The Fluke 2640A and Fluke 2645A NetDAQ Networked Data Acquisition Units are 20-channel front ends that operate in conjunction with NetDAQ Logger for Windows to form a data acquisition system. They are “modern” evolution of the older Fluke “Hydra” 2620A/2625A/2635A DAQs.

These rather obscure NetDAQ instruments are somewhat similar to more common Agilent 34970A scan dataloggers with DMM integrated function or similar systems like older Keithley 2700 DAQ or newer DAQ6510. Fluke units somewhat resemble repackaged Fluke 45 or 8840/8842A with scanner capabilities. Below are manuals for 2640/2645A.

Fluke 2640A,2645A NetDAQ User manual

Fluke 2640A,2645A NetDAQ Service manual w/schematics

The 2640A/2645A instruments can measure DC voltages, AC voltages, resistance, temperature, frequency, and DC current. Temperature measurement function can use thermocouples or resistance-temperature detectors (RTDs). To measure other parameters, use an appropriate transducer. The instrument also includes ten computed channels for custom calculations based on measured values. Our friend fenugrec got one of these interesting units for teardown and investigation, so we can now enjoy the high-resolution photographs of the internal hardware. Be sure to check his website.

Photographs

All photos are available in full resolution on click.

It’s nice to have Ethernet interface instead of more traditional GPIB IEEE-488. That make connection and communication to this instrument much cheaper and simpler.

NetDAQ using proper vacuum fluorescent display, just the way we love it in legacy test equipment.

It’s interesting to confirm presence of the glorious Linear Technologies LTFLU-1ACH REFAMP zener IC in this instruments, same as many other Fluke measurement products. The main key difference between the REFAMP such this LTFLU-1 or older Motorola SZA263 and more traditional LTZ1000/LTZ1000A zener chips are:

• REFAMP has no heater built-in unlike LTZ part.
• REFAMP has the temperature compensating transistor in series with the buried zener diode, not parallel as in the LTZ design.
• Therefore the LTFLU circuit needs a lot more temperature and current compensation work from designer than LTZ1000 circuits do.
• The LTFLU/SZA263 is in a 4-pin TO-can, the LTZ1000 in an 8-pin for additional signals. They are not compatible and need different bias circuit around them.

Fluke used REFAMP (Motorola SZA263 first, Linear LTFLU later) in many instruments, such as benchtop DMM, calibrators and DC Voltage Standards. Below is list of some examples.

Also for educational purpose schematic section with REFAMP LTFLU voltage reference from Fluke calibrator shown on figure below.

LTFLU-1 reference from Linear (now ADI) and Fluke hermetical resistor networks in 8842A DMM

Fluke 57LFC calibrator using same reference REFAMP and resistor networks Z3,Z4

So it’s a special part, that nobody can buy from Digikey. Yes, however, there are reference modules around SZA263/LTFLU and voltage reference chips available from time to time on secondary market. Even though nobody can say for sure if they are fakes or genuine parts, sort out with bad specification or gray market production. A tear-down of both parts revealed, that SZA263 is a two chip construction in one TO-package with separate silicon and zener diode, while LTFLU is a one chip design, both connected as a REFAMP with collector, base, emitter of the zener and anode of the silicon diode.

LTFLU-1CH die photo. Courtesy branadic (Dipl.-Ing. A. Bülau) from the EEVBlog

On the die photo we can see much more complex design than just diode and transistor. Different to Linear LTZ1000 which needs a separate stage to boost the zener voltage to 10V, the boost of the output on these REFAMPs is part of the zener circuit itself, realized in a bootstrap fashion. However, a 10V output requires an individually trimmed voltage divider for each device from the output to the base of the REFAMP. As found earlier the TC(Thermal Coefficient) of this divider is rather critical, as it is only dampened by a factor of ~3, while all other resistors of the circuit are less critical and their TC(Thermal Coefficient) is dampened by a factor of 150 … 500.