Keithley KPCMCIA-12AIAOH, User Manual

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KPCMCIA-12AIAOH User’s Manual Theory of Operation 3-3
Analog input multiplexer
The differential or single-ended configuration is selected via software.The expansion cards can
only be used on single-ended channels. The single-ended/differential selection should be the
same for all the internal channels (e.g., all four channels as differential, or all eight channels as
single-ended). Having some channels configured as single-ended and others as differential, pos-
sible as it is, may cause confusion and unexpected signal errors.
In a differential configuration, there are, at most, four channels. However, if you specify channels
4 to 7 in a differential configuration, the inputs to ground for system offset measurement will
short. The readings taken under such a circumstance can be used for offset correction.
The input multiplexers have built-in protection against overvoltage when the board is both pow-
ered on and powered down. The protection mechanism isolates the input from the rest of the board
as long as the input voltage is within the protection range of ±30V.
Programmable gain control
The KPCMCIA-12AIAOH PC card has an internal gain of 1, 10, 100, and 1000. The gain can be
changed “on the fly” from channel to channel when scanning through the channels. There is a pro-
grammable gain instrumentation amplifier with gains of 1, 10, 100, and 1000. The internal gain
selection is specified in the scan list entry via software.
The settling time of the analog front end meets the speed requirement. However, if the amplifier
is saturated, it may need a longer time to recover, which may cause distortion in the input signal
to the A/D converter. Amplifier saturation should be avoided (use low gains, attenuate the input
signal, etc.).
Scan list
One entry to the scan list contains a 16-bit word, or two 8-bit bytes. It specifies the internal chan-
nel selection and gain selection (in the high byte, or MSB), the external channel and gain selection
(in the low byte, or LSB), and other control and configuration settings.
The external selections are used for channels on the expansion cards, while internal selections are
used for channels on the KPCMCIA-12AIAOH PC card.
The expansion cards are not included as part of the KPCMCIA-12AIAOH data acquisition sys-
tem. However, they can be purchased separately from Keithley.
The number of entries in the scan list ranges from 1 to 2048. No dependencies are implied among
the entries of the scan list. You may choose any valid gain combinations for any channel, internal
or external. The channels can be scanned in any order as required, repeated or not, with the same
or different gain for each entry.
The differential/single-ended control bit (at bit 14, MSB) should be the same for all the entries in
the scan list, which is most likely the case for normal operations. Single-ended configurations
should be selected if one or more expansion cards are connected to the KPCMCIA-12AIAOH PC
card.
The synchronous sample hold bit (or SSH at bit 6, LSB) is reserved for the expansion cards.