Download 50 Ohm Driver Manual

Multi-Channel 50 ohm driver
The unit is intended to drive 50 ohm characteristic impedance co-ax cables which are terminated in
50 ohms and thus preserve the pulse shape of the signal. The source of the driving pulse will
normally be from a NI interface card.
Four isolated inputs are also provided. (Black BNCs)
Commercial 250ma line driver amps THS6022 are used as the drivers and HCPL-0900 digital isolators
provide fast isolation from the NI ground noise.
The NI card is reported to float high when it has finished outputting data and therefor latches are
used to buffer the NI card and need to be programed transparent for data transmission and then
latched after data transmission. IO bit D39 from the NI card is used as the latch enable control.
Spurious noise from the NI card when not outputting was also reported and therefor D39 is input to
a PIC16F688 controller so that this unit may be programmed to filter out the NI glitches if required.
Initially the PIC is not used (see modifications on schematic) as the group reported that a simple
control of the latch via the D39 was proving successful provided LTCH ENABLE is held low by 5K1
resistor to DGNDwhen NI card is not outputting data . Therefore with D39 high the latch is
transparent and data can be written to the output BNCs and when D39 is low the data on the
output BNCs is latched at the state when D39 went low. Please see the timing diagram for the latch
enable and data output signals for correct use of the unit. The four input channels are not latched
and connect straight to the NI card bits D35 to D38.
Eight 2pin jumpers are positioned on each board. When this jumper is in place a 10V high is available
at the driver output for driver loads terminated in 50ohms (the drivers have a 47ohm back
termination resistor in their output thus giving a 5V high at the load). When the jumper is removed a
5V high is available at the driver output for driving high impedance loads.
NB. The jumpers must be removed/replaced with the power to the unit turned off in order to
prevent the drivers oscillating.
Driving loads not terminated in 50ohms may cause distortion of the signal the severity of the
distortion is a function of the pulse width, load impedance and cable length.
If removal of the jumpers is inconvenient then a parallel resistor at the load could be used to bring
the effective load resistance down to 50ohms. However if very long steady state outputs are
required the removal of the jumper and maintaining a high load impedance will reduce power
dissipation in the driver of that output.
The two switches and jumper on the back plane PCB should NOT be altered. These are for set up and
servicing purposes only.
NB. The drivers are capable of driving properly terminated cables with pulses as short as 10nS
however the quality of the output signal will only be as good as the input signal. Care should be
taken when using the NI cable especially in conjunction with the NI break out box as these can
significantly degenerate the quality of fast pulses.