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M3i.48xx / M3i.48xx-exp Manual
Setting up the inputs
Analog Inputs
The following example shows a setup of path and input range of a two channel card.
Please note that this is a general example and the number of input channels may not match your card channels.
Input termination
The Spectrum analog acquisition cards of the M3i series offer an input path with
fixed 50 ohm termination (HF path, 50 ohm path) as well as a second input path
with all features to be programmed by the user (buffered path). If the HF path with
fixed 50 ohm termination is activated this register will have no functionality.
The buffered input path can be terminated separately with 50 Ohm by software
programming. If you do so, please make sure that your signal source is able to
deliver the higher output currents. If no termination is used, the inputs have an im-
pedance of 1 Megaohm. The following table shows the corresponding register to
set the input termination.
Input coupling
All inputs can be set separately switched to AC or DC coupling. Please refer to
the technical data section to see the signal frequency range that is available for
the different settings.
Using the AC coupling will eliminate all DC and low frequency parts of the input
signal and allows best quality measurings in the frequency domain even if the DC
level of the signal varies over the time.
The following table shows the corresponding register to set the input coupling.
AC/DC offset compensation
When using the HF-Path of the input channel, an offset voltage will be visible in case DC coupling is selected for the channel and the signal
source is externally AC coupled. This offset can be compensated for by setiing the compensation registers:
Anti aliasing filter (Bandwidth limit)
All inputs have a separate selectable anti aliasing filter (bandwidth limit) that will
cut of any aliasing effectes and that will reduce signal noise.
Please note that this bandwidth limit filter will also cut of any distortion or high
frequency spurious signals parts that are within the frequency spectrum of the
input.
Please refer to the technical data section to see the cut off frequency and the type
of filter used. The following table shows the corresponding register to activate
the bandwidth limit.
spcm_dwSetParam_i32 (hDrv, SPC_PATH0 , 0); // Set up channel0 to input path 0 (buffered)
spcm_dwSetParam_i32 (hDrv, SPC_AMP0 , 1000); // Set up channel0 to the range of ± 1.0 V
spcm_dwSetParam_i32 (hDrv, SPC_PATH1 , 1); // Set up channel1 to input path 1 (HF, 50 ohm terminated)
spcm_dwSetParam_i32 (hDrv, SPC_AMP1 , 500); // Set up channel1 to the range of ± 0.5 V
Register
Value
Direction
Description
SPC_50OHM0
30030
read/write
A „1“ sets the 50 ohm termination for channel0. A „0“ sets the termination to1 MOhm.
SPC_50OHM1
30130
read/write
A „1“ sets the 50 ohm termination for channel1. A „0“ sets the termination to1 MOhm.
Register
Value
Direction
Description
SPC_ACDC0
30020
read/write
A „1“ sets the AC coupling for channel0. A „0“ sets the DC coupling (default is DC)
SPC_ACDC1
30120
read/write
A „1“ sets the AC coupling for channel1. A „0“ sets the DC coupling (default is DC)
Register
Value
Direction
Description
SPC_ACDC_OFFS_COMPENSATION0
30021
read/write
A „1“ enables the compensation. A „0“ disables the compensation (default).
SPC_ACDC_OFFS_COMPENSATION1
30121
read/write
A „1“ enables the compensation. A „0“ disables the compensation (default).