Hardware and software around National Instruments PXI and modern PXIe variants are part of embedded instrumentation with modularity and flexibility in mind. One could have whole lab set of instruments packed into single compact half-rack or full rack 19” enclosure.

Typical add-on cards include digital and analog I/O interfaces, power supplies, signal generators, signal analyzers, RF and timing modules and even complex instruments like vector network analyzers. Many different vendors make hardware that is compatible with PXI/PXIe systems.

There are also DMMs represented in this form-factor. They mimic most of the capabilities from mid-range benchtop instruments such as traditional benchtop HP 34401A or 34465A but in a much smaller modular format. NI PXI DMM do not have any user display or controls but instead presented as virtual instrument in LabView environment. Currently NI offers range of NI DMMs such as:

• PXI-4065 – 6½-digit DMM, ±300 V, priced at US$ 2085
• PXIe-4080 – 6½-digit isolated DMM, ±300 V, 1.8 MSPS digitizer, priced at US$ 4434
• PXIe-4081 – 7½-digit isolated DMM, ±1000 V, 1.8 MSPS digitizer, priced at US$ 6800
• PXIe-4082 – 6½-digit isolated DMM, ±300 V, L and C functions, 1.8 MSPS digitizer, priced at US$ 5912

This pricing somewhat comparable to benchtop instruments such as Tektronix/Keithley DMM7510 (at US$ 5530) or Keysight 34470A (at US$ 4417)

Previous generation obsoleted NI PXI-4071 has similar specs to current 4081 but utilize older parallel interface to older NI chassis. Interesting fact – PXI-4071 utilizes best in class buried ovenized zener reference from Linear Technologies (today Analog Devices) – LTZ1000ACH. This reference is a work-horse typically found in expensive laboratory grade 8½-digit DMMs such as Keysight 3458A or Keithley 2002.

Design of the NI PXI-4071 compact 7½-digit DMM module.

All photos are very high resolution, please enjoy.

Image 1: Back side of the analog front-end, 367 Megapixel

Plastic cap is a dead giveaway of precision zener standard location. All the magical slots in the PCB help with voltage insulation and parasitic leakage currents mitigation. It’s quite impressive considering very compact design of all the circuitry and 1000 volts rating for this DMM.

Image 2: Front side of the analog front end, 89.6 Megapixel

LTZ1000ACH was manufactured in 2006, week 11. It’s respective opamp here is U2, AD706J. Perhaps if there is enough interest by our readers, I could power up the reference domain of this PCB and capture some parameters such as tempco, noise and oven temperature of this zener reference design? R5, R10, R11, R12 are perhaps precision resistors for LTZ1000 operation. It’s quite interesting choice not seeing typical wirewound or metal foils here, but perhaps NI decided it’s good enough to meet the specs of PXI-4071. After all this is not metrology-grade 8½-digit performer.

White relays are AXICOM IM42, B3GB4.5Z and solid-state AQV258. Large SSOP chip with Vishay logo is a custom resistor network with part-code 187202A-01. R36 nearby is another precision foil resistor from VPG – 75 kΩ.

Image 3: Front side of the digital back-end, 372 Megapixel

Big fat Analog Devices ADSP-21065L is 198 MFLOPS SHARC processor with SDRAM interface and 32-bit FPU implemented here for the handling the communication and data processing. Bus controller is located on the backside of this PCB, with custom NI ASIC U49. Pair of Xilinx Spartan XC2S100 FPGAs handle timing, data and control for all the measurement circuitry in DMM. U22 is out-guard controller, while U30 is placed in-guard on the isolated floating side. Isolation datalink is handled by U31 IL716 optocoupler, U14 PC817. Isolated power handled by U39 and T1 transformer. We can also find LTC1414CGN 14-bit ADC for various small tasks.

Image 4: Back side of the digital back end, 365 Megapixel

I’ve also captured transparent image displaying internal routing of the analog front-end card:

Image 5: Routing and inner design

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