GENERAL The Keithley Model 155 is a completely solid state, rugged, battery-operated combination Null Detector-Microvoltmeter. It measures from 1 microvolt full scale to 1000 volts in 19 Ix and 3x steps and has 150 nanovolts resolution. The recorder output, accurate to 1% of full scale exclusive of noise and drift, extends the versatility of the instrument. FEATURES. a. Excellent immunity to ac interference allows the Model 155 to detect dc signals in the presence of large ac voltages. The Microvoltmeter, Null Detector has greater than 14 0 dB CMRR and 10 0 dB HMRR {refer to specifications in Table 1). Also, hook up to source is simple and quick. Unshielded leads may generally be used without degrading performance. b. The Model 155 Null Detector can recover from 100-volt overloads within 5 seconds on the 30-micro-volt range. Up to 1200 volts peak may be applied momentarily on any range -without damaging the instrument . c. Stability is better than 0.5 microvolt per 24 hours after warm-up with a reasonably constant ambient temperature. The long-terra drift is non-cu-mulative. d. The ten-turn EERO Control permits easy adjustment of instrument zero. It also provides up to at least -25 microvolts suppression, ’which allows measuring submicrovolt changes in signals up to the limit of the suppression. e. Zero Check position on the Power Switch allows convenient zeroing of the instrument by shorting the input. This means the measuring circuit need not be disturbed by disconnecting' and shorting the input cables. f. High input resistance of 1 to 100 megohms coupled with less than 10"-^ volt per ohm zero shift with source resistance permits measurement accuracy even with high resistance sources. g._ The Model 155 is designed for battery operation to minimize ground loop and high frequency pick-up problems in sensitive voltage measurements. Four internally mounted zinc-carbon batteries provide greater than 1000 hours operation. The Model 15 5 may also be operated from external supplies. These may be connected to the barrier strip on the rear panel. Power requirement is ±15 to ±25 volts at 2 milliamperes. h. For line operation, the Model 1554 Power Supply is a convenient accessory. It attaches to the rear panel of the instrument and its output may be connected to the barrier strip provided also on the rear panel. A switch on the supply provided selectable line or battery operation. Line operation with the Model 1554 maintains excellent floating characteristics and negligible coupling to line. i. Four binding posts on the front panel provide fast and convenient input connection. A ±1 volt at 1 milliampere output is provided on the rear panel for convenient connection to recorders or other readout devices. Accuracy is ±2% of full scale at the meter and ±1% at the recorder output. A large 4-1/2 inch taut-band meter is provided for ease of readout. j. The Model 155 is completely solid state, utilizing a MOS-FET chopper in its input circuit. The solid-state chopper requires little power to drive, which gives long battery life and permits the use of inexpensive batteries. It also has low noise and increased sensitivity. k. Another significant design characteristic of the Model 155 is its electrical and mechanical ruggedness. All components except for the meter, battery, input and output connectors are mounted on a single printed circuit. For calibration or servicing the circuit board, may be conveniently and quickly removed from the instrument. l. Circuit isolation from chassis ground is greater than 10 ohms shunted by 0.01 microfarad. This high isolation generally eliminates the need for guarding the Model 155. APPLICATIONS. a. As a null detector the Model 15 5 can be used with potentiometers, bridges, ratio devices and comparator circuits. b. As a microvoltmeter it is ideal for measuring semiconductor resistivity, thermopile and thermocouple potentials, Hall-effect potentials, contact resistances, biologically generated emfs, electrochemical potentials, and strain gauge outputs. c. Other applications include monitoring power supply stability, DTA measurements, resistance thermometry and thermal conductivity measurements. d. The Model 15 5 is useful as a general purpose instrument in the research laboratory as well as in electronic development work and process control applications . e. Its excellent sensitivity limits the need for expensive potentiometer systems to measure microvolt level signals accurately. It can measure a signal of 30 microvolts or less as accurately as some complex potentiometer systems.