User's Manual

485SDA103798 Manual 19
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Chapter 4 - A/D
This chapter will deal with manipulating an A/D reading and cover
some of the aspects that were not explained in the A/D Connections
chapter.
Sampling Rate
The A/D converter has a conversion time of around 20
microseconds, however the sampling rate is limited by the serial
communications. The maximum sampling rate for a single channel
is around 120 samples per second (9600 baud). This rate drops to
25 samples per second when sampling all of the channels. When
reading an A/D input, the 485SDA10 takes four readings and returns
the average (0.5 and greater are rounded up) of these readings.
This averaging filters out noise.
A/D Input Range
The A/D input range on the 485SDA10 is from 0 to +5V DC. If it
is possible for your device to output a voltage that doesn’t fall in this
range, steps must be taken to ensure that the voltage remains
between 0 and +5V DC. Voltages outside this range will damage
the unit.
Reference Inputs
The A/D reference inputs set the top and bottom of the data
range. A/D Ref Input- sets the bottom of the data range.
A/D Ref Input+ sets the top of the data range. Since these inputs
are directly related to the data range, it is important that a precision
reference is used. The 485SDA10 has a 5V DC +/- 0.5% reference
available. The voltage on A/D Ref Input+ must be at least 2.5V DC
greater than A/D Ref Input-. The voltage difference between
A/D Ref Input+ and A/D Ref Input- is referred to as the Reference
Range.
Reference Range = (A/D Ref Input+) - (A/D Ref Input-)
Typically A/D Ref Input- is connected to Analog ground and
A/D Ref Input+ is connected to +5V DC. Figure 2.1 in Chapter 2
shows the typical connections for a reference range of 0 to 5V DC.