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ANALOG INPUT SECTION 11

Page 12-1

Figure 11-1 Analog Input and Reference Adj.

INTRODUCTION

ANALOG INPUT SECT ION 11

There are eight single ended analog to digital (A-D)

input channels on the RPC-220. These channels are used

to measure voltages from transducers, transmitters, 4-20

ma current loops, thermistors, etc. Input voltage range

is 0 to 5 volts with 10 bit (1024) count resolution. Input

impedance is 100K ohm s to ground . C onversio n time is

about 28 micro-seconds. Reference is adjustable to 5.00

±.20 volts.

This section begins with basic information to connect and

use analog inputs. Later, descriptions of how to measure

voltages other than 5 volts and calibration are presented.

CONNECTING ANALOG INPUTS

All analog inputs interface through connector J1. A

STB-20 terminal board and CM A-20 ribbon cable can be

used to provide screw terminal interface. Additional

components, such as resistors and capacitors, may be

connected directly to the screw term inals.

For greatest accuracy, connect unused inputs to ground.

In environments with high voltage or static electricity,

unused inputs can bleed over to other channels.

R4 may be adjusted to trim accuracy and/or maximum

voltage to your system. See Calibration later in this

section for more information.

ACQUIRING ANALOG DATA

Analog data is acquired in polled or interrupt fashion.

Polled mode takes about 50 micro-seconds/channel when

written in C. A-D conversion time is about 28 micro-

seconds. Read INPUT SIGNAL CONSIDERATIONS

below for signal requirements.

INPUT SIGNAL CONSIDERATIONS

Slew Rate

Slew rate is the rate of signal change. It is usua lly

expressed as volts/second, but other units are also used.

Ideally, signals presented to the RPC-220 are steady, DC

signals. However, this is not a real world situation.

Input signals are always changing.

To get a good reading from the ADC, the input signal

should be relatively stable during the sampling time.

Sampling time begins w hen conversion star ts. T o obtain

1/2 LSB (least significant bit) stability, the input signal

should not change more than 2.44 mv in 3.3 micro-

seconds or a rate of 750 volts/second.

The effect of a fast slew rate is a probable conversion

result of 0x3ff. The problem is that a rapidly changing

signal couples through to the input of the internal ADC

comparator, saturating it. This is true when any

channel, selected or not, changes rapidly. The input

signal may change rapidly before any conversion. The

critical time when the signal must be stable is during

conversion.

A way to limit slew rate is to connect an RC filter to the

analog inputs. The time constant should be 500 micro-

seconds or more. If a 5 volt P-P sine wave signal is

applied, the maximum frequency is about 600 Hz.

Signal Source

The output resistance (impedance) of the signal source

should be as low as possible. If the source impedance is

too high, there will be a voltage drop over the source

resistance.

The voltage over the output impedance is primarily due

to the 100K ohm pull down resistors at the analog inputs.

For 1/2 LSB accuracy, source impedance should be 50

ohms or less.

The resistor network R1 can be removed to allow an

increase in source resistance. If you do this, then all

unused inputs must be grounded.

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