ADPD4200 datasheet(32/93 Pages) AD

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Data Sheet

ADPD4200

APPLICATIONS INFORMATION

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Rev. 0 | 32 of 93

of charge that can be stored on the photodiode capacitance is

31.5 pC.

In addition, consider the maximum amount of charge the integrator

of the ADPD4200 can integrate. The integrator can integrate up to

7.6 pC. When this charge is referred back to the input, consider the

TIA gain. When the TIA gain is at 200 kΩ, the input referred charge

is at a 1:1 ratio to the integrated charge on the integrator. For

100 kΩ gain, it is 2:1. For 50 kΩ gain, it is 4:1. For 25 kΩ gain,

it is 8:1.

For the previous example using a photodiode with 70 pF capaci-

tance, use a 50 kΩ TIA gain and set the float timing such that, for

a single pulse, the output of the ADC is at 70% of full scale, which

is a typical operating condition. Under these operating conditions,

5.3 pC integrates per pulse by the integrator for 21.2 pC of charge

accumulated on the photodiode capacitance. The amount of time

to accumulate charge on CPD is inversely proportional to CTR.

TIA gain settings of 100 kΩ or 200 kΩ may be required based

on the CTR of the measurement and how much charge can be

accumulated in a given amount of time. Ultimately, the type of

measurement being made (ambient or pulsed LED), the photodiode

capacitance, and the CTR of the system determine the float times.

Pulse Connect Modulation

Pulse connect modulation is useful for ambient light measurements

or any other sensor measurements that do not require a synchro-

nous 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 accumula-

tion 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 30 shows a timing

diagram for pulse connect modulation type measurements.

Modulation of Stimulus Source

The ADPD4200 has operating modes that modulate the VC1 and

VC2 signals. These modes are useful for providing a pulsed stimu-

lus 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 29, where one of the VCx pins is connected to one side of

the capacitor and the other side is connected to the TIA input.

Figure 29. 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 31).

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 20 shows the relevant registers for

modulating the stimulus to the sensor.

ADPD4200 Datasheet(PDF) 32 Page - Analog Devices

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