This message was originally posted by me on the HP Agilent Yahoo Group but I thought the information may also be of interest to fellow members of this forum.Hi to all HP LogiCDart owners or potential owners.
I purchased two LogicDarts recently from the USA. They came without the infamous
probes so were effectively expensive paperweights.
Rumour had it that the LogicDart cannot be used without the HP OEM probes sets
as the probes contained active electronics and all manner of horrors. These
rumours were, to my knowledge, unproven.
Logic Dart probe sets are selling for around $150 per pair so there was an
incentive for me to attempt to build some clones for my own use. I soon
discovered that the LogicDart probe information available on the NET was limited
and mostly incorrect guesses
. People believed HP had placed the high speed
comparators in the probes...this is incorrect. The three comparators are
MAX902's and are located in the LogicDart itself, behind some attenuation and
protection circuitry.
I have been experimenting with a DIY probe solution for a week and have decided to share my discoveries.
This is an 'early days' project but the results to date have been excellent and
I am about to move from the first prototype to a more refined probe for use in
my lab. I welcome any and all comment and if anyone manages to improve upon my
design, please share your data with us so that we may all benefit :-)
Probe connector on LogicDart viewed end on with `flat' at the top
Top Left = Ground / 0V
Bottom Left = Vin
Bottom Right = Vout(active for Resistance,Continuity and diode modes)
Top Right = Not Used
The male 4 pin connector for a probe lead may be constructed using a 0.1" pitch
Molex twin row PCB header or male data connector salvaged from an IDE hard drive
or similar. Such 0.1" pin headers are common inside computers. A shell will need
to be constructed but this is purely a personal choice and hardware task. 10mm
OS diameter Plastic tube with a flat filed on it or a salvaged 3.5mm audio plug
shell could be used.
20pF Trimcap
|---------| |---------|
Vin -----------|-/\/\/\/\---/\/\/\/\---|-/\/\/\/\------|-----Tip---
750k0 36k0 2k0 |
Vout-----------------------------------------------|
0v ----------------------------------------------------- 0v clip
All resistors are 1% tolerance 0.6W metal film types.
LogicDart connector Vin measured dc resistance = 281k5
Measured LogicDart probe series dc resistance = 788k0 (measured)
20pF trimcap required for probe compensation
Total input impedance of the OEM probe + LogicDart is stated as approx 1M / 13pF
The probe series resistance has been measure and tested to confirm that 788k0 is
the required value for correct calibration. It forms half of a potential
divider, the other half being inside the LogicDart.
Vout provides the drive voltage for resistance, continuity and diode ranges and
was measured at 4V52 dc. I believe this to be a high impedance supply.
The Vout supply inside the LogicDart incorporates a series RF inductor and
GM54GI protection diode (forward biased) in series with the output pin.
The LogicDart probes are passive, containing only resistance and capacitance.
The probe cable from the head to the connector is twisted pair to reduce
capacitance effects.
I do not own or have access to the OEM probes but resistance measurements were
provided by a helpful person who does own one. The readings were confirmed by
practical test using a DIY prototype probe assembly.
Since building this prototype, I have discovered an excellent source of HP probe
information at the URL shown below:
http://www-d0.fnal.gov/trigger/stt/commiss/agilent/agilentprobing.pdfThe resistor values in the document do not map across to the LogicDart probes as
the Logic probes detailed are 100k input impedance rather than 1M. The theory of
construction mirrors my prototype however
I hope this information is of use and look forward to any comments or
improvement suggestions that members may have. I have yet to test the probe to
33MHz but that will come later when I have a more refined probe build to keep
stray capacitance under control
Needless to say, I presently have the prototype working very well, and
accurately with one of my LogicDarts. The original outlay for the new Logic Dart
was $105 which compares favourably to HP's original $800 price tag, and for less
than $5 of components it can be made to work without the OEM probes.
Happy days