Setting up the inputs | Analog Inputs |
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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
This results in a much better use of the converters resolution.
On all acquisition boards from Spectrum you have the possi- bility 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
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.
Register | Value | Direction | Description |
| Offset range |
SPC_OFFS0 | 30000 | r/w | Defines the input’s offset | and therfore shifts the input of channel0. | ± 100 % in steps of 1 % |
SPC_OFFS1 | 30100 | r/w | Defines the input’s offset | and therfore shifts the input of channel1. | ± 100 % in steps of 1 % |
SPC_OFFS2 | 30200 | r/w | Defines the input’s offset | and therfore shifts the input of channel2. | ± 100 % in steps of 1 % |
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SPC_OFFS3 | 30300 | r/w | Defines the input’s offset | and therfore shifts the input of channel3. | ± 100 % in steps of 1 % |
SPC_OFFS4 | 30400 | r/w | Defines the input’s offset | and therfore shifts the input of channel4. | ± 100 % in steps of 1 % |
SPC_OFFS5 | 30500 | r/w | Defines the input’s offset | and therfore shifts the input of channel5. | ± 100 % in steps of 1 % |
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SPC_OFFS6 | 30600 | r/w | Defines the input’s offset | and therfore shifts the input of channel6. | ± 100 % in steps of 1 % |
SPC_OFFS7 | 30700 | r/w | Defines the input’s offset | and therfore shifts the input of channel7. | ± 100 % in steps of 1 % |
When writing a program that should run with different board families it is useful to just
Register | Value | Direction | Description |
SPC_READMAXOFFSET | 3100 | read | Reads out the maximum offset that can be used to compensate an signal offset. Value in ±percent. |
To give you an example how the registers of the input range and the input offset are to be used, the following example shows a setup to match all of the four signals in the second input offset figure to match the desired input range. Therefore every one of the four channels is set
44 | MC.31xx Manual |
