Remote Technologies RPC-320 manual Converting Analog Measurements, Amplifiers

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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