Considerations for Current Inputs
The AIM-16 card has provisions for reading input current. Reserved space and board connections are available for 16 precision resistors. To utilize this provision, it is only necessary to install a resistor of the proper value and set the gain appropriately for each channel to be used for current input.
When all inputs are designated for sensing 4-20mA current, it is beneficial to increase resolution by providing offset. To accomplish this, install a jumper in the OFST position of the programming point labelled "OFST STD". Alternatively, if the A/D card operates on ±10V range, set DIP switch #2 "on" ; i.e. OSH (offset-high) position.
If the AIM-16 that you have is model AIM-16I, then it already has the current-sensing resistors installed. Further, it has also been modified so that maximum resolution is provided at the A/D converter. The modifications that are made include introduction of an offset and addition of a gain adjustment pot.
To better understand the modifications, consider the following. If the A/D converter input range is ±5V, the AIM-16 amplifier gain is 1, and a 250-ohm resistor is used, then a 4-20mA current input would yield voltages of +1V to +5V at the AIM-16 output. Only 40% of the A/D converter range would be used! To accomplish better resolution, it is necessary to introduce an offset and amplifier gain. For example, if a -5V offset is introduced and the amplifier gain is increased to 2, then the output would be -3V to +5V. But that is still only 80% of the A/D converter's range
So, to cover the full range (and thus realize maximum resolution), an additional offset is required to accommodate the 4mA "live zero". On the AIM-16I, standard 200-ohm (0.1%) resistors are used in place of 250-ohm resistors. This would yield +2.496V to +12.48V after amplification. An offset is applied and the net output is -5.004V to +4.980V. The amplifier gain is adjusted slightly with gain adjust pot RP5 to compensate for the fact that 200 ohms is slightly low.
Considerations for Other Voltage Inputs
The input voltage to AIM-16 should be within a range of ±10V. Any gain selected should allow the output to stay within the full-scale range of the host analog-to-digital converter. If you know that the input voltage falls within those limits but don't know exactly what it is, software controlled auto-ranging capability is available via the gain/curve setup routine provided with the AD12-8 setup software.
