Keithley 485 Datasheet

LOW-CURRENT/HIGH-RESISTANCE METERS

485 Picoammeter

• 100fA sensitivity

• 200µV maximum voltage burden

• Analog output

• Log I readings

• Optional IEEE-488 interface

ACCESSORIES AVAILABLE

OUTPUT OPTIONS

4853 IEEE-488 Interface (485)

CABLES

4801 Low Noise BNC Input Cable,

1.2m (4 ft)

4802-10 Low Noise BNC Input Cable,

3m (10 ft), unterminated

4803 Low Noise Cable Kit

ADAPTER

4804 Male BNC to Female Triax Adapter

RACK MOUNT KITS

1010 Single Rack Mount Kit

1017 Dual Rack Mount Kit

POWER

1758 Rechargeable Battery Pack

See page 235 for descriptions of all accessories.

The Model 485 Autoranging Picoammeter provides 100fA sensitivity with 41⁄2-digit resolution in a low-cost, highly sensitive, easy-to-use instrument. The 485 measures DC current on seven ranges covering 10 decades from 100fA to 2mA. The input can withstand overloads as high as 1000V (with 100kΩ limiting resistor) for flexibility in a wide range of applications in test, research, and student labs. An analog output linearly converts the incoming current to voltage for hard copy output or control loop applications.

In addition to 100fA sensitivity, the 485 has both excellent accuracy and low voltage burden. One-year accuracy on the most sensitive range is an impressive 0.4%. The 485’s input voltage drop (burden) is actively constrained by feedback techniques to less than 200µV. Thus the 485 makes high accuracy current measurements even in circuits with very low source voltages.

The 485 has several features that facilitate measuring low current. In the autorange mode, the 485 maximizes resolution. The REL button makes readings relative to the baseline (the reading prior to touching the button). The LOG button converts the display to the logarithm (base 10) of the absolute value of the measured current. Digital calibration is performed from the front panel or over the bus.

A 100-point data store buffer collects and stores measurements at one of six automatic reading rates from three per second to one per hour, or manually with the STORE button. Minimum and maximum readings are continuously updated at three per second in the data store mode.

The addition of the Model 4853 IEEE-488 Interface to the 485 provides fully programmable computer control. For isolation from the power line or for portability, the 485 can be battery powered with the Model 1758 Rechargeable Battery Pack.

ORDERING INFORMATION

485 Autoranging Picoammeter

485/1758 Autoranging Picoammeter with

Rechargeable Battery Pack

485/4853 Autoranging Picoammeter with

IEEE-488 Interface

These products are available with an Extended Warranty. See page 635 for complete ordering information.

QUESTIONS?

1-800-552-1115 (U.S. only)

Call toll free for technical assistance,

product support or ordering information, or

visit our website at www.keithley.com.

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LOW-CURRENT/HIGH-RESISTANCE METERS

485 Picoammeter

MODEL 485

RANGE RESOLUTION ±(%rdg + counts)* (10%–90%) (50 or 60Hz) INPUT**

2 nA 100 fA 0.4 + 4 60 ms 70 dB 350 V DC

20 nA 1 pA 0.4 + 1 60 ms 70 dB 350 V DC

200 nA 10 pA 0.2 + 1 6 ms 65 dB 350 V DC

2 µA 100 pA 0.15 + 1 3 ms 65 dB 350 V DC

20 µA 1 nA 0.1 + 1 3 ms 65 dB 50 V DC

200 µA 10 nA 0.1 + 1 1 ms 65 dB 50 V DC

2 mA 100 nA 0.1 + 1 1 ms 55 dB 50 V DC

*When properly zeroed. **With no limiting resistance: 1000V DC with external 100kΩ series resistance.

INPUT VOLTAGE BURDEN: <200µV. RANGING: Manual or autoranging. AUTORANGING TIME: Average 250ms per range change. SETTLING TIME AT DISPLAY: <1 second to within 2 counts

on fixed range.

CONVERSION PERIOD: 300ms. TEMPERATURE COEFFICIENT (0°–18°C & 28°–50°C): ±(0.1

× applicable accuracy specification)/°C.

MAXIMUM COMMON MODE VOLTAGE: 30V rms, DC to 60Hz

sine wave.

ANALOG OUTPUT: Output Voltage: +1V = –10000 counts,

except +100mV = –10000 counts on 2nA range. Output Resistance: 1kΩ.

VOLTAGE BURDEN CAN CAUSE ERRORS AT ANY CURRENT LEVEL

ACCURACY NORMAL MODE

(1 Year) ANALOG REJECTION MAXIMUM

18°-28°C RISE TIME RATIO CONTINUOUS

REL: Pushbutton allows zeroing of on-range readings. Allows

relative readings to be made with respect to baseline value. Front panel annunciator indicates REL mode.

DATA STORE and MIN/MAX: 100 reading storage capacity;

records data at one of six selectable rates from 3 readings/second to 1 reading/hour, or by manual triggering. Also detects and stores maximum and minimum readings continuously while in the DATA STORE mode.

LOG: Displays logarithm (base 10) of the absolute value of the

measured current (examples: –3.000 = ±1mA; –6.301 = ±0.5µA).

Figure 1

The voltage burden is the terminal voltage of an ammeter. An ideal ammeter will not alter the current flowing in a circuit when connected in place of a conductor. Thus, it must have zero resistance and therefore zero voltage burden.

Digital multimeters use the shunt ammeter technique shown in Figure 1 to measure current.The measurement method is to

BURDEN

200mV

Range

≤ 200mV

develop a voltage across a sensing resistor. The resistor is chosen such that 200mV corresponds to the maximum current reading on a selected range. The voltage burden specification

Figure 2

is the 200mV developed across the sensing resistor.

Feedback picoammeters such as the 485 and Keithley electrometers use a technique in which the voltage burden is the input voltage of an op amp, as shown in Figure 2.

The output voltage of the op amp is precisely related to the input current. Since input voltage is output voltage divided by

BURDEN

–

≤ 0.2mV

BURDEN

op amp gain (typically 100,000), the voltage burden is only microvolts. The maximum specified voltage burden of the 485 is only 0.2mV.

Figure 3

200mV or 2V Range

IEEE BUS IMPLEMENTATION

(485 OPTION 4853)

MULTILINE COMMANDS: DCL, SDC, GET,

GTL, UNT, UNL, SPE, SPD.

UNILINE COMMANDS: IFC, REN, EOI, SRQ,

ATN .

INTERFACE FUNCTIONS: SH1, AH1, T5,

TE0, L4, LE0, SR1, RL2, PP0, DC1, DT1, C0, E1.

PROGRAMMABLE PARAMETERS: Range,

Zero Check, REL, LOG, Trigger, Calibration, EOI, SRQ, Status, Output Format, Terminator.

GENERAL

DISPLAY:41⁄

OVERRANGE INDICATION:“OL” displayed. CONNECTORS: Input: BNC.

EMC: Conforms to European Union Direc-

SAFETY: Conforms to European Union

OPERATING ENVIRONMENT: 0°–50°C, <70%

STORAGE ENVIRONMENT: –25° to 60°C. POWER: 105–125V or 210–250V (switch

DIMENSIONS, WEIGHT: 85mm high ×

ity, range, and status indication.

Analog Output: Banana jacks.

tive 89/336/EEC.

Directive 73/23/EEC (meets EN61010-1/ IEC 1010).

RH up to 35°C; linearly derate 3% RH/°C up to 50°C.

selected), 90–110V available, 50–60Hz, 12VA.

235mm wide × 275mm deep (3½ in × 9¼ in × 10¾ in). Net weight 1.8kg (4 lb).

-digit LCD, 0.5 in height; polar-

An example of the problems caused by high voltage burden is shown in Figure 3. In measuring the emitter current of a transistor, the DMM causes a very significant error (200mV out of 300mV) while the 485 voltage burden creates negligible error (0.2mV out of 300mV). Even though the basic measurement is well within the range of a DMM, the 485 makes a more accurate measurement since, due to its low voltage burden, the 485 is much closer to an ideal ammeter.

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300mV – V

BURDEN

0.7V I V

Ideal Ammeter: V

485: V

DMM: V

BURDEN

= 0 = 0% error

≤ 0.2mV ≤ 0.07% error

≤ 200mV ≤ 67% error

QUESTIONS?

1-800-552-1115 (U.S. only)

Call toll free for technical assistance,

product support or ordering information, or

visit our website at www.keithley.com.