Rabbit Smartcat, User Manual

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User’s Manual 29
3.5 A/D Converter Inputs
The single 14-channel A/D converter chip used in the BL2100 has a resolution of 12 bits
(models BL2100 and BL2120 only). Eleven of the 14 channels are available externally,
and three are used internally for the reference voltages: 4.096 V (V
ref
), 2.048 V (V
ref
/2),
and Analog Ground. These internal voltages can be used to check the functioning of the
A/D converter chip.
The A/D converter chip only measures voltages between 0 V and the applied reference
voltage. Therefore, each external input has circuitry that provides scaling and buffering.
All 11 external inputs are scaled and buffered to provide the user with an input impedance
of 1 M

and a range of -10.24 V to +10.24 V.
Figure 18 shows the buffered A/D converter inputs.
Figure 18. Buffered A/D Converter Inputs
The op-amp is powered from the +V supply. The 1 M

and 200 k

resistors set the gain
(scale factor), which is 0.2 in this case. This results in a dynamic input range of 0.2 × 20.48 V
or 4.096 V. The center point of this range is set by the 1.707 V reference voltage. With the
reference set to 1.707 V, the center point is at 0 V and the input voltage can range from
-10.24 V to +10.24 V. To maintain the best accuracy, the input range should be limited to
-10.0 V to +10.0 V.
The A/D converter inputs are factory-calibrated and the calibration constants are stored in
flash memory. You may calibrate the A/D converter inputs at a later time using the software
functions described in Section 4.4.4, “A/D Converter Inputs.” The
GETCALIB.C
and the
SAVECALIB.C
sample programs in the Dynamic C
SAMPLES\BL2100\
Calib_Save_Retrieve
folder illustrate how to retrieve and save calibration data.
100 nF
200 k W
To ADC
1 M W
R
IN
1 nF
ADCIN0
AGND
+ V
ADCIN1
ADREF