ADPD107 datasheet(41/66 Pages) AD

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Data Sheet

ADPD105/ADPD106/ADPD107

Rev. A | Page 41 of 66

until two or three of the four channels reach a minimum value

(note that TIA is in an inverting configuration). All four channels

do not reach this minimum value because, typically, 3 μs LED

pulse widths are used and the ADC samples the four channels

sequentially at 1 μs intervals. This procedure aligns the ADC

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 26 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 26, 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 sweep SLOTx_AFE_OFFSET. If SLOTx_AFE_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 26). To measure this value, select

no photodiode 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 and 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 26 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 ADPD105/ADPD106/

ADPD107, 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 ADPD105/ADPD106/ADPD107 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, especially 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. Setting the cathode to 1.3 V

places 0 V across the photodiode because the anode is always at

1.3 V while in operation. Setting the cathode to 1.8 V 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 the ADC 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.

Measuring TIA Input Shunt Resistance

A resistance to develop between the TIA input and another

supply or ground on the PCB is an example of another problem

that can occur. These resistances can force the TIA into saturation

prematurely. This premature saturation, in turn, takes away

dynamic range from the device in operation and adds a Johnson

noise component to the input. To measure these resistances, place

the device in TIA ADC mode in the dark and start by measuring

the TIA ADC offset level with the photodiode inputs disconnected

(Register 0x14, Bits[11:8] = 0 or Register 0x14, Bits[7:4] = 0).

From this, subtract the value of TIA ADC mode with the

darkened photodiode connected and convert the difference

into a current. If the value is positive, and the ADC signal

decreased, the resistance is to a voltage higher than 1.3 V, such

as VDD. Current entering the TIA causes the output to drop. If

the output difference is negative due to an increase of codes at

the ADC, current is being pulled out of the TIA and there is a

shunt resistance to a lower potential than 1.3 V, such as ground.

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