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Ohms Current Source
The ohms current source (schematic shown on page 9-10) flows from the
Input HI terminal to the Input LO terminal for both the 2-wire and 4-wire
ohms functions. Each current value is generated by forcing a stable,
precise voltage across a stable resistance. The value of the current
becomes part of the range gain constant stored during calibration.
The +7 V reference voltage is used to generate a stable reference current
with
U201A. R201 and R202 are the resistance references for the current
sources as shown in the table below. The
IREF current is used to
produce a precise voltage drop across the 28.57 k
Ω resistor in U102D-4.
The
IREF generated using R202 produces an approximate 5 V drop
across the 28.57 k
Ω resistor. The IREF generated using R201 produces
an approximate 0.5 V drop. This voltage is used to force a reference
voltage across the selected current source range resistor (5 k
Ω, 50 kΩ,
500 k
Ω, 1 MΩ) by U201B. The resulting precision current flows through
JFET Q202 and protection circuit Q203 to Q211, and CR202 to relay K102
where it is switched to the Input HI terminal for ohms measurements.
The protection circuits are designed to protect the ohms current source
from inadvertently applied voltages in excess of
±1000 V. Protection
from large positive voltages is provided by the reverse breakdown
voltage of
CR202. Protection from large negative voltages is provided by
the sum of the collector to base breakdown voltages of
Q203, Q205, Q207,
and
Q209. Bias for these transistors is provided by Q211 and R203 to
R206 while negative overvoltages are applied.
Ohms Range Current
Open Circuit
Voltage
Compliance
Limit
Reference
Isource R
U102D
100 Ω
1 kΩ
10 kΩ
100 kΩ
1 MΩ
10 MΩ
100 MΩ [1]
1 mA
1 mA
100
µA
10
µA
5
µA
500 nA
500 nA [1]
9 V
9 V
9 V
9 V
9 V
14 V
5 V
2.5 V
2.5 V
4 V
4 V
8 V
10 V
R202
R202
R202
R202
R202
R201
R201
5 k
Ω
5 kΩ
50 kΩ
500 kΩ
1 MΩ
1 MΩ
1 MΩ
[1] Measured in parallel with the internal 10 M
Ω resistor.
Chapter 5 Theory of Operation
Ohms Current Source
98
- Agilent 34401A 1
- 6 ½ Digit Multimeter 1
- Safety Information 3
- Additional Notices 5
- DECLARATION OF CONFORMITY 6
- Multimeter 7
- The Front Panel at a Glance 8
- Display Annunciators 10
- The Rear Panel at a Glance 11
- In This Book 12
- Contents 13
- Specifications 17
- DC Characteristics 18
- Measuring Characteristics 19
- Chapter 1 Specifications 19
- AC Characteristics 20
- General Information 24
- HOLD Sensitivity: 24
- Product Dimensions 25
- ≤ 150 µV 26
- ≤ 15 µV 26
- ≤ 1.5 µV 26
- Measurement Error 27
- Sensitivity 28
- Resolution 29
- Accuracy 29
- Transfer Accuracy 30
- 24-Hour Accuracy 30
- 90-Day and 1-Year Accuracy 30
- Temperature Coefficients 30
- Quick Start 33
- ERROR annunciator turns on 35
- To Adjust the Carrying Handle 38
- To Measure Voltage 39
- To Measure Resistance 39
- To Measure Current 40
- To Test Continuity 41
- To Check Diodes 41
- To Select a Range 42
- To Set the Resolution 43
- To Make dB Measurements 46
- To Make dBm Measurements 47
- To Trigger the Multimeter 48
- Using an External Trigger 48
- -H.DDD,DDD EFFF 50
- -045.23 mVDC 50
- Agilent System II 51
- Menu Tutorial 53
- Front-Panel Menu Reference 55
- A Front-Panel Menu Tutorial 57
- Chapter 3 Menu Tutorial 58
- On/Off 59
- MEAS MENU is your first 61
- A: , B: , etc.) 61
- Shift < 61
- Auto/Man 63
- Calibration 65
- Procedures 65
- Calibration Procedures 66
- Agilent Calibration Services 67
- Calibration Interval 67
- Recommended Test Equipment 69
- Application 69
- Recommended Equipment 69
- Accuracy Requirements 69
- Test Considerations 70
- Self-Test 71
- ERROR annunciator 71
- Quick Performance Check 72
- Q in the 72
- Zero Offset Verification 73
- Input HI-LO and Sense HI-LO 73
- Gain Verification 75
- AC FILTER slow (MEAS MENU) 76
- Calibration Security Code 79
- Be careful 80
- Calibration Count 81
- Calibration Message 81
- Zero Adjustment 83
- Zero Adjustment Procedure 83
- Gain Adjustment 85
- Gain Adjustment Procedure 86
- – 100 kHz 86
- –10 Vdc Adjustment Procedure 88
- 500 Vdc Adjustment Procedure 89
- RAM U505 90
- MEAS MENU) 90
- ± 400 µA ac 90
- Input Frequency 91
- AC Filter Selected 91
- Settling Time 91
- Understanding Resolution 92
- -045.23 mVDC 93
- Error Messages 95
- out of range 96
- Calibration Error Messages 96
- Theory of 97
- Operation 97
- Theory of Operation 98
- Block Diagram 99
- Front/Rear Selection 100
- Function Switching 101
- DC Amplifier 102
- Input I and 103
- Ohms Current Source 104
- AC Circuit 105
- A-to-D Converter 107
- Floating Logic 109
- Earth-Referenced Logic 111
- Power Supplies 112
- Front Panel 113
- Service 115
- Operating Checklist 117
- Types of Service Available 118
- Repackaging for Shipment 119
- Surface Mount Repair 120
- RS-232 connector 121
- GPIB interface 121
- Installation Procedure 122
- Troubleshooting Hints 123
- Unit is inoperative 123
- Unit fails self-test 123
- Current input is inoperative 124
- Power supply problems 124
- Readings indicate OVERLOAD 124
- Power Supply Minimum Maximum 124
- Readings are inaccurate 125
- Readings show excessive noise 125
- Self-Test Procedures 126
- Power-On Self-Test 126
- Self-Test Descriptions 126
- Replaceable Parts 131
- To Order Replaceable Parts 132
- Backdating and Part Changes 132
- Chapter 7 Replaceable Parts 133
- Agilent 34401A Mainframe 140
- Manufacturer’s List 141
- Backdating 143
- Chapter 8 Backdating 145
- Agilent 34401A Multimeter 145
- Schematics 148
- Binder Edge (LH Page) 151
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