Converting Analog Measurements | Remote Technologies RPC-320

ANALOG INPUT

CONVERTING ANALOG

MEASUREMENTS

Inputs are converted to "real numbe rs" by perfor ming scaling calculations in the program. The AIN function returns values from 0 to 4095 . To change these numbers into something more meaningful, use the following formula:

var = K * AIN(n)

n is the analog channel to read. K is the scaling constant. K is obtained by dividing the highest number in the range of units by the maximum AIN count (4095).

Example 1: To measure the results of an A/D conversion in volts and the voltage range is 0 to 5V, divided 5 by 4095 to obtain K.

K = 5/4095

K = .001221

Your program could look something like:

1000 C = .001221 * AIN(N)

Example 2: Y ou want to measure a 0 to 200 PSI pressure transducer w ith a 0 to + 5V output. Divide 200 by 4095 to obtain the constant K.

K = 200 / 4095

K = .0488

The code can then look like:

1000 B = .0488 * AIN(0)

Measuring 4-20 mA current loops

Curr ent loops is a convenient way to transmit a value and still assure the integrity of the signal. If the line should break, a 0 volt (or nearly so) is returned.

A 4-20 ma curr ent loop is converted to 1 - 5V by placing a 250 ohm resistor across the input of the chan nel to ground.

Cur rent loop r eadings ar e conver ted to engineer ing units by performing scaling as described earlier. Since the measur ement r ange is 1 to 5V , the count ran ge is reduced by 20% to 3276.

If pressure were measured:

SECTION 10

K = 200/3276

K = .06105

There is one addition factor. Since the lowest value read is 1 V, this offset is subtracted from all readings. A 1 V offset is 1/5 of 4095 counts, or 819. The program line then becomes:

200 A=.06105*(AIN(N)-819)

Note that if the current loop line breaks, a negative value is returned.

AMPLIFIERS

Two operational amplifiers are available to signal condition inputs. Each amplifier is configured as shown below.

Figure 10-2 Amplifier circuit

Amplifiers are accessed through header connector H1. Pin out is as follows:

H1 pin

Function

1Temper ature output from U14

2To channel 0 analog input

3Non -inverting input, amplifier A

4Output from amplifier A

5Inverting input, amplifier A

6Approximately + 7V supply (5 ma maximum)

7Ground

8Ground

9Non -inverting input, amplifier B

10	Approximately - 7V supply (5 ma
	maximum)
11	Inverting input, amplifier A
12	Output from amplifier A

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