Data Sheet ADPD188GG
Rev. B | Page 35 of 61
FLOAT MODE
The ADPD188GG has a unique operating mode, float mode,
that allows excellent SNR at low power in low light situations.
In float mode, the photodiode is first preconditioned to a
known state and then the photodiode anode is disconnected
from the receive path of the ADPD188GG for a preset amount
of float time. During the float time, light falls on the
photodiode, either from ambient light, pulsed LED light, or a
combination of the two depending on the operating mode.
Charge from the sensor is stored directly on the capacitance of
the sensor. At the end of the float time, the photodiode switches
back into the receive path of the ADPD188GG and an inrush of
the accumulated charge occurs, which is subsequently integrated
by the integrator of the ADPD188GG, allowing the maximum
amount of charge to be processed per pulse with the minimum
amount of noise added by the signal path. The charge is
integrated externally on the capacitance of the photodiode for
as long as it takes to acquire maximum charge, independent of
the amplifiers of the signal path, which adds noise to the signal.
Amplifier and ADC noise values are constant for a given
measurement. For optimal SNR, it is desirable to have a greater
amount of signal (charge) per measurement. In normal mode,
because the pulse time is fixed, the charge per measurement can
be increased only by increasing the LED drive current. For high
light conditions, this is sufficient. In low light conditions,
however, there is a limit to the available current. In addition,
high current pulses can cause ground noise in some systems.
Green LEDs have lower efficiency at high currents, and many
battery designs do not deliver high current pulses as efficiently.
Float mode allows the user the flexibility to increase the
amount of charge per measurement by either increasing the
LED drive current or by increasing the float time. This
flexibility is especially useful in low current transfer ratio (CTR)
conditions, for example, 10 nA/mA, where normal mode
requires multiple pulses to achieve an acceptable level of SNR.
In float mode, the signal path bypasses the BPF and uses only
the TIA and integrator. In normal mode, the shape of the pulse
is known (typically either 2 µs or 3 µs) and is consistent across
devices and conditions. The shape of the signal coming through
the BPF is also predictable, which allows a user to align the
integrator timing with the zero crossing of the filtered signal. In
float mode, the shape of the signal produced by the charge
dump can differ across devices and conditions. A filtered signal
cannot be reliably aligned; therefore, the BPF cannot be used.
In float mode, the entire charge dump is integrated in the
negative cycle of the integrator and the positive cycle cancels
any offsets.
Float Mode Measurement Cycle
Figure 37 shows the float mode measurement cycle timing
diagram, and the following details the points shown:
• The precondition period is shown prior to Point A. The
photodiode is connected to the TIA, and the photocurrent
flows into the TIA. The photodiode anode is held at 0.9 V
(Register 0x42 and Register 0x44, Bits[5:4] = 0x2 sets TIA_
VREF = 0.9 V). The photodiode is reverse biased to a
maximum reverse bias of ~250 mV by setting Register 0x54,
Bit 7 = 1 and Register 0x54, Bits[9:8] = 0x2 (for Time Slot
A). At this point, the output of the TIA (TIA_OUT) =
TIA_VREF − (IPD × RF), where IPD is the current flowing
from the PD into the ADPD1080/ADPD1081 input, and
the integrator is off.
• At Point A, the photodiode is disconnected from the
receive path. Light continues to fall on the photodiode
producing a charge that accumulates directly on the
photodiode capacitance. As the charge accumulates, the
voltage at the floating photo-diode anode rises. The TIA is
disconnected from the input to the ADPD188GG so that
no current flows through the TIA, and the TIA output is at
TIA_VREF. Just prior to Point B, the integrator resets to 0.
In the Float Mode for Synchronous LED Measurements
section, the LED pulses during the time period between
Point A and Point D. Float times of <4 µs are not allowed.
• At Point B, the integrator begins its positive integration
phase. Small dc offsets between the TIA output and the
integrator reference causes the integrator output to ramp
up for positive offsets or ramp down for negative offsets.
The photodiode continues to accumulate charge during
this period.
• At Point C, the integrator begins its negative integration
phase. This reversal in polarity begins to cancel any signal
caused by offsets. This offset cancellation continues through
Point F, where all offsets are cancelled completely.
• At Point D, the photodiode switches into the receive path
where all the charge that has accumulated on the photodiode
capacitance during the float time is dumped into the TIA. The
typical charge dump time is less than 2 µs. As the current
flows through the TIA, the output of the TIA responds
with a large negative signal. Because the integrator is in the
negative integration phase at this point, the output of the
integrator rises as the input current to the device integrates
back to total charge. Between Point D and Point E, any
light incident on the photodiode produces additional
photocurrent, which is immediately integrated by the
integrator as charge.
• At Point E, the TIA disconnects from the receive path and
the TIA output returns to TIA_VREF. Between Point E
and Point F, the integrator completes the negative
integration phase and cancellation of the offsets.
• At Point F, the integrator output is held until sampled by the
ADC.