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Data Sheet

ADPD103

Rev. B | Page 29 of 52

sampling time with the LED pulse to measure the total amount of

light falling on the photodetector (for example, background light +

LED pulse).

If this minimum value is above 0 LSB, the TIA is not saturated.

However, take care, because even if the result is not 0 LSB,

operating the device near saturation can quickly result in

saturation if light conditions change. A safe operating region is

typically at ¾ full scale and lower. Use Table 17 to determine

how the input codes map to ADC levels on a per channel per

pulse basis. These codes are not the same as in normal mode

because the band-pass filter and integrator are not unity-gain

elements.

Coarse Ambient Light Measurement

Using the typical values in Table 17, TIA_ADC mode can be used

to measure or quantify the amount of background or ambient light

present on the photodetector. The settings are the same in the

method described in the Protecting Against TIA Saturation in

Normal Operation section, except the timing used in the

normal operating mode is sufficient for this mode. There is no

need to sweepAFE_OFFSET. IfAFE_OFFSET is in the same place

as the normal mode operation, the TIA_ADC mode does not

return the same value, regardless of whether the LED is on or off.

In TIA_MODE, the dark level is a high level near 13,000 LSBs

per channel per pulse (see Table 17). To measure this value,

select no PD by writing a 0x0 to Register 0x14, Bits[11:8] for

Time Slot B or Register 0x14, Bits[7:4] for Time Slot A. This

setting internally opens the photodiode connection. This gives a

baseline LSB value that coincides with a zero signal input.

After Register 0x14 is restored to its normal value, while

connecting the photodiode to the TIA, this TIA_ADC result

can be subtracted from the open photodiode case to yield a

background light measurement. Use Table 17 to translate this

measurement into an input photocurrent. Use this result for

coarse absolute measurements only, because it is typically only

accurate to within 10%.

Measuring PCB Parasitic Input Resistance

During the process of mounting the ADPD103, undesired

resistance can develop on the inputs through assembly errors or

debris on the PCB. These resistances can form between the

anode and cathode, or between the anode and some other

supply or ground. In normal operation, the ambient rejection

feature of the ADPD103 masks the primary effects of these

resistances, making it very difficult to detect them. However,

even at 1 MΩ to 10 MΩ, such resistance can impact performance

significantly through added noise or decreased dynamic range.

TIA_ADC mode can be used to screen for these assembly issues.

Measuring Shunt Resistance on the Photodiode

A shunt resistor across the photodiode does not generally affect

the output level of the device in operation because the effective

impedance of the TIA is very low. This is especially true if the

photodiode is held to 0 V in operation. However, such resistance

can add noise to the system, degrading performance. The best

way to detect photodiode leakage, also called photodiode shunt

resistance, is to place the device in TIA_ADC mode in the dark

and vary the operation mode cathode voltage. When the cathode is

at 1.3 V, this places 0 V across the photodiode because the anode is

always at 1.3 V while in operation. When the cathode is at 1.8 V,

this places 0.5 V across the photodiode. Using the register settings

in Table 3 to control the cathode voltage, measure the TIA_ADC

value at both voltages. Next, divide the voltage difference of 0.5 V

by the difference of theADC result after converting it to a current.

This result is the approximate shunt resistance. Values greater

than 10 MΩ may be difficult to measure, but this method is useful

in identifying gross failures.

Table 17. Analog Specifications for TIA_ADC and Digital Integrate Modes

Parameter

Test Conditions/Comments

Typ

Unit

TIA_ADC/Digital Integration Saturation Levels

Values expressed per channel, per sample

TIA Feedback Resistor:

25 kΩ

38.32

μA

50 kΩ

19.16

μA

100 kΩ

9.58

μA

200 kΩ

4.79

μA

TIA_ADC Resolution

Values expressed per channel, per sample

TIA Feedback Resistor:

25 kΩ

2.92

nA/LSB

50 kΩ

1.5

nA/LSB

100 kΩ

0.73

nA/LSB

200 kΩ

0.37

nA/LSB

Output with No Input Current

ADC offset (Register 0x18 to Register 0x21) = 0x0

13000

LSB

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