B&B Electronics 4 Channel Input Buffer Board SDAIBB Manual do Utilizador descarregar pdf (Página 15)

Documentation Number UD128A8D4602 Manual Chapter 1 11

B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104

B&B Electronics Ltd – Westlink Comm. Pk. – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax 353-91-792445

1. The pairs are locked with the even numbered inputs being the

positive input and the odd numbered inputs being the negative

(AD0 & AD1, AD2 & AD3, etc.)

2. The UD128A8D does not produce a negative output if the plus

input goes below the minus. The plus input always has to be

higher.

3. There is no common mode voltage tolerance. Both inputs must be

in the 0-5V range and should still have a ground connection.

4. The full output range is still set by the reference. If you want to

measure a differential signal that has less span than 0-5V, you

must change the reference.

5. The minus input cannot go above 1.25V.

• The differential input is mainly for getting rid of ground noise.

• Digital and Analog grounds are tied together at one point on the

PCB and should not be used interchangeably to avoid noise

problems.

• When using the analog inputs to measure a 0-20mA signal with a

resistor to ground, it has to be the last element in the loop,

returning current to the supply. If more than one measuring device

is used, the UD128A8D must be last or the loop must be isolated

from ground. (See Fig. 2.16)

• To obtain the best accuracy of the UD128A8D using the built-in 5V

reference, you must use a 249 Ohm resistor as the sense resistor

for 0-20 or 4-20mA measurement. (See Fig. 2.16) The voltage for

20mA current is 4.98 Volts or 4080 counts. The voltage supply for

the current loop must be more than 5V plus all other voltage drops

in the loop.

• The digital count for 4-20mA is 204 counts per mA. All the

816 counts under 4mA (0.996 volts) are below scale, so

useable range is from 816 to 4080, 3264 counts over the

16mA range. This is the best possible from 4096 steps

without using additional external circuitry.

Analog Outputs

• Analog output commands get second priority to Analog inputs

during timed sampling. If there is not enough time to set an analog

output, an error will be reported from the hardware. If there is time

to set an analog output, it will be set between reads of the A/D. All

commands have the same priority when timed sampling is not

taking place. Digital I/O can be done during Analog Sampling, but

may be slower depending on the sampling rate

• The Analog Output reference is fixed at 5V. Factory hardware

modifications are required to set a lower reference.

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