA_VREF to 1.14 V.

0x0104, Bits[9:8]

AFE_TRIM_VREF_x

Set to 0x2 to set TIA_VREF = 0.9 V.

0x0104, Bits[10]

VREF_PULSE_x

Set to 0x1 to pulse TIA_VREF.

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