B&B Electronics RS-485 manual Sampling Rate, Input Range, Reference Inputs

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.

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