CN-0537

Circuit Note

Rev. A | Page 4 of 8

The photodetector current is calculated using the

ADPD188BI digital output, as shown in Equation 7.

1

_

PD

I

Code Q

PULSE COUNT

=

××

(7)

where:

IPD is the photodetector current in nA.

Code is the 32-bit output code in LSB.

Q is the ADC resolution in nA/LSB.

PULSE_COUNT is the number of LED pulses for the time slot.

The ADC resolution of the ADPD188BI is dependent on the

setting of the LED pulse width (Register 0x30 for Time Slot A

and Register 0x35 for Time Slot B) and the setting of the

internal transimpedance amplifier (TIA) gain (Register 0x55).

Refer to the ADPD188BI data sheet for the ADC resolution

using different TIA gain values.

Calculating the LED Efficiency

The efficiencies of integrated LEDs (ηLEDx) can be calculated

using Equation 3.

ηLED = ηN × k × C

(8)

where:

ηN is the nominal LED efficiency in W/A (0.38 for the blue

LED and 0.22 for the infrared LED).

k is the LED derating factor (for the blue LED, this is

calculated using Equation 9 and for the infrared LED, k = 1.0).

C is a scalar value to compensate for device-to-device

differences.

Unlike the infrared LED, the efficiency of the blue LED

decreases nonlinearly as the drive current is increased. As

such, the derating factor must first be calculated using

Equation 9.

k = A0 + A1ILED + A2(ILED)2 + A3(ILED)3

(9)

where:

A0 = 9.8976 × 10−1

A1 = −5.1448 × 10−3

A2 = 2.0287 × 10−5

A3 = −2.9645 × 10−8

Reading the ADPD188BI Gain Calibration Values

As shown in Equation 8, the ADPD188BI has a scalar value

that compensates for device variation, which is the gain

calibration for each LED and is calculated by using

Equation 10 to Equation 16.

_

LED

Gain I

INTER

C

NOMINAL SCALAR

×

+

=

(10)

For the blue LED

21 (

_

112) 21

256

GAIN COEFFICIENT

Gain

×−

=

+

(11)

INTER = 8 × (INTER_COEFFICIENT – 80)

(12)

NOMINAL_SCALAR = 21 × ILED + 753

(13)

For the infrared LED

42 (

_

112) 42

256

GAIN COEFFICIENT

Gain

×−

=

+

(14)

INTER = 5 × (INTER_COEFFICIENT – 80)

(15)

NOMINAL_SCALAR = 42 × ILED + 156

(16)

Note, these calibration equations are valid for devices with

Module ID 33. For the calibration equations for devices with

Module ID 30 and Module ID 31, refer to the AN-2033.

LED Temperature Compensation

The full loop response of the ADPD188BI is affected by the

ambient temperature. For the blue channel, this issue is further

complicated because the shape of the temperature response

curve can also vary, depending on the amount of LED current

used. Figure 5 illustrates the temperature effect on the relative

output response across the ADPD188BI operating temperature

range, using the common ILED settings of 100 mA and 175 mA

for the blue LED. For the infrared channel, the temperature

response curve is independent of the LED current.

Figure 5. Temperature Effect on the ADPD188BI Output Response

Relative to 25°C

To determine the value of the relative response, the ability to

measure the ambient temperature in real time is required. On

the CN-0537, a temperature and humidity sensor monitors

the conditions inside the chamber, next to the ADPD188BI.

Compensation for the temperature effect can then be done by

dividing the ADPD188BI output by the relative response for

the current ambient temperature, taken from the appropriate

curve in Figure 5.

25

T

C

T

Code

Code

RelativeResponse

° =

(17)

where:

Relative ResponseT is the relative response at the current

ambient temperature.

CodeT is the output code at the current ambient temperature.

Code25°C is the output code at temperature 25°C.

1.050

0.850

0.875

0.900

0.925

0.975

1.025

0.950

1.000

–60

100

60

20

80

40

0

–20

–40

TEMPERATURE (°C)

BLUE LED = 175mA

BLUE LED = 100mA

IR LED