ADPD4100 datasheet(34/101 Pages) AD

34 / 101 page

ADPD4100/ADPD4101

Data Sheet

Rev. 0 | Page 34 of 101

Group

Time Slot A

Bit Field Name

Description

LED Settings

0x0105, Bit 15 and Bit 7;

0x0106, Bit 15 and Bit 7

LED_DRIVESIDEx_x

Select LED for time slot used.

0x0105, Bits[14:8], Bits[6:0];

0x0106, Bits[14:8], Bits[6:0]

LED_CURRENTx_x

Set LED current for selected LED.

0x0109, Bits[7:0]

LED_OFFSET_x

Sets start time of first LED pulse in 1 μs increments.

0x0109, Bits[15:8]

LED_WIDTH_x

Sets width of LED pulse in 1 μs increments.

0x010D, Bits[15:12]

LED_DISABLE_x

In any given sequence of four pulses, disable the LED pulse in

the selected position. Selections are active high (that is, disable

LED if 1) and the LSB of this register maps to the first pulse. For

a sequence of four pulses, it is recommended to turn on the

LED in the second and third pulses by writing 0x9 to this

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.

Float Mode Limitations

When using float mode, the limitations of the mode must be

well understood. For example, a finite amount of charge can

accumulate on the capacitance of the photodiode, and there is a

maximum amount of charge that can be integrated by the

integrator. Based on an initial reverse bias of 215 mV on the

photodiode and assuming that the photodiode begins to

become nonlinear at ~200 mV of forward bias, there is ~450 mV

of headroom for the anode voltage to increase from its starting

point at the beginning of the float time before the charge ceases

to accumulate in a linear fashion. It is desirable to operate only

in the linear region of the photodiode (see Figure 38). To verify

that float mode is operating in the linear region of the

photodiode, the user can perform a simple check. Record data

at a desired float time and then record data at half the float

time. The recommended ratio of the two received signals is 2:1.

If this ratio does not hold true, the photodiode is likely

beginning to forward bias at the longer float time and becomes

nonlinear.

The maximum amount of charge that can be stored on the

photodiode capacitance and remain in the linear operating

region of the sensor is estimated by

Q = CPDV

where:

Q is the integrated charge.

CPD is the capacitance of the photodiode.

V is the amount of voltage change across the photodiode before

the photodiode becomes nonlinear.

For a typical discrete optical design using a 7 mm2 photodiode

with 70 pF capacitance and 450 mV of headroom, the maximum

amount of charge that can store on the photodiode capacitance

is 31.5 pC.

FLOAT TIME (µs)

RECOMMENDED

FLOAT MODE

OPERATING REGION

PD BEGINS TO

FORWARD BIAS

Figure 38. Integrated Charge on the Photodiode (PD) vs. Float Time

In addition, consider the maximum amount of charge the

integrator of the ADPD4100/ADPD4101 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 capacitance, 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, the

integrator integrates 5.3 pC per pulse 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.

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