Devin Alex

A Multi-Product Calibrator (MPC)—epitomized by the legendary Fluke 5700A, 5720A, and modern 5730A series—is the foundational workhorse of electrical metrology laboratories. Serving as a multi-functional primary calibration source, an MPC synthesizes direct current voltage (DCV), alternating current voltage (ACV), direct and alternating current (DCI/ACI), and multi-decade precision resistance with sub-part-per-million (ppm) uncertainties across an extraordinary dynamic range.

Achieving 24-hour absolute uncertainties as low as ± 3.5 ppm for DC voltage and ± 9 ppm for resistance requires revolutionary analog engineering: ovenized buried Zener references (LTZ1000), hermetic Evanohm resistor networks, multi-decade pulse-width modulation (PWM) DACs, 4-wire remote Kelvin sensing, active driven guard shielding, and the ground-breaking paradigm of Artifact Calibration (“Art Cal”).

Instrument Architecture & Core Output Disciplines

The internal architecture of a flagship multi-product calibrator coordinates several isolated functional subsystems to generate precision electrical stimulus across a massive envelope:

Measurement Discipline	Dynamic Output Range	Frequency Bandwidth	Typical Best 1-Year Spec
DC Voltage (DCV)	0 to ± 1100 V (continuous)	DC	± 7 ppm of reading + 0.4 ppm of range
AC Voltage (ACV)	220 µV to 1100 V	10 Hz to 1.2 MHz	± 45 ppm (mid-band 10 Hz – 20 kHz)
DC Current (DCI)	0 to ± 2.2 A (20 A w/ boost)	DC	± 35 ppm of reading
AC Current (ACI)	9 µA to 2.2 A (20 A w/ boost)	10 Hz to 100 kHz	± 120 ppm of reading
Resistance Standards	1 Ω to 100 MΩ (18 discrete values)	DC to 10 Hz	± 12 ppm (at 10 kΩ)

Calibrator outputs are fully floating and isolated from chassis ground via high-voltage optical couplers and isolated toroidal power transformers, capable of sustaining up to 1000 V common-mode voltage safely.

Internal Physical Reference Standards

At the heart of every high-tier multi-product calibrator are highly stable internal physical standards that provide long-term autonomous stability between external calibration cycles:

Buried Zener Reference (Linear Tech LTZ1000)

Rather than using surface zener junctions subject to surface contamination and crystal lattice recombination noise, modern calibrators use the legendary Linear Technology LTZ1000 sub-surface buried zener.

Sub-Surface Avalanche Breakdown: Breakdown occurs 10 to 15 µm beneath the silicon surface within bulk crystal, eliminating 1/f flicker noise ($< 1.2\text{ µV}_{\text{p-p}}$ from 0.1 to 10 Hz).
On-Chip Isothermal Oven: Integrated heater and temperature-sensing transistors maintain the silicon substrate at a constant elevated temperature ($T_{\text{die}} \approx 65^\circ\text{C}$), rendering the 7.2 V native output immune to ambient room fluctuations.
Drift Rate: Less than 1.0 to 1.5 ppm per year, providing an ultra-stable anchor for the scaled 10 V and 1.0 V internal reference rails.

Hermetic Evanohm & Foil Resistor Networks

Precision resistance synthesis relies on proprietary hermetically sealed bulk metal foil and oil-filled Evanohm / Karma alloy wirewound resistor arrays.

Temperature Tracking: Individual resistors are physically co-packaged within oil-filled cans or copper blocks. While absolute temperature coefficients are < 0.5 ppm/°C, ratio tracking between divider arms is matched to better than 0.05 ppm/°C.
Hermetic Integrity: Nitrogen-purged hermetic glass-to-metal sealing isolates resistance elements from atmospheric humidity changes, eliminating oxidation and dielectric absorption drift.

Multi-Decade DAC Architectures & Linearity

Generating arbitrary voltages from nanovolts up to 1100 V with 0.1 ppm resolution cannot be accomplished with conventional commercial DAC ICs. Multi-product calibrators rely on proprietary PWM (Pulse-Width Modulation) interpolation paired with auto-calibrating Kelvin-Varley dividers:

Chopper-Stabilized PWM DAC Operation

A high-speed digital synthesizer alternates CMOS switches between the calibrated $+10\text{ V}$ reference and ground at an exact repetition rate (e.g., 50 kHz). The DC average voltage is strictly determined by the duty cycle $D = t_{\text{on}} / T$:

$V_{\text{out}} = V_{\text{ref}} \cdot \left( \frac{t_{\text{on}}}{T} \right) = V_{\text{ref}} \cdot \left( \frac{N_{\text{clock cycles}}}{N_{\text{period}}} \right)$

Because time intervals can be measured and generated digitally with quartz-crystal precision, the PWM architecture exhibits perfect intrinsic differential linearity. Multi-pole active Bessel/Butterworth low-pass filters eliminate switching ripple, leaving a pure, drift-free DC potential with linearity errors under 0.1 ppm.

The Artifact Calibration (“Art Cal”) Philosophy

Historically, calibrating a multi-function calibrator required transporting the massive instrument to a primary standards lab and connecting dozens of external standards over days of manual adjustments. In 1988, Fluke introduced Artifact Calibration (Art Cal), fundamentally transforming electrical metrology:

The Three External Artifacts

Under the Art Cal philosophy, the entire multi-product calibrator—encompassing all DC voltage, AC voltage, DC current, AC current, and resistance ranges—is fully recalibrated and verified to original factory specifications using only three external artifacts:

1. DC Voltage Standard

10.000000 V

(Fluke 732B / 732C Zener)

2. Low Resistance Standard

1.0000000 Ω

(Fluke 742A-1 Standard)

3. High Resistance Standard

10.000000 kΩ

(Fluke 742A-10k Standard)

Internal Ratio Transfer Mechanisms

When the operator connects the three artifacts during an automated Art Cal cycle, the calibrator uses precision internal null detectors, 10:1 ratio bridges, and solid-state transfer sensors to autonomously calibrate its internal matrix:

DC Voltage Step-Up & Step-Down: The external 10 V reference updates the internal 10 V LTZ1000 standard. The calibrator's auto-calibrated Kelvin-Varley divider steps 10 V down to 1 V, 100 mV, and 10 mV ranges. A precision bootstrapping high-voltage amplifier then transfers the calibration up to 100 V and 1100 V ranges.
Resistance Matrix Synthesis: Using the external 1 Ω and 10 kΩ standards, internal auto-balancing bridge circuits step through each decade resistor, calibrating all 18 discrete values from 1 Ω to 100 MΩ.
AC Voltage & Current Transfer: An internal patented solid-state thin-film thermal RMS sensor (or electronic RMS sensor) transfers the newly calibrated DC voltage levels to alternating current voltage (ACV) and current across the full frequency spectrum up to 1.2 MHz.

Measurement Signal Integrity: 4-Wire Sensing & Active Guarding

4-Wire Remote Sensing (Kelvin Connection)

In high-accuracy resistance and low-voltage calibration, current-carrying test leads introduce lead resistance ($R_{\\text{lead}}$) and thermal Seebeck electromotive forces (EMFs) at copper-brass terminals.

$V_{\text{sensed}} = V_{\text{UUT}} \quad \text{since} \quad I_{\text{sense}} \approx 0$

By separating the circuit into Force (Current) leads and high-impedance Sense (Potential) leads right at the terminals of the unit under test (UUT), lead drop errors are completely eliminated from the measurement loop.

Active Guard Shielding & CMRR

Common-mode voltages (induced by AC power line hum and ground loops) cause parasitic leakage currents to flow through internal insulation resistance to earth ground.

An MPC implements a physical conductive internal Guard shield completely enclosing the sensitive analog circuitry. An active unity-gain buffer drives this Guard shield to the identical potential as the LO terminal ($V_{\\text{guard}} = V_{\\text{LO}}$). With zero potential difference across the inner insulation boundary, leakage currents drop to zero, yielding a Common-Mode Rejection Ratio (CMRR) exceeding 140 dB at DC.

Multi-Product Calibrators & Artifact Calibration