500-003124-000
3-5
3.6.1
Low Pass Filters
The VMIVME-3124 provides passive single-pole low-pass input filters on all
inputs. The normal -3 dB cut-off frequency is 50 kHz. This provides some high frequency
noise protection for the board. The board may also be configured with an add on daughter
board option. The daughter board provides a low-pass filter with a cut-off frequency of
40 Hz. This lower cut-off provides some protection from local 60 Hz AC line noise. This
option is normally installed by the factory at the time of the order. It can be removed by
the user at a later date if new applications are implemented. If the board is running in
single-ended mode, the user must be sure to install the two zero ohm SIP jumpers in
locations E1 and E2. This allows the low-pass filter to perform in the single-ended
configuration. Adding the 40 Hz daughter board automatically sets the 40 Hz bit in the
CSR register.
3.6.2
Input Multiplexers
The board has two-tiers of analog multiplexing. Each of the 32 inputs is selected
using one of four 8x1 first-tier analog input multiplexers. The second-tier multiplexer is
configured as a 4x2 board. It selects one or two of the first-tier output signals to provide
the single-ended or differential mode of operation. The second-tier multiplexer also
selects the BIT reference voltage if enabled in the Control/Status Register. The
second-tier output is differentially transferred to the Programmable Gain Amplifier (PGA).
3.6.3
Current Inputs
The VMIVME-3124-2BC and -3BC models include current termination resistors
using a daughter board installed in the place of the 40 Hz filter. This allows a current signal
to be terminated to ground. The voltage developed across a resistor is read by the
VMIVME-3124. The optional -2BC board provides resistors value at 250
±0.01%. The
optional -3BC board provides resistors value at 500
±0.01%. To support this option the
5 kHz low pass filter is not installed at the factory, all other functionality of the board
remains the same.
NOTE:
THE OPTIONAL -2BC AND -3BC BOARDS ARE CONFIGURED FOR 32-CHANNEL SINGLE-ENDED
MODE ONLY. DIFFERENTIAL INPUTS ARE NOT SUPPORTED.
3.7
PROGRAMMABLE GAIN INSTRUMENTATION AMPLIFIER
Once an input channel has been selected and routed through the input
multiplexers, it enters the PGA as a differential input. The differential amplifier rejects
common mode noise and delivers a scaled, single-ended output to the ADC. The PGA can
be hardware jumpered for gains of 1, 10, and 100. This allows for inputs as low as
50 mV
up to
10 VDC in range. There is no increase in acquisition time for any change in the gain
setting.