AFE4490 datasheet(16/70 Pages) TI

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F ´

Rx Sample Time

LED2

SLED2

CONVLED2

Amb

SLED1_amb

SLED1

SLED2_amb

CONVLED1

CONVLED2_amb

CONVLED1_amb

LED1

Ambient-cancellation current can be set digitally using SPI interface.

CPD

TIA

Stage 2

Gain

Buffer

û ADC

ADC

ADC Clock

ADC Convert

ADC Output Rate

PRF Sa/sec

I-V Amplifier

Amb cancellation DAC

Buffer

Filter

ADC

Ambient

DAC

AFE4490

SBAS602B – DECEMBER 2012 – REVISED FEBRUARY 2013

www.ti.com

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 27. The feedback resistor of the amplifier (RF)

is programmable to support a wide range of photodiode currents. Available RF values include: 1 MΩ, 500 kΩ,

250 k

Ω, 100 kΩ, 50 kΩ, 25 kΩ, and 10 kΩ.

Figure 27. Receiver Front-End

The RF amplifier and the feedback capacitor (CF) form a low-pass filter for the input signal current. Always ensure

that the low-pass filter 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 CF 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: 1, 1.5, 2, 3, and 4. 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.

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 CR. Similarly, the LED1 signal is sampled on the CLED1

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 CLED2_amb and CLED1_amb.

The sampling duration is termed the Rx sample time and is programmable for each signal, independently.

Sampling can start after the I-V amplifier output is stable (to account for LED and cable settling times). The Rx

sample time is used for all dynamic range calculations; the minimum time supported is 50 µs.

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