Spectrum M2p.59 Series, Руководство по оборудованию, Страница: 76 / 190

76 (c) Spectrum Instrumentation GmbH
Setting up the inputs Analog Inputs
Input offset
In most cases the external signals will not be symmetrically re -
lated to ground. If you want to acquire such asymmetrical sig -
nals, it is possible to use the smallest input range that matches
the biggest absolute signal amplitude without exceeding the
range.
The figure at the right shows this possibility. But in this exam -
ple you would leave half of the possible resolution unused.
It is much more efficient if you shift the signal on-board to be
as symmetrical as possible and to acquire it within the best
possible range.
This results in a much better use of the converters resolution.
On this acquisition boards from Spectrum you have the pos -
sibility to adjust the input offset separately for each channel.
The example in the right figure shows signals with a
range of ±1.0 V that have offsets up to ±1.0 V. So relat -
ed to the desired input range these signals have offsets
of ±100 %.
For compensating such offsets you can use the offset reg -
ister for each channel separately. If you want to compen -
sate the +100 % offset of the outer left signal, you would
have to set the offset to -100 % to compensate it.
As the offset levels are relatively to the related input
range, you have to calculate and set your offset again
when changing the input’s range.
The table below shows the offset registers and the possi -
ble offset ranges for your specific type of board.
When writing a program that should run with different board families it is useful to just read-out the possible offset than can be programmed.
You can use the following read only register to get the possible programmable offset range in percent
Table 28: Spectrum API: input offset registers
Register Offset range
SPC_OFFS0 30000 r/w Defines the input’s offset and therefore shifts the input of channel0. ± 100 % in steps of 1 %
SPC_OFFS1 30100 r/w Defines the input’s offset and therefore shifts the input of channel1. ± 100 % in steps of 1 %
SPC_OFFS2 30200 r/w Defines the input’s offset and therefore shifts the input of channel2. ± 100 % in steps of 1 %
SPC_OFFS3 30300 r/w Defines the input’s offset and therefore shifts the input of channel3. ± 100 % in steps of 1 %
SPC_OFFS4 30400 r/w Defines the input’s offset and therefore shifts the input of channel4. ± 100 % in steps of 1 %
SPC_OFFS5 30500 r/w Defines the input’s offset and therefore shifts the input of channel5. ± 100 % in steps of 1 %
SPC_OFFS6 30600 r/w Defines the input’s offset and therefore shifts the input of channel6. ± 100 % in steps of 1 %
SPC_OFFS7 30700 r/w Defines the input’s offset and therefore shifts the input of channel7. ± 100 % in steps of 1 %
Table 29: Spectrum API: registers to read out the possible input offset settings range
Register Value Direction Description
SPC_READOFFSMIN0 4200 read Minimum programmable offset for input range 0 in percent
SPC_READOFFSMAX0 4100 read Maximum programmable offset for input range 0 in percent
SPC_READOFFSMIN1 4201 read Minimum programmable offset for input range 1 in percent
SPC_READOFFSMAX1 4101 read Maximum programmable offset for input range 1 in percent
.. ... ... ...
Image 46: Spectrum API: using the input offset shifting to optimize the usage of the input range
Image 47: Spectrum API: effects of different input offset setting