Datasheet

V = 2
DIFF
´
I
PLETH
´
R
F
R
I
+ I
AMB
´
R
F
R
I
- I
CANCEL
´ R
G
Rf
Cf
Rf
Cf
Rx
V
DIFF
Ri
Ri
Rg
Rg
I
PLETH
+ I
AMB
I
CANCEL
Value of I
CANCEL
set using
the SPI interface.
I
CANCEL
AFE4490
www.ti.com SBAS602F –DECEMBER 2012–REVISED OCTOBER 2013
Using the set value, the ambient cancellation stage subtracts the ambient component and gains up only the pleth
component of the received signal, as shown in Figure 56. The amplifier gain is programmable to 0 dB, 3.5 dB,
6 dB, 9.5 dB, and 12 dB.
Figure 56. Front-End (I-V Amplifier and Cancellation Stage)
The differential output of the second stage is V
DIFF
, as given by Equation 2:
where:
• R
I
= 100 kΩ,
• I
PLETH
= photodiode current pleth component,
• I
AMB
= photodiode current ambient component, and
• I
CANCEL
= the cancellation current DAC value (as estimated by the host processor). (2)
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