Datasheet
Device
Digital Control for Ambient-Cancellation DAC
Rx
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
SBAS602F –DECEMBER 2012–REVISED OCTOBER 2013
www.ti.com
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.
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 55.
Figure 55. Ambient Cancellation Loop (Closed by the Host Processor)
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