B&B Electronics RS-485 manual Appendix B Analog Input Impedance

Appendix B: Analog Input Impedance

When interfacing with an A/D converter, it is important that the device you are connecting can drive the A/D input. To determine if your device can drive an A/D input, there are three factors you must consider:

∙Output impedance of the device

∙Input impedance of A/D

∙A/D sampling time

The goal is to have the voltage at the A/D input settle to a voltage close to the output voltage of the device in a time frame that is less than the A/D sampling time. (Close to means a value significantly less than the resolution of the A/D). If the voltage does not settle fast enough, errors will occur in the reading, resulting in a loss of resolution.

The next section, titled “Simplified Analog Input Analysis,” contains information from Texas Instruments data sheet on the TLC1543. The TLC1543 is the A/D converter that is used on the 485SDA10. This section provides a simplified calculation which can be used to determine the maximum output impedance the device can have to settle the A/D input to a voltage within one half LSB.

For the 485SDA10:

tc = 100us

Using this information:

Rs ≤ 170kΩ

If the output impedance of your device is 170kΩ, you should figure an additional error of ½ LSB.

It should be pointed out that this is a simplified analysis and there other several other factors that must be considered (pin capacitance, noise immunity, etc.). The data sheet for the TLC1543 states that “The driving source impedance should be less than or equal to 1kΩ.” B&B Electronics recommends placing a voltage follower between the 485SDA10 and any device with output source impedance greater than 1kΩ.

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