Datasheet
R C
F
´ £
F
Rx Sample Time
10
LED2
S
LED2
CONV
LED2
Amb
Amb
S
LED1_amb
S
LED1
S
LED2_amb
CONV
LED1
CONV
LED2_amb
CONV
LED1_amb
LED1
Rx
Ambient-cancellation current can be set digitally using SPI interface.
C
F
C
F
R
F
R
F
C
PD
+
TIA
+
Stage 2
Gain
R
G
R
G
+
Buffer
ûADC
ADC
ADC Clock
ADC Convert
ADC Output Rate
PRF Sa/sec
I-V Amplifier
Amb cancellation DAC
BufferFilter ADC
Ambient
DAC
AFE4490
www.ti.com
SBAS602F –DECEMBER 2012–REVISED OCTOBER 2013
RECEIVER CHANNEL
This section describes the receiver channel functionality.
Receiver Front-End
The receiver consists of a differential current-to-voltage (I-V) transimpedance amplifier that converts the input
photodiode current into an appropriate voltage, as shown in Figure 54. The feedback resistor of the amplifier (R
F
)
is programmable to support a wide range of photodiode currents. Available R
F
values include: 1 MΩ, 500 kΩ,
250 kΩ, 100 kΩ, 50 kΩ, 25 kΩ, and 10 kΩ.
Figure 54. Receiver Front-End
The R
F
amplifier and the feedback capacitor (C
F
) form a low-pass filter for the input signal current. Always ensure
that the low-pass filter RC time constant has sufficiently high bandwidth (as shown by Equation 1) because the
input current consists of pulses. For this reason, the feedback capacitor is also programmable. Available C
F
values include: 5 pF, 10 pF, 25 pF, 50 pF, 100 pF, and 250 pF. Any combination of these capacitors can also be
used.
(1)
The output voltage of the I-V amplifier includes the pleth component (the desired signal) and a component
resulting from the ambient light leakage. The I-V amplifier is followed by the second stage, which consists of a
current digital-to-analog converter (DAC) that sources the cancellation current and an amplifier that gains up the
pleth component alone. The amplifier has five programmable gain settings: 0 dB, 3.5 dB, 6 dB, 9.5 dB, and 12
dB. The gained-up pleth signal is then low-pass filtered (500-Hz bandwidth) and buffered before driving a 22-bit
ADC. The current DAC has a cancellation current range of 10 µA with 10 steps (1 µA each). The DAC value can
be digitally specified with the SPI interface. Using ambient compensation with the ambient DAC allows the dc-
biased signal to be centered to near mid-point of the amplifier (±0.9 V). Using the gain of the second stage
allows for more of the available ADC dynamic range to be used.
The output of the ambient cancellation amplifier is separated into LED2 and LED1 channels. When LED2 is on,
the amplifier output is filtered and sampled on capacitor C
R
. Similarly, the LED1 signal is sampled on the C
LED1
capacitor when LED1 is ON. In between the LED2 and LED1 pulses, the idle amplifier output is sampled to
estimate the ambient signal on capacitors C
LED2_amb
and C
LED1_amb
.
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