ADPD1081 datasheet(44/74 Pages) AD

44 / 74 page

ADPD1080/ADPD1081

Data Sheet

Rev. C | Page 44 of 74

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 29 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 BPF and integrator are not unity-gain elements.

Measuring PCB Parasitic Input Resistance

During the process of mounting the ADPD1080/ADPD1081,

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 ADPD1080/ADPD1081 masks the primary effects

of these resistances, making it difficult to detect them. However,

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

significantly through added noise or decreased dynamic range.

TIA ADC mode can 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 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.

Table 29. Analog Specifications for TIA ADC Mode

Parameter

Test Conditions/Comments

Typ

Unit

TIA ADC Saturation Levels

Values expressed per channel, per sample

25 kΩ gain

38.32

µA

50 kΩ gain

19.16

µA

100 kΩ gain

9.58

µA

200 kΩ gain

4.79

µA

TIA Linear Range

25 kΩ gain

42.8

µA

50 kΩ gain

21.4

µA

100 kΩ gain

10.7

µA

200 kΩ gain

5.4

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 Without Input Photocurrent

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

13,000

LSB

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