Remote-processing RPC-210 Manuel d'utilisateur télécharger pdf (Page 44)

ANALOG I/O BASIC SECTION 10

Page 10-2

When I/O range is 5V max., set W12[1-2]. If the I/O

range is 2.5V max. , set W12[2-3].

When W 12 is set to [1-2], a higher, but noisier supply

voltage is applied to the buffers. The result is D/A

outputs are slightly noisier (8 bits or so) and A/D

readings are noisier.

WARNING:

Buffers obtain power from the RS-232 driver when

W12 is set to [1-2] (5V range). If one or both serial

ports are under heavy load, the supply voltag e will

probably drop, causing the 5V reference to go out of

reference. This will result in inaccurate A/D and

D/A readings in the 5V range.

The 5V max. range is provided for uses that do not need

high conversion accuracy and where wiring is simplified

by directly connecting to the inputs. Highest accuracy

and best per form ance is when A/ D and D /A is set to

2.5V max. range.

ACQUIRING ANALOG DATA

Analog data is acquired using the AIN(n) function. n is

the channel number in the range of 0 to 8. The function

returns a number from 0 to 4095. Channel 8 returns the

temperature from the reference in the CPU.

The AIN command makes 8 conversions then averages

them. This tends to reduce noise, possibly eliminating

the need for filter capacitors on the inputs.

The input range for all channels is set by jumper W11.

When installed, the input range is 0 to 5V volts. This is

adjustable to ±0.05 volts. W hen W11 is removed, the

range is 0 to 2.5 ± 0.05V . T his voltage is not ad justable

as it is set by the CPU.

AIN210-1. BAS conve rts all chann els and contentiously

prints the result.

Acquiring after low power or sleep modes

You need to allow at least 1/4 second after exiting from

low power or sleep modes to stabilize pow er. This

includes modes where the RS-232 port is powered off.

You can include power up reset time, normally about 1/4

second, in your wait time calculations.

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-210 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.

Since RPBASIC-52 takes 8 readings for averaging, the

signal should be stable for at least 200 micr o-Seconds.

Signal Source

The output resistance (impedance) of the signal source

should be as low as possible. A low impedance source

helps keep noise low and reduces errors due to source

resistance.

Voltage er rors are primarily due to the 1 M ohm pull

down resistors at the analog inputs. For ½ LSB

accuracy, source impedance should be 500 ohm s or less.

Inputs are buffered through a high impedance op-amp.

Input impedance is on the order of several hundred

meg-ohms. Resistor network R11 can be rem oved to

allow an increase in source resistance. If you do this,

then all unused inputs must be grounded.

TEMPERATURE MEASUREMENT

The ADUC812 A -D re serves channel 8 for an on-chip

temperature sensor. This sensor is used to get an

ambient temperature reading around the CPU.

The sensor typically outputs a 600 mV signal @ 25°C.

This voltage is not specified, however, and could vary

from one unit to another. Voltage typically changes by -

3.0 mV/ °C. T he sensor is on the CP U chip, so it is

good for measuring ambient temperature in that area.

AIN-1.BAS prints the temperature as a voltage.

CONVERSION ERRORS

The A/D and D/ A converters have 12 bit resolution.

Note that resolution is not necessarily the same as

accuracy. The converters have sources of errors that

greatly affect accuracy.

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Remote-processing RPC-210 Manuel d'utilisateur Page 44

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