Rabbit BL2020, User Manual

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User’s Manual 27
3.5 A/D Converter Inputs
The single 14-channel A/D converter used in the BL2000 has a resolution of 12 bits (models
BL2000 and BL2020) or 10 bits (models BL2010 and BL2030). 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.
The A/D converter only measures voltages between 0 V and the applied reference voltage.
Therefore, each external input has circuitry that provides scaling and buffering. The first
four external inputs are scaled and buffered to provide the user with an input impedance of
1 M
Ω
and a range of -10.24 V to +10.24 V. The remaining five or seven inputs are not
buffered, but are scaled to provide inputs that can range from 0 V to +49 V.
Figure 17 shows the buffered A/D converter inputs.
Figure 17. Buffered A/D Converter Inputs
The op-amp is powered from the +V supply. The 1 M
Ω
and 200 k
Ω
resistors set the gain
(scale factor), which is 0.2 in this case. This results in a dynamic input range of 4.096 V ÷
0.2 or 20.48 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 five or seven unbuffered inputs have an impedance of 12 k
Ω
and a scale factor of
0.0833, which provides for an input voltage range of 0 V to 49.15 V. Accuracy is main-
tained over the specified voltage range from 0 V to 48 V DC.
The analog inputs can also be used as digital inputs when required. In this case a lower
quality 10-bit D/A converter can be used, and the software would assign a 1 or 0 to a volt-
age based on whether it is above or below a particular threshold. See the
digIn
function
description for more information.
100 pF
200 k W
To ADC
1 M W
R
IN
100 pF
ADC0
AGND
+ V
ADC1