TopCurrent is a high-speed, auto-ranging precision current adapter designed for accurate current measurement with standard voltage measurement tools. It achieves a typical precision of 0.05% or better across all ranges (to be confirmed) and switches ranges in approximately 2.5 µs—comparable to the Joulescope—making it well suited for fast current-switching. Burden voltage remains extremely low during operation, including when switching ranges, with a maximum of 20 mV, while maintaining wide bandwidth. The goal is to deliver this level of performance in a device that is also affordable and accessible for hobbyists, students, and small businesses.

TopCurrent measures currents from 1 nA to 3 A across three ranges (nA, µA, and mA). It operates without firmware (no MCU, FPGA etc.), ADCs, calibration and without any manual intervention. The full version includes an aluminum enclosure with grounded PCB end panels, together acting as a partial Faraday cage. For low-noise (and oscilloscope) measurements, a BNC connector is provided, serving as both a mechanical anchor and a low-impedance ground connection to the enclosure.

Measure More Than Before

High-speed, low-burden, auto-ranging current measurement equipment with nine decades of range is typically expensive. TopCurrent provides similar capabilities by converting voltage measurement instruments into precision current measurement tools, limited only by the performance of the connected multimeter or oscilloscope. A current applied at the input is converted to a voltage output with a scale of 1 unit of current = 1 mV per range. So, for example, 1 nA produces 1 mV at the output when in the nA range. Measurement equipment can be connected using gold-plated safety banana jacks or the BNC connector.

TopCurrent is designed to be highly adaptable and expandable. It handles accuracy, speed, and auto-ranging, allowing users to focus on customization.

The Hammond enclosure features a removable sliding top panel that allows space for an additional DIY PCB, such as one with a display and microcontroller (not included). Op-amp noise at the lowest current ranges can be reduced by adding a low-pass filter when using an oscilloscope. Range-switching speed can be increased by adjusting or removing one or more of the clock-setting capacitors, though this may cause instability at range boundaries and requires careful tuning. Numerous DIP-spaced, breadboard-compatible through-hole pads are provided for debugging and expansion.

Features & Specifications

Current Ranges

2.5 mA to 3,000 mA (3,300 mA if battery voltage > 3.3 V):

• Typical burden voltage: 20 µV / mA
• Included ADC supports up to 2,500 mA

2.5 µA to 2,500 µA:

• Typical burden voltage: 10 µV / µA

0 to 2,500 nA:

• Typical burden voltage: 10 µV / nA

Typical burden voltage during range switching: 20 mV

Range Switching Speed

Upward switching (to higher current):

• <2.5 µs to fully open the ranging MOSFETs @3.3 V

Downward switching:

• Min. ~380 µs, Max ~800 µs to fully open the ranging MOSFETs

• Defaults to mA range at power-up

• Anti-lock circuitry prevents the device from remaining in an incorrect range

Accuracy and Performance

Resolution (nA range):

• 100 pA (3.5-digit DMM)
• 10 pA (4.5-digit DMM)
• 1 pA (5.5-digit DMM)

Typical accuracy:

• <±0.05% on µA and nA ranges (above ±40 nA / µA)
• <±0.1% on mA range
  • Up to 0.01% typical accuracy higher in the range (above +-50 mA)
• Output offset voltage: <50 µV
• Bandwidth: >300 kHz (-3 dB)

Temperature drift:

• <10 ppm / °C (µA and nA ranges)
• <15 ppm / °C (mA range)

Power

• Battery type: 3.7 V lithium-polymer, JST-PH + JST-SH connector (battery not included)
• Typical runtime: ~24 hours with a 500 mAh battery
• USB-PD compatible
• 5x LED indicators (charging, nA range, µA range, mA range, 3.3 V power stability)

Inputs:

• 2x Gold-plated shrouded (safety) banana jacks, compatible with banana-to-BNC adapters
• 1x USB Type-C (for charging only)

Outputs:

• 2x Gold-plated shrouded (safety) banana jacks
• female 50 Ω BNC connector

Absolute Maximum Ratings

• Voltage: 20 V
• Current: 3.3 A

Dimensions: 7.98 cm × 12.3 cm × 2.6 cm (3.14 in × 4.83 in × 1.0 in)
Weight: ~225 g (7.93 oz), excluding battery

Open Source

• Front and end panel KiCad files will be fully open source
• A complete schematic PDF will be released after launch
• License: TBD (possibly Creative Commons BY-SA)
• These design files will be available in a public GitHub repository

We hope the open design allows users to extend TopCurrent with external hardware such as a display or MCU.

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