ADPD4100 datasheet(43/101 Pages) AD

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Data Sheet

ADPD4100/ADPD4101

Rev. 0 | Page 43 of 101

Protecting Against TIA Saturation with TIA Ceiling

Detection

While the TIA ADC mode monitoring helps to avoid

saturation, there may be cases where the voltage at the output

terminals of the TIA may exceed the typical operation levels

that ensure no current saturation for some amount of time

while the actual measurement is in the process. If the current

fed into the inputs of the TIA is high enough, it could result in

exceeding the typical operating points of TIA depending on the

TIA gain and the TIA reference voltage settings. In this case,

measurements can be distorted or may not be done. High levels

of photodiode current or high levels of ambient current may

cause the output voltages of the TIA to go above a limit that is

close to saturation.

The ADPD4100/ADPD4101 have a TIA ceiling detection

feature that uses voltage comparators at the outputs of the TIA

stage to detect whether the TIA output voltage exceeds a certain

range. This range ensures the TIA output to be far enough from

the ceiling of the total TIA range by some margin. Therefore, if

the TIA output voltage exceeds the range even for a small amount

of time, the user is informed about the status of voltage at the

TIA outputs and can take action to avoid a possible saturation.

Voltage comparators compare the positive and negative output

terminals to a certain threshold voltage, which sets the effective

TIA output voltage range, and send an output bit indicating

whether the TIA output voltage is beyond the range.

To enable this feature, TIA_CEIL_DETECT_EN_x must be set

to 1. Each time slot has its own TIA_CEIL_DETECT_EN_x bit

that can be controlled individually. The feature is turned on for

Channel 1, and Channel 2 if Channel 2 is also enabled within

the time slot. When the comparators are enabled, the TIA ceiling

detection information is latched onto the INT_TCLN1_x bits in

interrupt output turns to 1 if either output exceeds the threshold

voltage.

Figure 50 illustrates the internal circuitry for TIA ceiling

detection. VTIAO+ and VTIAO− represent the positive and negative

outputs of the TIA, respectively, and VTH is the threshold voltage

that the TIA output voltages are being compared to.

INT_TCLN1_x

(INT_TCLN2_x FOR CHANNEL 2)

TIA_CEIL_DETECT_EN_x

VTH

VTIAO+

VTH

VTIAO–

Figure 50. Schematic of TIA Ceiling Detection Circuit

Due to different TIA reference voltage settings and different

TIA gains, different amounts of input current can result in

exceeding the threshold. Higher TIA reference voltage

(TIA_VREF) and lower TIA gain increase the input current

level at which the TIA output voltage exceeds the threshold

voltage and, therefore, increase the range in terms of current.

Table 26 shows the typical input current needed to trigger TIA

ceiling detection for different TIA gain and different TIA_VREF

while using 2 µs LED pulses in continuous connect mode.

Table 26. Typical Input Currents to Trigger TIA Ceiling

Detection

TIA Gain

(kΩ)

Input Current (µA) at

TIA_VREF = 0.9 V

Input Current (µA) at

TIA_VREF = 1.3 V

21.9

33.6

10.6

16.7

100

5.2

8.4

200

2.6

4.2

Figure 51 illustrates the trigger points for TIA ceiling detection

with respect to different TIA_VREF values. Trigger points for

different TIA gains can be found in Table 26.

0,0

TIA _VREF = 1.3V

TIA _VREF = 0.9V

INPUT CURRENT (µA)

TIA OUTPUT VOLTAGE (V)

Figure 51. Illustration of Trigger Points for TIA Ceiling Detection for TIA_VREF = 0.9 V and TIA_VREF = 1.3 V, Points Marked by Black Crosses

ADPD4100 Datasheet(PDF) 43 Page - Analog Devices

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