Repair and teardown of picoampmeter Keithley Model 6485

Contents

Intro

What is a picoammeter, and how it’s useful?

Measuring low currents or high resistance often demands a lot more than a general purpose digital multimeter can do. Generally, DMMs lack the sensitivity required to measure currents less than few µA. Even at higher currents, a DMM’s input voltage drop (voltage burden) of hundreds of millivolts can make accurate current measurements impossible. Electrometer can measure low currents very accurately, but the circuitry needed to measure extremely low currents, combined with functions like voltage, resistance, and charge measurement, can increase an electrometer’s cost significantly. The Model 6485 picoammeter is a way to get low current sensitivity near that of an electrometer, without paying full cost of one. MSRP for Model 6485 is set for $1930 USD.

Meter exterior and specification

Model 6485 Picoammeter have eight current measurement ranges and able to measure currents from 20fA to 20mA, with up to 1K/readings/second. Meter provide 10fA resolution and good sensitivity, making it a good fit to characterize low current devices and signals. On higher end its 20mA range can be useful for common applications such as measuring 4-20mA sensor loops.

Regular DMMs typically employ shunt circuitry to convert input current into voltage, while Keithley Model 6485 is using different approach with a feedback active amplifier. This design reduces voltage burden by several orders of magnitude, resulting in a voltage burden <200µV on the lower ranges. This low voltage burden allow picoammeter function much more like an ideal ammeter, so it can make low current measurements with high accuracy, even in circuits with very low source voltages. This can be very useful for battery powered applications and low-voltage/low-power portable electronics design and testing.

Distilled specification features:

  • Relatively affordable instrument with very low <200µV burden voltage, low current measurement capability.
  • 5½-digit (10fA on lowest 2nA range) resolution.
  • Up to 1000 readings/second
  • Rear panel standard BNC Input
  • Scaled Analog Output for external datalogging/recording without use of GPIB/PC.
  • Input is protected up to 220V overload
  • Easy to use front panel design
  • REL and LOG functions to perform relative readings with respect to a baseline value or display the logarithm of the absolute value of the measured current
  • Resistance calculations capability by dividing an externally sourced voltage value by the measured current
  • Built-in Keithley Model 485 emulation mode
  • Remote interface for control are GPIB IEEE-488 and RS-232

Documentation and manuals

Model 6482

Model 6485

Model 6487

6485 from tin

Comparison to Model 6487 unit from EEVBlog member razperik

Other materials

Author: Illya Tsemenko
Published: Oct. 21, 2016, 11:17 a.m.
Modified: March 21, 2017, 7:06 p.m.

References

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