46
Connection of signals.
band up to 10 MHz are always present in the real situation, and also in the situation of
the user's failure to apply a differential connection (and adjustment to the differential
mode), and therefore use the valuable property of a differential input – effectively
suppressing common-mode interference.
Physical cause #2.
The high frequency of the 2 MHz channel switch (when tuned to
multichannel mode) with L-502 default settings may not only be a great advantage of the L-502
ADC (in the high-speed ADC input interrogation), but it can also be a problem as it takes a short
setting time for the measuring circuit (less than 500 ns after the switch's own charge is injected into
the measurement circuit). This cause is fundamental for all ADC with input commutator,
manifested as a switching disturbance.
4.5.2. Conditions for correct connection and settings L-502.
1.
If you need to use no more than 16 ADC channels, always select the differential
connection mode and the L-502 settings.
2.
With a differential connection, the X and Y circuits of each channel always lead in pairs
(twisted pair, shielded pair).
3.
The L-502 signal circuit must be shielded. On the screen connection, see
4.
For differential connection, when using a pair cable, to maximize common-mode
rejection of the L-502, the output impedances from the remote source along the X and Y
circuits should be balanced, (connection example - p.
5.
The AGND L-502 circuit must be connected to the common circuit of the signal source
(if there are several signal sources to the common junction of the common signal source
wires).
6.
If you need to configure the L-502 for multichannel operation, you first need to optimize
the signal-to-noise ratio of the ADC in a single-channel operation mode with a
maximum data acquisition frequency of 2 MHz, in which in the signal spectrum you will
see all the frequencies of the interference (possibly from "mirror frequencies", if
interference above 1 MHz), which means that you have a tool in your hands to find the
sources and causes of these interferences (before going to multi-channel mode and
averaging mode, which will be much more difficult to understand with sources of
interference).
7.
When using ("default") in the multi-channel operation mode of the maximum
switching frequency (2 MHz), the output impedance
of the L-502
signal circuit
must
be from 0 to 50
Ω (in the frequency band up to 10 MHz), and this voltage source
must be linked to connector L-502 by wires of zero length.
This is achieved either by
directly connecting the output of the signal source itself (50 Ω) or by connecting its load
resistor (up
to 50 Ω) directly to the cable part of the L
-502 signal connector (when
connecting an agreed 50-ohm line or a current shunt to 50 ohms of the external current
measurement circuit) .
8.
When using a multichannel mode of non-zero length of wires to the voltage source
and (or) with an output resistance of the signal circuit of more than 50 ohms, the
program setting of the signal conditioning time n
su
> 1 (s.
) should be applied.
