Keithley 428 Programmable Current Amplifier

The Keithley 428 Programmable Current Amplifier converts low-level AC or DC currents into proportional voltages for digitization and display by oscilloscopes, waveform analyzers, and data acquisition systems. Using a feedback current circuit, it achieves sub-picoampere noise levels and rapid response times across nine selectable gain ranges. The 428-PROG variant integrates IEEE-488 control for automated measurement workflows in research and industrial environments.

– Technical Specifications

Gain & Dynamic Range
• Adjustable gain: 10³ V/A to 10¹¹ V/A in decade increments
• Input noise: 1.2 fA RMS
• Rise time: 2 µs to 300 ms (selectable); 2 µs at 10³–10⁴ V/A gain
• Bandwidth: 175 kHz

Programmable Features
• Bias voltage: ±5 V range, 2.5 mV resolution, ±(1.1% reading + 25 mV) accuracy
• Low-pass filter: 10 µs to 300 ms (±25%) in 1, 3, 10 sequence or OFF; 12 dB/octave attenuation; second-order Bessel response
• Current suppression: Up to 5 mA with automatic mode
• Zero correct: <50 µV typical offset nulling

Input/Output
• Output: ±10 V, 1 mA; <100 Ω impedance DC–175 kHz
• Input voltage burden: <200 µV (18–28°C) for <100 µA; <10 mV for ≥100 µA
• Temperature coefficient: 20 µV/°C
• Maximum input overload: 100 V (10⁴–10¹¹ V/A ranges); 10 V (10³ V/A range)

IEEE-488 Performance (428-PROG)
• Zero correct & auto suppression: <3 seconds
• Save/recall configuration: <500 ms
• All other commands: <40 ms

– Key Features

• Nine amplification ranges enable speed/noise trade-offs for diverse measurement needs
• Integrated bias supply (±5 V) eliminates external supply requirement
• Second-order Bessel filter minimizes noise without rise-time penalty on high-gain settings
• Switchable 6 dB/octave roll-off supports scanning tunneling microscope feedback loops
• Zero check and offset correction functions optimize accuracy
• All parameters programmable except IEEE-488 address

– Typical Applications

• Photodiode and photodetector signal conditioning
• Dark current characterization and suppression
• Electron microscopy and scanning probe measurements
• Semiconductor device characterization
• Low-level optical and radiation detection

– Compatibility & Integration

The 428-PROG model features IEEE-488 (GPIB) programming for integration into automated test systems and data acquisition networks. Command execution speed supports high-throughput measurement sequences.