ADPD4100 datasheet(35/101 Pages) AD

35 / 101 page

Data Sheet

ADPD4100/ADPD4101

Rev. 0 | Page 35 of 101

Pulse Connect Modulation

Pulse connect modulation is useful for ambient light measure-

ments or any other sensor measurements that do not require a

synchronous stimulus. This mode works by preconditioning the

sensor to some level selected by the PRECON_x bits and then

only connecting the sensor to the input of the TIA during the

modulation pulse. When not connected to the TIA, the sensor

is connected to a low input impedance node at the TIA_VREF

voltage. Any sensor current during this time is directed into the

AFE. Therefore, no charge accumulates on the sensor. This lack of

charge accumulation is in contrast to float mode, which fully

disconnects the sensor between modulation pulses. The

MOD_TYPE_x bits must be set to 0x2 for pulse connect mode.

The advantage of using this mode for nonsynchronous sensor

measurements is that it allows the user to take advantage of the

noise performance benefits of the full signal path using the BPF

and integrator. Figure 40 shows a timing diagram for pulse

connect modulation type measurements.

Modulation of Stimulus Source

The ADPD4100/ADPD4101 have operating modes that

modulate the VC1 and VC2 signals. These modes are useful for

providing a pulsed stimulus to the sensor being measured. For

example, a bioimpedance measurement can be made where one

electrode to the human is being pulsed by the VC1 or VC2

output and the response is measured on a second electrode

connected to the TIA input. This mode is also useful for a

capacitance measurement, as shown in Figure 39, where one of

the VCx pins is connected to one side of the capacitor and the

other side is connected to the TIA input.

TIA

TIA_VREF

INx

VCx

INT

RINT

Figure 39. Modulate Stimulus for Capacitance Measurement

The BPF is bypassed for this measurement. When a stimulus

pulse is provided on the VCx pin, the capacitor response is a

positive spike on the rising edge that then settles back toward

TIA_VREF, followed by a negative spike on the falling edge of

the stimulus pulse. The integration sequence is centered such

that the positive and negative integration sequences completely

integrate the charge from the positive and negative TIA

responses, respectively (see Figure 41).

Pulsing of the VC1 and VC2 pins is controlled by the

VCx_PULSE_x, VCx_ALT_x, and VCx_SEL_x bits while

timing of the modulation is controlled by the MOD_OFFSET_x

and MOD_WIDTH_x bits. Table 21 shows the relevant registers

for modulating the stimulus to the sensor.

Mutual Capacitance-Based Proximity Measurement

One of the applications of the ADPD4100/ADPD4101 based on

capacitance measurement is the proximity measurement. A

mutual capacitance-based proximity measurement application,

for example, is based on modulation of the stimulus source in

principle, and is shown in Figure 39. However, this application

requires two electrodes along with the circuit shown in Figure 39,

one of which is connected to an input of the ADPD4100/

ADPD4101 and the other connected to VC1 or VC2. The

capacitor between INx and VCx, in this case, is the representation

of the capacitance formed between two electrodes, instead of a

physical capacitor.

A proximity event represents the proximity of tissue to the two

electrodes mentioned. At the proximity event, a capacitance of

ΔC is formed between the electrodes and human tissue. ΔC varies

due to the varying proximity of tissue to the electrodes. Therefore,

total capacitance, which is the sum of the capacitance of the

capacitance between two electrodes represented as C and the

capacitance ΔC formed between human tissue and the electrode,

changes.

PRECONDITION

SENSOR

CONNECT TIA

TO SENSOR

–

INTEGRATOR

SEQUENCE

ADC CHANNEL 1

ADC CHANNEL 2

(IF ENABLED)

START OF TIME SLOT

MOD_OFFSET_x

PERIOD

(AUTOMATICALLY CALCULATED)

MOD_WIDTH_x

INTEG_OFFSET_x

INTEG_WIDTH_x

TIA OUTPUT

BPF OUTPUT

INTEGRATOR

OUTPUT

REPEAT NUM_REPEAT_x TIMES

PRE_WIDTH_x

(DEFAULT 8µs)

CONVERSION(S)

Figure 40. Timing Diagram for Pulse Connect Modulation

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