Datasheet
0
0.5
1
1.5
2
2.5
3
3.5
0 20 40 60 80 100 120 140 160
%inputcharge
OutputVoltage
IdealP
PracticalP
PracticalM
IdealM
REF5025
10uF
Vin = ~3.3V Vout = 2.5
GND
300
ohm
2.7k
1.5k+47ohm
OPA2376
100 ohm
100 ohm
2.25 V to AFE REFP
0.85 V to AFE REFM
1uF
1uF
Filter for each AFE
To filterinputsfor
other AFE
reference
3.3V
AFE0064
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........................................................................................................................................................................................ SLAS672 –SEPTEMBER 2009
AFE TRANSFER CHARACTERISTICS
The plot above shows AFE transfer characteristics in integrator down mode. (For integrator up mode the P and
M plots are interchanged.) AFE output is linear in the charge range bound by the rectangle shown.
The four corners of the rectangle in clockwise direction, starting with bottom left corner are as follows:
(0%, 0.85 V), (0%, 2.25 V), (100%, 2.25 V), (100%, 0.85 V) where REFP = 2.25 V and REFM = 0.85 V.
Beyond this range, the AFE output still responds to input charge however linearity is not specified. Linearity
deteriorates as the output reaches close to the rails.
One can detect overrange once the output is beyond the linear rectangle and select a higher AFE range. It is
also recommended to clamp the ADC input once it crosses 100% FS.
AFE REFERENCE DRIVING
Figure 33 shows generation of the 0.85 V and 2.25 V references for an AFE. Note that the device uses internal
buffers on the reference inputs. As a result, it is possible to share a reference to multiple AFEs in a system.
However, it is recommended to use a separate 100-Ω, 1-µF LPF for each individual AFE. Use 1% tolerance
resistors for dividing 2.5 V to 2.25 V and 0.85 V.
Figure 33. Typical Reference Generation and Driving Circuit for the AFE0064
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