KEI 414A INST RevB(Model414A) 电路图.pdf

INSTRUCTION MANUAL MODEL 4 1 4 A PICOAMMETER 0 COPYRIGHT 1976, K E I T H L E Y INSTRUMENTS, INC. PRINTED MAR. 1978, CLEVELAND, OHIO, U.S.A. RadioFans.CN 收音机爱 好者资料库 CONTENTS MODEL 414A CONTENTS 1. 2. 3. 4 . 5. 6. 7. 1 4 6 7 9 14 17 ii 0575 RadioFans.CN 收音机爱 好者资料库 MODEL 414A ILLUSTRATIONS I LLU ST RAT I 0 N S Fig. No. T i t l e Page 3 3 5 6 10 12 13 21 21 0974 iii RadioFans.CN 收音机爱 好者资料库 S PEC IF ICAT ION S MODEL 414A S P E C I F ICAT 1 0 N S RANGE: 10-lo ampere full scale to l o 2 ampere in seventeen lx and 3x ranges, positive or negative currents. ACCURACY: *4% of full scale on 3 x 10-9 to 10-10 ampere ranges. ZERO DRIFT: Less than 0.5% of full scale per week plus 0.05% per C on any range after a 10-minute warm-up and with source voltages greater than 1 volt. INPUT VOLTAGE DROP: Less than 1 millivolt for full-scale meter deflection on any range when properly zeroed. EFFECTIVE INPUT RESISTANCE: Less than 0.1 ohm on ampere range, increasing to less than lo7 ohms on 10-lo ampere range. RISE TIME, MAXIMUM: Seconds, from 10% to 90%. Range, ampere Across Input 10.10 0.75 10.9 0.15 10.8 0.13 10-7 0.09 10 - 6 0.05 10.5 and above With up to 500 pF less than 0.001 MAXIMUM INPUT OVERLOAD: Transient: 1000 volts for up to 3 seconds using a Keithley (or other 10 mA limited) High Voltage Supply. Continuous: 600 volts on 10-10 to 10-6 ampere ranges, de- creasing to 12 volts on the ampere range. CONNECTORS: Input: Teflon-insulated UHF type. Output: Amphenol 8OPC2F. RECORDER OUTPUT: Output: il volt or 1 milliampere for full-scale meter deflection. Output polarity is opposite input polarity. Noise: 414s: Less than 0.3% of full scale peak-to-peak. 414A: Less than 1 % of full scale peak-to-peak. POWER: 105-125 or 210-250 volts (switch selected), 50-60 Hz, 5 watts. DIMENSIONS, WEIGHT: 5%w high x 8% wide x 10 deep: net weight, 8 pounds. ACCESSORIES SUPPLIED: Mating input and output con- nectors. iv 0974 MODEL 414A GENERAL DESCRIPTION SECTION 1. GENERAL DESCRIPTION 1-1. GENERAL. a. The Keithley Model 414A is a completely solid- state picoammeter which measures currents over 17 ranges from LO- to ampere full scale. Accuracy is *2% of full scale on the to of full scale on the 3 x lo- to lo- ampere ranges. am ere ranges and *4% b. The Picoammeter employs matched MOS FET transis- tors in the input followed by a differential amplifier stage, a transistor driver and a transistor output stage. Negative feedback is used for stability and accuracy. 1-2. FEATURES. b. A unique circuit provides complete overload pro- tection for Model 414A without compromising the out- standing features of the MOS FET input. The Picoam- meter .will withstand transient overloads up to 1000 volts without damage, and overload recovery is almost instantaneous. a. The time and temperature stability of the 414A Picoammeter is unmatched by any other inexpensive pico- ammeter. The Model 414A will operate for days without requiring rezeroing. Zero drift with time is less than 0.5% of full scale per week. Temperature drift is equally small - a change of 10C affects the reading less than 0.5% of full scale on the most sensitive range. c. Fast warm-up is an inherent characteristic of the Picoammeter. It can be used almost immediately on its most sensitive range. For maximum stability, how- ever, about 10 minutes warm-up time should be allowed. d. One volt or 1 milliampere at full-scale deflec- tion on all ranges is provided to drive chart recorders. The 1 milliampere recorder output will drive the Keith- ley 370 Recorder directly. When potentiometric record- ers are used, the output voltage can be conveniently divided by a shunt resistor and adjusted for full scale with the rear panel Calibration potentiometer. 0671 1 GENERAL DESCRIPTION TABLE 1-1 MODEL 414.4 Front Panel Controls Control Functional Description Paragraph RANGE Switch Selects full-scale current range instrument is to measure. 2-2 METER Switch Turns instrument on; selects meter polarity; checks instrument zero. 2-2,2-4 ZERO ADJ. Control Zeroes meter on any range. 2-2 Pilot Light Glows to indicate instrument is on. - TABLE 1-2 Rear Panel Controls Control Functional Description Paragraph INPUT Receptacle Connects input to source. Receptacle is a Teflon-insulated UHF connector. 2-1, 2-2 OUTPUT Receptacle Connects output to monitoring device. 2-4 1 MA - 1 V Switch Selects output of instrument: 1 milliampere or 1 volt. 2-4 1 MA CAL Control Adjusts output from 0.95 to 1.05 mA. 2-4 117-234 V Switch Sets instrument for 117 or 234 volt ac power line. 2-2, 2-5 I Fuse 3AG Slow-Blow. 117-volt - .125 A; 234-volt - .062 A. 2-2. 2-5 2 0671 GENERAL DESCRIPTION MODEL 414A METER MlOl METER ZERO METER I - I POWER LAMP DS201 ZERO A D J CONTROL R123 METER SWITCH S l O l RANGE s 1 0 2 - SWITCH FIGURE 1. Front Panel Controls. INPUT 5102 t 1 SWITCH S 103 OUTPUT 5103 117-234V U SELECTOR s 2 0 1 F S E IMACAL F 2 0 1 CONTROL R133 - - FIGURE 2. Rear Panel Controls. 0671 3 0 PE RAT ION SECTION 2. OPERATION MODEL 414A PICOAMMETER 2-1. INPUT CONNECTIONS. Use the following precautions when using the Picoammeter on the more sensitive ranges. a. The INPUT Receptacle of the Model 414A is a Tef- lon-insulated UHF connector. The center terminal is the high impedance terminal, and the outer shield is case ground. b. Carefully shield the input connection and the cur- rent source being measured, since power line frequencies are well within the pass band of the Picoammeter on a l l ranges. Unless shielding i s thorough, pickup may cause definite meter disturbances. c. Use high resistance, low-loss materials - such as polyethylene, polystyrene or Teflon - for insula- tion. The insulation resistance of t e s t leads and fix- tures should be several orders of magnitude higher than the source resistance. Excessive leakage w i l l reduce accuracy. Any coaxial cable used should be a low- noise type which employs a graphite coating between the dielectric and the surrounding shield braid. d. Any change i n the capacitance of the measuring circuit to ground w i l l cause disturbances in the read- ing, especially on the more sensitive ranges. Make the measuring setup as rigid as possible, and t i e down con- necting cables to prevent their movement. If a contin- uous vibration is present, it may appear a t the output as a sinusoidal signal and other precautions may be necessary to isolate the instrument and the connecting cable from the vibration. NOTE Keep the shield cap on the INPUT Receptacle when the Picoarmneter is not in a circuit. 2-2. OPERATING PROCEDURES. a. Check the fuse and the 117-2341 V Switch for the proper line voltage. b. Connect the power cord t o the power source. c. Set the RANGE Switch t o ampere, the METER S i t c h t o (+). Within seconds the meter needle should read zero. Zero the meter with the ZERO ADJ. Control. After a few moments increase the current sensitivity b ampere range. Continue zeroing with the ZERO A D J Con- trol. The instrument is now ready t o use. advancing the RANGE Switch i n decade steps to the 10- 18 d. I f long term measurements are t o be made, allow the instrument t o warm up for a t least 10 minutes. e. Attach the current source t o the INPUT Receptacle and turn the METER Switch t o the polarity of the input signal, + or -. Increase sensitivity with the RANGE 1. When the RANGE Switch i s s e t to 10, 1, 0.1, etc. positions, use the upper meter scale. Full scale current range is equal to the RANGE Switch set- ting. 2. When the RANGE Switch is s e t to 3, 0.3, 0.03, etc. positions, use the lower meter scale. F u l l scale current range i s equal to the RANGE Switch setting . 2-3, MEASUREMENT CONSIDERATIONS. a. The Picoammeter employs the fast method of cur- rent measurement - the measuring resistor is between the amplifier input and output in the feedback loop. This method largely neutralizes the effect of input capacity and greatly increases the response speed. Also, the input voltage drop is reduced to a maximum of one millivolt on any range. b. Rise time varies with the current range and the input capacity (see specifications, Table 1). The rise time, though, is not affected with up to 500 picofarads across the input; however, it i s better to place the Picoammeter nearer the current source than to the data reading instrument, Transmitting the input signal through long cables - with greater than 500 picofarads of capacitance - w i l l increase response time and meter noise. c. The internal resistance of the unknown source should not be less than the reciprocal of the current range being used, otherwise the zero stability w i l l be affected. The instrument w i l l s t i l l be operable, how- ever, but the s t a b i l i t y w i l l be degraded by the amount given by equation 1. Stability = 0.5%/week x (R, + Rf)/Rs Equation 1. where Rf i s the feedback resistance in ohms; Rs is the source resistance in ohms. For example, i f the source to be measured has a resis- tance of 105 ohms and the current i s lom6 then the feed- back resistor w i l l be 106 ohms. This means that the gain of the Picoammeter is 106/105 = 10. s t a b i l i t y of .5%/week w i l l be .5% X 10 = 5%/week, and the offset due to temperature w i l l be .05% X lo.= .5%/OC. This i s the reason that it is advantageous to have the source resistance a t least equal to the feed- back resistor. Then the zero d. Overload Protection. A unique circuit provides complete overload protection for the Model 414A with- out compromising the features of the M O S FET input. Recovery is instantaneous for most overloads. 1. A t the ampere range and below the Picoam- meter can withstand overloads of up to 1000 volts for 3 seconds and continuous overloads of up to 600 volts without damage. 2. Above amperes, the max. continuous overload Switch u n t i l the greatest on scale deflection i s achieved. is a function of rated power dissapation in the resistor. 4 0671 MODEL 414A OPERATION TABLE 4. Allowable Overloads on Ranges Above 10-6 Ampere. I Max. Continuous Max. Continuous Range Voltage Overload Current Overload 10-5 A 10-3 A 10-4 A 10-2 A 300 V 120 v 30 V 12 v 1 m A 4 m A 10 mA 40 mA I J 3. For maximum protection, use a Keithley Model 240A Voltage Supply, or some other 10 milliampere current limited supply, in combination with the Picoammeter. 2-4. RECORDER OUTPUTS. a. For recording with the Model 414A, use the Keith- ley Model 370 Recorder for ease, economy, versatility and performance. The Model 370 is a pen recorder with 10 chart speeds and 1% linearity. The Model 370s in- put cable has a connector which mates directly with the OUTPUT Connector on the Picoammeter; this avoids interface problems often encountered between a measur- ing instrument and a recorder. The Picoammeter output, when set to the 1 mA position, will drive the 370; no preamplifier is needed. No special wiring is required. b. Other recorders, oscilloscopes and similar in- struments can be used with the Model 414A. The Pico- ammeter has two outputs, 2 1 volt and 2 1 milliampere, to amplify signals for recorders, oscilloscopes and similar instruments. These can be used on all ranges. c. 1-Volt Output. Connect oscilloscopes and pen recorder amplifiers to the OUTPUT Receptacle. Pin no. 1 is the high terminal and pin no. 2 is ground. Set the 1 mA - 1 V Switch to 1 V. The Picoarmeter output is now 2 1 volt for full scale meter deflection on any range. Internal resistance is approximately 1 kilohm. Noise is less than.l% peak-to-peak of full scale. The METER Switch does not reverse the output polarity. Output polarity is always opposite input polarity. Mode 1 414A output (5103) $ 1 kR t Recorder FIGURE 3. for Driving 50 and 100-Millivolt Recorders. Divider Circuits Across Picoammeter Output d. 1-Milliampere Output. Connect 1-milliampere instruments to the OUTPUT Receptacle. Pin no, 1 is the high terminal. Set the 1 mA - 1 V Switch to 1 mA. The output is approximately 1 milliampere for full- scale meter deflection on any range. apply a known full scale signal to the Picoammeter and adjust the 1 mA CAL Control until the recorder reads full scale. Check the recorder and meter zero and repeat adjustment if necessary. The METER Switch does not reverse the output polarity which is always oppo- site input polarity. For exact output, e. For servo rebalance recorders, use a divider across the Picoammeter OUTPUT Receptacle. See Figure 3. Set the OUTPUT Switch to 1 mA. Use the.1 mA CAL Control to trim the output for full-scale recorder de- flections. Operation is the same as for current outputs. 2-5. 234-VOLT OPERATION. The instrument is shipped for use with a 117-volt power source unless otherwise ordered. To convert the Picoammeter for 234-volt sources, use a screwdriver to change the slide switch on the back panel to the 234-volt position. Change the fuse from 0.125 ampere to .062 ampere. No other adjustment is necessary. To switch from 234 to 117- volt operation, reverse the procedures. 0671 5 CIRCUIT DESCRIPTION SECTION 3. CIRCUIT DESCRIPTION MODEL 414A 3-1. GENERAL. The Keithley Model 414A is a linear dc amplifier with a f u l l scale sensitivity of either 1 or 3 volts. By using the front panel controls, shunt re- sistors are selected to make measurements over a total of 17 current ranges. (Refer to schematic diagram 22223D for circuit designations.) 3-2. AMMETER OPERATION. a. The amplifier has matched insulated-gate field- effect input transistors followed by a differential transistor stage, a transistor amplifier and a tran- sistor output stage. Figure 4 shows the simplified circuit for the Picoammeter. FIGURE 4. Simplified Diagram for h e t e r Circuit. b. I f it is assumed that the input voltage drop, ein, is negligible, then a l l the input current, iin, flows through the measuring resistor, R, and eo = -iin R,. Equation 2. But the output voltage, eo, is also equal to the input voltage times the amplifier gain. eo = -A ein. Equation 3. Therefore, from equations 2 and 3 we get A ein = iin R,. Equation 4. From whence, ein/iin = Rs/A. Equation 5. where ein/iin is the effective input resistance. Thus, the input voltage drop is kept a t a small frac- tion, 1/A, of the output voltage, and the effective in- put resistance is 1 / A of the measuring resistor. 3-3. AMMETER CIRCUIT. a. Two balanced insulated-gate field-effect tran- s i s t o r s , 1 0 1 and Q102, are used for the amplifier in- put. Resistors R117 and R118 and circuit designation 21850B (see schematic) protect the gate of transistor Q l O l , the active field-effect transistor, from overloads. The gate of Q102 is returned t o amplifier ground, b. Turning the METER Switch t o ZERO CK position places a short from the input to the output and zeroes the instrument. c. A differential amplifier stage, transistors Q103 and 4104, drives an amplifier stage