A-64
T.O.C.
ALPHA
INDEX
NUM.
INDEX
INST.
D ATA
ACQ.
SERVICE
GLOSSARY
SALES
REPS
LIT.
REQ.
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MAIN
MENU
LOW CURRENT MEASUREMENTS
480 and 485 Picoammeters
•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 (485)
See page A-231 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 100kΩlimiting 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.
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
480 Picoammeter
480/1788 Picoammeter with Rechargeable
Battery Pack
These products are available with an
Extended Warranty. See section C for com-
plete ordering information.
QUESTIONS?
1-800-552-1115 (U.S. only)
Call toll free for technical assistance,
product support or ordering information.
MODEL 480
• LED display
• 1pA resolution
• Analog output
• Optional battery power
The Model 480 is an easy-to-use, sensitive
ammeter that resolves 1pA at a low price.
Simple controls, LED display, shielded
BNC input, high normal mode rejection, and excellent overload specifications make it easy to use. The 480
measures DC current from 1pA per digit (2nA full range) to 2mA, with 3
the full span of low-current measurements. The input is simply connected to the circuit under test, the appropriate range selected, and the current read from the LED display.
1
⁄2-digit precision. Seven ranges cover
T.O.C.
ALPHA
INDEX
NUM.
INDEX
INST.
D ATA
ACQ.
SERVICE
GLOSSARY
SALES
REPS
LIT.
REQ.
PRINT
MAIN
MENU
LOW CURRENT MEASUREMENTS
480 and 485 Picoammeters
A-65
MODEL 485 ACCURACY NORMAL MODE
(1 Year) ANALOG REJECTION MAXIMUM
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 100kΩseries resistance.
18°-28°C RISE TIME RATIO CONTINUOUS
MODEL 480 ACCURACY NORMAL MODE
(1 Year) ANALOG REJECTION MAXIMUM
RANGE RESOLUTION ±(%rdg + counts)* (10%–90%) (50 or 60Hz) INPUT**
2 nA 1 pA 0.8 + 4 70 ms 70 dB 600 V DC
20 nA 10 pA 0.8 + 3 70 ms 70 dB 600 V DC
200 nA 100 pA 0.5 + 3 7 ms 60 dB 600 V DC
2 µA 1 nA 0.5 + 3 7 ms 60 dB 600 V DC
20 µA 10 nA 0.5 + 3 4 ms 50 dB 75 V DC
200 µA 100 nA 0.5 + 3 1 ms 50 dB 75 V DC
2 mA 1 µA 0.5 + 3 1 ms 50 dB 20 V DC
*Up to 70% R.H.
INPUT VOLTAGE BURDEN: <200µV.
RANGING: 480: Manual. 485: Manual or autoranging.
AUTORANGING TIME (485): Average 250ms per range
change.
SETTLING TIME AT DISPLAY: <1 second to within 2 counts
on fixed range.
CONVERSION PERIOD: 485: 300ms. 480: 400ms.
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 (485): Output Voltage: +1V = –10000
counts, except +100mV = –10000 counts on 2nA range.
Output Resistance: 1kΩ.
18°-28°C RISE TIME RATIO CONTINUOUS
ANALOG OUTPUT (480): Output Voltage: 1V = 1000 counts.
Output Resistance: 1kΩ.
REL (485): 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 (485): 100 reading storage capac-
ity; 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 (485): Displays logarithm (base 10) of the absolute value
of the measured current (examples: –3.000 = ±1mA; –6.301
= ±0.5µA).
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:
485:
41⁄2-digit LCD, 0.5 in height; polarity,
range, and status indication.
480: Four 0.5 in LED digits with appro-
priate decimal point and polarity indication.
OVERRANGE INDICATION: 485: “OL” dis-
played. 480: Display blinks all zeros above
1999 counts.
CONNECTORS: Input: BNC.
Analog Output: Banana jacks.
OPERATING ENVIRONMENT: 0°–50°C,
<70% RH up to 35°C; linearly derate 3%
RH/°C up to 50°C.
STORAGE ENVIRONMENT: –25° to 60°C.
POWER: 105–125V or 210–250V (switch
selected), 90–110V available, 50–60Hz,
12VA (20VA on 480).
DIMENSIONS, WEIGHT: 85mm high ×
235mm wide × 275mm deep (3½ in × 9¼
in × 10¾ in). Net weight 1.8kg (4 lb).
VOLTAGE BURDEN CAN CAUSE ERRORS
AT ANY CURRENT LEVEL
Figure 1
I
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 devel-
V
BURDEN
R
S
V
BURDEN
200mV
Range
≤ 200mV
op 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 is
the 200mV developed across the sensing resistor.
Feedback picoammeters such as the 480/485 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 op amp gain (typically 100,000), the voltage burden is only microvolts. Maximum specified voltage burden of the 480/485 is only 0.2mV.
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 480/485 voltage burden
creates negligible error (0.2mV out of 300mV). Even though the basic measurement is well within the range of a
DMM, the 480/485 makes a more accurate measurement since, due to its low voltage burden, the 480/485 is much
closer to an ideal ammeter.
Figure 2
I
–
V
BURDEN
+
Figure 3
300mV – V
BURDEN
1V
0.7V I V
Ideal Ammeter: V
480/485: V
DMM: V
R
S
A
V
BURDEN
BURDEN
BURDEN
BURDEN
200mV or
V
2V Range
O
≤ 0.2mV
BURDEN
= 0
= 0% error
≤ 0.2mV
≤ 0.07% error
≤ 200mV
≤ 67% error
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