The Keithley 191 is a 5½-digit (200,000 counts) manual-ranging digital multimeter engineered for precise DC voltage and resistance measurements across laboratory and field applications. Built on a microprocessor-based architecture, it combines single-slope and charge-balance analog-to-digital conversion with automatic non-linear digital filtering to minimize noise and stabilize display response. The instrument delivers 1 µV DC resolution with 0.005% ±2 digits basic accuracy, and resistance measurement down to 1 mΩ across six ranges extending to 20 MΩ. A pushbutton null function corrects for lead resistance in 2-wire ohms and bucks thermal EMFs in low millivolt measurements.
## Technical Specifications
**Measurement Ranges & Resolution**
• DC Voltage: 1 µV to 1200 V across 5 ranges; 1 µV resolution; 0.005% ±2 digits accuracy
• Resistance: 1 mΩ to 20 MΩ across 6 ranges; 1 mΩ resolution
• AC Voltage (Model 1910 option): 10 µV to 1000 V across 4 ranges
**Display & Conversion Speed**
• Six 0.5-inch LED digits with decimal point
• DC Volts: 1 reading/second; Ohms: 3 readings/second; AC Volts: 2 readings/second
• Settling time: <1.3 seconds to 0.005% of final reading for zero to full-scale step input
**Electrical Characteristics**
• DC Voltage input impedance: 2 MΩ shunted by 80 dB DC, 50/60 Hz (1 kΩ in either lead)
• NMRR: >60 dB at 50/60 Hz
• Input LO to power line ground isolation: >1000 MΩ shunted by ~30 pF
• Maximum input between LO and power line ground: 1400 V peak, 50/60 Hz
• Power consumption: 25 VA maximum
• Line power: 105-125 VAC or 210-250 VAC selectable; 50/60 Hz
**Physical & Environmental**
• Dimensions: 95 mm H × 235 mm W × 257 mm D
• Weight: 2.3 kg
• Connectors: 5-way binding posts
• Operating temperature: 0°C to 50°C; humidity 0-80% RH to 35°C
• Warm-up time: 1 hour to rated accuracy
## Key Features
• Microprocessor-controlled measurement with automatic non-linear filtering
• Null pushbutton with annunciator for zeroing and thermal EMF rejection
• Programmable multimeter functionality
• High CMRR and NMRR for noise-immune measurements
## Typical Applications
Precision DC voltage and resistance characterization; thermal EMF compensation in low-level DC measurements; resistance verification across component and circuit-board testing.
## Compatibility & Integration
Optional Model 1910 AC Voltage module; Model 1901 Plug-in Current Adapter for DC current measurements.
