AFE4403 datasheet(23/97 Pages) TI

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+1.2 V

+1 V

+0.6 V

0 V

-1 V

-1.2 V

ADC max

(Differential)

TIA max

(Differential)

Ideal Operating

Point

TIA min

(Differential)

ADC min

(Differential)

AFE4403

www.ti.com

SBAS650B – MAY 2014 – REVISED JULY 2014

Feature Description (continued)

For example, for a photodiode current of Iin = 1 µA and a TIA gain setting of RF = 100 kΩ, the differential output

of the TIA is equal to 200 mV. The TIA has an operating range of ±1 V, and the ADC has an input full-scale

range of ±1.2 V (the extra margin is to prevent the ADC from saturating while operating the TIA at the fullest

output range). Furthermore, because the PPG signal is one-sided, only one half of the full-scale is used. TI

recommends operating the device at a dc level that is not more than 50% to 60% of the ADC full-scale. The

margin allows for sudden changes in the signal level that might saturate the signal chain if operating too close to

full-scale. Signal levels are shown in Figure 38:

Figure 38. Signal Levels in TIA and ADC

On startup, a gain calibration algorithm running on the microcontroller unit (MCU) can be used to monitor the dc

level and adjusts the LED current and TIA gain to get close to the target dc level. In addition to a target dc level,

a high and low threshold (for example 80% and 20% of full-scale) can be determined that can cause the

algorithm to switch to a different TIA gain or LED current setting when the signal amplitude changes beyond

these thresholds.

In heart rate monitoring (HRM) applications demanding small-form factors, the sensor size can be so small (and

the signal currents so low) that they do not occupy even 50% of full-scale even with the highest TIA gain setting

of 1 M

Ω, which is the case for signal currents that are less than 300 nA. As such, experimentation with various

use cases is essential in order to determine the optimal target value, as well as high and low thresholds. Also, by

enabling the stage 2 and introducing additional gain (up to 12 dB), a few extra decibels of SNR can be achieved.

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AFE4403 Datasheet(PDF) 23 Page - Texas Instruments