Page 10
Input Connections
Page 10
Blue Chip Technology Ltd.
01271014.doc
For Single Ended Mode
LKH
D
S
b Differential Mode Termination
This section is only relevant if you are using the analogue inputs in differential
mode.
The ADC uses solid-state multiplexers. These devices will not operate properly
if either signal voltage input e or - 12 volts. These limits define the
common mode voltage range. The board has a differential input option and will
accept signals which are floating with respect to the computer system ground.
But these signals must still lie within the common mode voltage range (CMVR)
in order to obtain satisfactory operation. In some cases it may be necessary to
tie external signals to the computer ground via suitable resistors to hold them
within CMVR. Failure to do so may result in erratic readings or damage.
The board provides links (LKE and LKD) which may be used to pull the input
signals towards ground. The 1M resistor value generally proves satisfactory,
but lower values may be needed in electrically noisy environments.
In many cases external signals are related to mains ground and the computer is
also earthed so that signals automatically lie within CMVR.
However, in the case of the Amstrad PC1512 computer, there is no built-in
mains earth connection and it may be necessary to provide one in order to
obtain proper performance of the ADC board.
Whether the differential signals are terminated or not is selected by two links
marked LKE and LKD on the ADC card.