AFE4403 datasheet(21/97 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

ûADC

ADC

ADC Clock

ADC Convert

ADC Output Rate

PRF Sa/sec

I-V Amplifier

Amb cancellation DAC

Buffer

Filter

ADC

Ambient

DAC

AFE4403

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SBAS650B – MAY 2014 – REVISED JULY 2014

8.3 Feature Description

8.3.1 Receiver Channel

This section describes the functionality of the receiver channel.

8.3.1.1 Receiver Front-End

The receiver consists of a differential current-to-voltage (I-V) transimpedance amplifier (TIA) that converts the

input photodiode current into an appropriate voltage, as shown in Figure 36. The feedback resistor of the

amplifier (RF) is programmable to support a wide range of photodiode currents. Available RF values include:

1 M

Ω, 500 kΩ, 250 kΩ, 100 kΩ, 50 kΩ, 25 kΩ, and 10 kΩ.

The device is ideally suited as a front-end for a PPG (photoplethysmography) application. In such an application,

the light from the LED is reflected (or transmitted) from (or through) the various components inside the body

(such as blood, tissue, and so forth) and are received by the photodiode. The signal received by the photodiode

has three distinct components:

1. A pulsatile or ac component that arises as a result of the changes in blood volume through the arteries.

2. A constant dc signal that is reflected or transmitted from the time invariant components in the path of light.

This constant dc component is referred to as the pleth signal.

3. Ambient light entering the photodiode.

The ac component is usually a small fraction of the pleth component, with the ratio referred to as the perfusion

index (PI). Thus, the allowed signal chain gain is usually determined by the amplitude of the dc component.

Figure 36. 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 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 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)

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