Thursday, April 15, 2010

PLC Analog Output

Analog outputs PLC much simpler than analog inputs PLC. To set an analog output an integer is converted to a voltage. This process is very fast, and does not experience the timing problems with analog inputs. But, analog outputs subject to quantization errors.
Figure below gives a summary of the important relationship. This relationship is almost identical to those of the A/D converter.
AD converter
Assume we are using an 8 bit D/A converter that outputs values between 0V to 10V. We have a resolution of 256, where 0 results in an output of 0V and 255 results in 10V. The quantization error will be 20mV. If we want output of a 6.234V, we would specify an output integer of 159, these would result in an output voltage of 6.235V. The quantization error would be 6.235V – 6.234V = 0.001V. You can see the calculation as below:
DA converter
The current output from a D/A converter is normally limited to a small value, typically less than 20mA. This is enough for instrumentation, but for high current loads such as motors, a current amplifier is needed. If the current limit is exceeded for 5V output, the voltage will decrease. If the current limit is exceeded for long periods of time the D/A outputs may be damaged.

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