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

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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.

485SDA103798 Manual

Appendix B

B-1

B&B Electronics -- 707 Dayton Rd. -- PO Box 1040 -- Ottawa, IL 61350

PH (815) 433-5100 -- FAX (815) 434-7094

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Contents Internet Electronics Mfg. Co. IncDesigned and Manufactured Ottawa, Illinois Table of Contents Tables Software485SDA103798 Manual Table of Contents Iii 485SDA10 Unit 485SDA10 FeaturesPacking List Software InstallationWindows 9x and Windows NT UninstallGetting Started WindowsA/D with Variable Resistor 485SDA10 Specifications Ref Input + Inputs #0-10Typical Connections Ref InputAnalog Ground Digital Outputs #0-2 Digital I/O Connections485SDA10 I/O Port Pinout Digital Inputs #0-2Typical Digital I/O Connections Digital GroundRS-485 Terminal Block Connections Serial Port ConnectionsTypical RS-485 2-wire Connection Power Supply Connections485SDA10 Commands CommandsEquivalent Values Reading A/D Channels CommandSyntax Read A/D Response AddrRA#Ch#MSBch#LSBch#-1MSB...ch0MSBch0LSB Read Digital I/O Response for Outputs Reading Digital I/O CommandAddrRD StatesSet Digital Output Data Byte Values Set Digital Output CommandSet Module Address Command AddrSOstatesRead Module Configuration Command Set Power-up States CommandSet Turn-around Delay Command Addressstatesdelay Reference Inputs Sampling RateInput Range Converting Data Data RangePage MSB$ = INPUT$1, #1 LSB$ = INPUT$1, #1 Read A/D CommandCommand$ = ! + CHR$addr + RA + CHR$channel Print #1, Command$Reply$ = INPUT$ 1, #1 Read Digital I/O CommandCommand$ = ! + CHR$addr + RD Digital I/O Mask Values Set Digital Output StatesAddr = Command$ = !0SO + CHR$statesCommand$ = ! + CHR$addr + SO + CHR$newaddr Set Module AddressAddr = 5 states = States = states and NOTmaskCommand$ = ! + CHR$addr + SC + CHR$delay Set Turn-around DelayRead Module Configuration Comp = not x and &HFF #addrRA00FF00FF01FE Table A-1 Extended Commands Appendix B Analog Input Impedance 1 2 LSB = R s + 1k Ω × 60pF × ln2048 Simplified Analog Input Analysis485SDA103798 Manual Appendix B Table C-1 Decimal to HEX to Ascii Table Appendix C Decimal to HEX to Ascii TableClass a Equipment
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RS-485 specifications

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