AFE4490 datasheet(17/70 Pages) TI

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Device

Digital Control for Ambient-Cancellation DAC

Digital

LED2 Data

ADC Output Rate

PRF Samples per Second

Ambient (LED2)

Data

LED1 Data

Ambient (LED1)

Data

SPI

Block

SPI

Interface

Host Processor

Ambient Estimation Block

Ambient information is available in the host

processor.

The processor can:

* Read ambient data

* Estimate ambient value to

be cancelled

* Set the value to be used by the ambient

cancellation DAC using the SPI of AFE

Front End

(LED2 ± Ambient)

Data

(LED1 ± Ambient)

Data

ADC

AFE4490

www.ti.com

SBAS602B – DECEMBER 2012 – REVISED FEBRUARY 2013

A single, 22-bit ADC converts the sampled LED2, LED1, and ambient signals sequentially. Each conversion

takes a maximum of 25% of the pulse repetition period (PRP) and provides a single digital code at the ADC

output. As discussed in the Receiver Timing section, the conversions are staggered so that the LED2 conversion

starts after the end of the LED2 sample phase, and so on. This configuration also means that the Rx sample

time for each signal is no greater than 25% of the pulse repetition period.

Note that four data streams are available at the ADC output (LED2, LED1, ambient LED2, and ambient LED1) at

the same rate as the pulse repetition frequency. The ADC is followed by a digital ambient subtraction block that

additionally outputs the (LED2 – ambient LED2) and (LED1 – ambient LED1) data values.

Ambient Cancellation Scheme

The receiver provides digital samples corresponding to ambient duration. The host processor (external to the

AFE) can use these ambient values to estimate the amount of ambient light leakage. The processor must then

set the value of the ambient cancellation DAC using the SPI, as shown in Figure 28.

Figure 28. Ambient Cancellation Loop (Closed by the Host Processor)

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