Keithley 485
The Keithley 485 Picoamperemeter is a dedicated picoamperemeter from the mid-1980s, from the same generation and model series as the better-known Keithley 197 "Microvolt DMM".
The 485 operates as a feedback (transimpedance) amperemeter (i.e. the input burden is constantly low at less than 0.2 mV), has a resolution of 4½ digits (19999), some rudimentary built-in logging capabilities, as well as an optional GPIB interface.
This instrument measures only (low) currents, in seven decadic ranges from 2 nA to 2 mA full scale. In the 2 nA range, the least significant digit resolves 100 fA (0.1 pA).
Key Specifications
| Current | 7 decade ranges, 2 nA to 2 mA full scale |
|---|---|
| Input voltage burden | ≤200 μV |
| Max. input | 350 V DC in 2 nA to 2 μA ranges, 50 V DC in 20 μA to 2 mA ranges |
| Display | 4½ digits, LCD |
| Analog output | +1 V (2 nA range: +100 mV) equivalent to –10000 counts |
| Data logger | 100 readings at at one of six selectable rates from 3 readings/second to 1 reading/hour, or manual triggering; also detects and stores maximum and minimum readings |
| Log function | Displays log10 of the absolute value, e.g. –3.000 for ±1 mA |
Options
- 1758 Rechargeable battery pack
- 4853 GPIB (IEEE-488) interface
Links
- Keithley 485 @ radiomuseum.org
Related instruments
Internals
The 485's input stage is a transimpedance amplifier built from a dual FET followed by an LM308 opamp, and decade feedback resistors from 1 kΩ to 100 MΩ. The resulting 2 V input signal to the A/D converter is buffered by a TLC271 opamp that is configured for ×1 gain except on the 2 nA range, where it supplies the extra ×10 gain needed.
The ADC is a charge-balancing single-slope design built with discrete parts and opamps, implemented in firmware.
The GPIB interface (Option 4853) communicates with the base instrument over an opto-isolated serial port. It uses a separate 6805 CPU.