ADPD188GG datasheet(32/61 Pages) AD

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ADPD188GG

Data Sheet

Rev. B | Page 32 of 61

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).

To ensure that the TIA does not saturate, a safe operating

region is typically at ¾ full scale and lower. Use Table 19 to

determine how the output 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.

Measuring PCB Parasitic Input Resistance

During the process of mounting the ADPD188GG, 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

ADPD188GG 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/EXT_INx 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 1 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.

Using the EXT_IN 1 and EXT_IN 2 Inputs with a Voltage

Source

The ADPD188GG can be used for voltage inputs. Voltage

inputs can be measured in normal mode or in TIA ADC mode.

If these inputs are not a result of stimulation from the LED

driver, TIA ADC mode is preferred. To understand the

conversion gain from a voltage through a series resistor, RS, the

current can be determined by following the schematic in Figure 34.

TIA_VREF

OPTIONAL

BUFFER

TIA

R_IN

EXT_INx

VIN

ADPD188GG

ADC

Figure 34. ADPD188GG Used for Voltage Inputs

Input Current = (VIN − TIA_VREF)/(RS + R_IN)

Values for R_IN are listed in Table 2. R_IN is not needed for

photodiode or other current inputs because the current of these

inputs are not a function of the input resistance. Conversion from

input current in amps to ADC codes (LSBs) follows Table 19 in

TIA ADC mode. Current conversion in normal mode is listed

in Table 2. The offset level shown in Table 19 represents the

expected code value with zero current input. The conversion

gain in nA/LSB can be added onto this for nonzero input currents.

Table 19. Analog Specifications for TIA ADC Mode and Digital Integrate Mode

Parameter

Test Conditions/Comments

Typ

Unit

TIA ADC Offset Level

Floating input (Input current = 0A); Register 0x43 and Register 0x45 = 0xAE65; Register 0x42

and Register 0x44, Bit 7 = 1, Register 0x58, Bit 7 = 1

TIA_VREF = Register 0x42 and Register 0x44, Bits[5:4] = 0 (1.14 V)

11400

LSB

TIA_VREF = Register 0x42 and Register 0x44, Bits[5:4] = 1 (1.01 V)

9700

LSB

TIA_VREF = Register 0x42 and Register 0x44, Bits[5:4] = 2 (0.89 V)

8100

LSB

TIA_VREF = Register 0x42 and Register 0x44, Bits[5:4] = 3 (1.27 V); recommended for PD inputs

13200

LSB

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