B&B Electronics RS-485 Simplified Analog Input Analysis, 1 2 LSB = R s + 1k Ω × 60pF × ln2048

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Simplified Analog Input Analysis

Using the equivalent circuit in Figure B-1, the time required to charge the analog input capacitance from 0 to Vs within ½ LSB can be derived as follows:

The capacitance charging voltage is given by

Vc = Vs (1 − e−tc RtCi )

where

Rt = Rs + ri

The final voltage to ½ LSB is given by

Vc (12 LSB) = Vs − (Vs 2048)

Equating equation 1 to equation 2 and solving for time tc gives

Vs − (Vs 2048) = Vs (1 − e−tc RtCi )

and

tc (12 LSB) = Rt × Ci × ln(2048)

(1)

(2)

(3)

(4)

Therefore, with the values given the time for the analog input signal to settle is

tc (1 2 LSB) = ( Rs + 1kΩ) × 60pF × ln(2048)

(5)

This time must be less than the converter sample time shown in the timing diagrams.

VI = Input Voltage at A0 - A10

VS = External Driving Source Voltage RS = Source Resistance

ri = Input Resistance

Ci = Equivalent Input Capacitance *Driving source requirements:

∙Noise and distortion for the source must be equivalent to the resolution of the converter.

∙RS must be real at the input frequency.

B-2

Appendix B

485SDA103798 Manual

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
Electronics Mfg. Co. Inc Designed and Manufactured Ottawa, IllinoisInternet Table of Contents Software Tables485SDA103798 Manual Table of Contents Iii 485SDA10 Features 485SDA10 UnitSoftware Installation Packing ListUninstall Getting StartedWindows Windows 9x and Windows NTA/D with Variable Resistor 485SDA10 Specifications Inputs #0-10 Ref Input +Ref Input Analog GroundTypical Connections Digital I/O Connections 485SDA10 I/O Port PinoutDigital Inputs #0-2 Digital Outputs #0-2Digital Ground Typical Digital I/O ConnectionsSerial Port Connections RS-485 Terminal Block ConnectionsPower Supply Connections Typical RS-485 2-wire ConnectionCommands 485SDA10 CommandsReading A/D Channels Command SyntaxEquivalent Values AddrRA# Ch#MSBch#LSBch#-1MSB...ch0MSBch0LSBRead A/D Response Reading Digital I/O Command AddrRDStates Read Digital I/O Response for OutputsSet Digital Output Command Set Module Address CommandAddrSOstates Set Digital Output Data Byte ValuesSet Power-up States Command Set Turn-around Delay CommandRead Module Configuration Command Addressstatesdelay Sampling Rate Input RangeReference Inputs Data Range Converting DataPage Read A/D Command Command$ = ! + CHR$addr + RA + CHR$channelPrint #1, Command$ MSB$ = INPUT$1, #1 LSB$ = INPUT$1, #1Read Digital I/O Command Command$ = ! + CHR$addr + RDReply$ = INPUT$ 1, #1 Set Digital Output States Digital I/O Mask ValuesCommand$ = !0SO + CHR$states Command$ = ! + CHR$addr + SO + CHR$newaddrSet Module Address Addr =States = states and NOTmask Addr = 5 states =Set Turn-around Delay Command$ = ! + CHR$addr + SC + CHR$delayRead Module Configuration #addrRA00FF 00FF01FEComp = not x and &HFF Table A-1 Extended Commands Appendix B Analog Input Impedance Simplified Analog Input Analysis 1 2 LSB = R s + 1k Ω × 60pF × ln2048485SDA103798 Manual Appendix B Appendix C Decimal to HEX to Ascii Table Table C-1 Decimal to HEX to Ascii TableClass a Equipment
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