Note: Unless otherwise indicated, this manual applies to all serial numbers. The Agilent Technologies 34970A combines precision measurement capability with flexible signal connections for your production and development test systems. Three module slots are built into the rear of the instrument to accept any combination of data acquisition or switching modules.
The Front Panel at a Glance Denotes a menu key. See the next page for details on menu operation.
The Rear Panel at a Glance 1 Slot Identifier (100, 200, 300) 2 Ext Trig Input / Alarm Outputs / Channel Advance Input / Channel Closed Output (for pinouts, see pages 83 and 128) 3 RS-232 Interface Connector 4 5 6 7 Power-Line Fuse-Holder Assembly Power-Line Voltage Setting Chassis Ground Screw GP-IB (IEEE-488) Interface Connector Use the Menu to: • Select the GP-IB or RS-232 interface (see chapter 2). • Set the GP-IB address (see chapter 2).
The Plug-In Modules at a Glance For complete specifications on each plug-in module, refer to the module sections in chapter 9. 34901A 20-Channel Armature Multiplexer • 20 channels of 300 V switching • Two channels for DC or AC current measurements (100 nA to 1A) • Built-in thermocouple reference junction • Switching speed of up to 60 channels per second • Connects to the internal multimeter • For detailed information and a module diagram, see page 164.
Chapter 1 Quick Start To Connect Wiring to a Module To Connect Wiring to a Module 1 Remove the module cover. 2 Connect wiring to the screw terminals. 20 AWG Typical 6 mm 3 Route wiring through strain relief. 4 Replace the module cover. Cable Tie Wrap (optional) 5 Install the module into mainframe. Channel Number: Slot Channel 20 Wiring Hints... • For detailed information on each module, refer to the section starting on page 163.
Chapter 1 Quick Start To Connect Wiring to a Module 1 Thermocouple Thermocouple Types: B, E, J, K, N, R, S, T See page 351 for thermocouple color codes. 2-Wire Ohms / RTD / Thermistor DC Voltage / AC Voltage / Frequency Ranges: 100 mV, 1 V, 10 V, 100 V, 300 V 4-Wire Ohms / RTD Ranges: 100, 1 k, 10 k, 100 k, 1 M, 10 M, 100 MΩ RTD Types: 0.00385, 0.00391 Thermistor Types: 2.
System Overview This chapter provides an overview of a computer-based system and describes the parts of a data acquisition system.
Chapter 3 System Overview Data Acquisition System Overview The system configuration shown on the previous page offers the following advantages: • You can use the 34970A to perform data storage, data reduction, mathematical calculations, and conversion to engineering units. You can use the PC to provide easy configuration and data presentation. • You can remove the analog signals and measurement sensors from the noisy PC environment and electrically isolate them from both the PC and earth ground.
Chapter 3 System Overview Signal Routing and Switching Multiplexer Switching Multiplexers allow you to connect one of multiple channels to a common channel, one at a time. A simple 4-to-1 multiplexer is shown below. When you combine a multiplexer with a measurement device, like the internal DMM, you create a scanner. For more information on scanning, see page 62.
Chapter 8 Tutorial System Cabling and Connections Shielding Techniques Shielding against noise must address both capacitive (electrical) and inductive (magnetic) coupling. The addition of a grounded shield around the conductor is highly effective against capacitive coupling. In switching networks, this shielding often takes the form of coaxial cables and connectors. For frequencies above 100 MHz, double-shielded coaxial cable is recommended to maximize shielding effectiveness.
Chapter 8 Tutorial System Cabling and Connections Sources of System Cabling Errors Radio Frequency Interference Most voltage-measuring instruments can generate false readings in the presence of large, high-frequency signals. Possible sources of high-frequency signals include nearby radio and television transmitters, computer monitors, and cellular telephones. High-frequency energy can also be coupled to the internal DMM on the system cabling.
Chapter 8 Tutorial System Cabling and Connections Thermal EMF Errors Thermoelectric voltages are the most common source of error in low-level dc voltage measurements. Thermoelectric voltages are generated when you make circuit connections using dissimilar metals at different temperatures. Each metal-to-metal junction forms a thermocouple, which generates a voltage proportional to the junction temperature difference.
Chapter 8 Tutorial Measurement Fundamentals Measurement Fundamentals This section explains how the 34970A makes measurements and discusses the most common sources of error related to these measurements. The Internal DMM The internal DMM provides a universal input front-end for measuring a variety of transducer types without the need for additional external signal conditioning.
Chapter 8 Tutorial Measurement Fundamentals Rejecting Power-Line Noise Voltages A desirable characteristic of an integrating analog-to-digital (A/D) converter is its ability to reject spurious signals. Integrating techniques reject power-line related noise present with dc signals on the input. This is called normal mode rejection or NMR. Normal mode noise rejection is achieved when the internal DMM measures the average of the input by “integrating” it over a fixed period.
Chapter 8 Tutorial Measurement Fundamentals Temperature Measurements A temperature transducer measurement is typically either a resistance or voltage measurement converted to an equivalent temperature by software conversion routines inside the instrument. The mathematical conversion is based on specific properties of the various transducers. The mathematical conversion accuracy (not including the transducer accuracy) for each transducer type is shown below.
Chapter 8 Tutorial Measurement Fundamentals RTD Measurements An RTD is constructed of a metal (typically platinum) that changes resistance with a change in temperature in a precisely known way. The internal DMM measures the resistance of the RTD and then calculates the equivalent temperature. An RTD has the highest stability of the temperature transducers. The output from an RTD is also very linear. This makes an RTD a good choice for high-accuracy, long-term measurements.
Chapter 8 Tutorial Measurement Fundamentals Thermocouple Measurements A thermocouple converts temperature to voltage. When two wires composed of dissimilar metals are joined, a voltage is generated. The voltage is a function of the junction temperature and the types of metals in the thermocouple wire. Since the temperature characteristics of many dissimilar metals are well known, a conversion from the voltage generated to the temperature of the junction can be made.
Chapter 8 Tutorial Measurement Fundamentals An ice bath is used to create a known reference temperature (0 °C). Once the reference temperature and thermocouple type are known, the temperature of the measurement thermocouple can be calculated. Internal DMM Ice Bath The T-type thermocouple is a unique case since one of the conductors (copper) is the same metal as the internal DMM’s input terminals. If another type of thermocouple is used, two additional thermocouples are created.
Chapter 8 Tutorial Measurement Fundamentals To make a more accurate measurement, you should extend the copper test leads of the internal DMM closer to the measurement and hold the connections to the thermocouple at the same temperature. Internal DMM Measurement Thermocouple Ice Bath Reference Thermocouple This circuit will give accurate temperature measurements. However, it is not very convenient to make two thermocouple connections and keep all connections at a known temperature.
Chapter 8 Tutorial Measurement Fundamentals In some measurement situations, however, it would be nice to remove the need for an ice bath (or any other fixed external reference). To do this, an isothermal block is used to make the connections. An isothermal block is an electrical insulator, but a good heat conductor. The additional thermocouples created at J1 and J2 are now held at the same temperature by the isothermal block.
Chapter 8 Tutorial Measurement Fundamentals Loading Errors Due to Input Resistance Measurement loading errors occur when the resistance of the device-under-test (DUT) is an appreciable percentage of the instrument’s own input resistance. The diagram below shows this error source.
Chapter 8 Tutorial Measurement Fundamentals Loading Errors Due to Input Bias Current The semiconductor devices used in the input circuits of the internal DMM have slight leakage currents called bias currents. The effect of the input bias current is a loading error at the internal DMM’s input terminals. The leakage current will approximately double for every 10 °C temperature rise, thus making the problem much more apparent at higher temperatures.
Chapter 8 Tutorial Measurement Fundamentals Resistance Measurements An ohmmeter measures the dc resistance of a device or circuit connected to its input. Resistance measurements are performed by supplying a known dc current to an unknown resistance and measuring the dc voltage drop. HI Runknown Itest To Amplifier and Analog-to-Digital Converter I LO The internal DMM offers two methods for measuring resistance: 2-wire and 4-wire ohms.
Chapter 8 Tutorial Measurement Fundamentals The 4-wire ohms method is used in systems where lead resistances can become quite large and variable and in automated test applications where cable lengths can be quite long. The 4-wire ohms method has the obvious disadvantage of requiring twice as many switches and twice as many wires as the 2-wire method.
Chapter 8 Tutorial Low-Level Signal Multiplexing and Switching Low-Level Signal Multiplexing and Switching Low-level multiplexers are available in the following types: one-wire, 2-wire, and 4-wire. The following sections in this chapter describe each type of multiplexer. The following low-level multiplexer modules are available with the 34970A.
Chapter 8 Tutorial Low-Level Signal Multiplexing and Switching One-Wire (Single-Ended) Multiplexers On the 34908A multiplexer, all of the 40 channels switch the HI input only, with a common LO for the module. The module also provides a thermocouple reference junction for making thermocouple measurements (for more information on the purpose of an isothermal block, see page 350).
Chapter 8 Tutorial Low-Level Signal Multiplexing and Switching Four-Wire Multiplexers You can make 4-wire ohms measurements using the 34901A and 34902A multiplexers. For a 4-wire ohms measurement, the channels are divided into two independent banks by opening the bank relay. For 4-wire measurements, the instrument automatically pairs channel n with channel n+10 (34901A) or n+8 (34902A) to provide the source and sense connections.
Chapter 8 Tutorial Low-Level Signal Multiplexing and Switching Signal Routing and Multiplexing When used stand-alone for signal routing (not scanning or connected to the internal DMM), multiple channels on the 34901A and 34902A multiplexers can be closed at the same time. You must be careful that this does not create a hazardous condition (for example, connecting two power sources together). Note that a multiplexer is not directional.
Chapter 9 Specifications DC, Resistance, and Temperature Accuracy Specifications DC, Resistance, and Temperature Accuracy Specifications ± ( % of reading + % of range ) [1] Includes measurement error, switching error, and transducer conversion error Range [3] Function Test Current or Burden Voltage DC Voltage 100.0000 mV 1.000000 V 10.00000 V 100.0000 V 300.000 V Resistance [4] 100.0000 Ω 1.000000 kΩ 10.00000 kΩ 100.0000 kΩ 1.000000 MΩ 10.00000 MΩ 100.
Chapter 9 Specifications To Calculate Total Measurement Error To Calculate Total Measurement Error Each specification includes correction factors which account for errors present due to operational limitations of the internal DMM. This section explains these errors and shows how to apply them to your measurements. Refer to “Interpreting Internal DMM Specifications,” starting on page 416, to get a better understanding of the terminology used and to help you interpret the internal DMM’s specifications.
Chapter 9 Specifications To Calculate Total Measurement Error Understanding the “ % of range ” Error The range error compensates for inaccuracies that result from the function and range you select. The range error contributes a constant error, expressed as a percent of range, independent of the input signal level. The following table shows the range error applied to the DMM’s 24-hour dc voltage specification.
Chapter 9 Specifications Interpreting Internal DMM Specifications Interpreting Internal DMM Specifications This section is provided to give you a better understanding of the terminology used and will help you interpret the internal DMM’s specifications. Number of Digits and Overrange The “number of digits” specification is the most fundamental, and sometimes, the most confusing characteristic of a multimeter.
Chapter 9 Specifications Interpreting Internal DMM Specifications Resolution Resolution is the numeric ratio of the maximum displayed value divided by the minimum displayed value on a selected range. Resolution is often expressed in percent, parts-per-million (ppm), counts, or bits. For example, a 61⁄2-digit multimeter with 20% overrange capability can display a measurement with up to 1,200,000 counts of resolution. This corresponds to about 0.0001% (1 ppm) of full scale, or 21 bits including the sign bit.
Chapter 9 Specifications Interpreting Internal DMM Specifications 24-Hour Accuracy The 24-hour accuracy specification indicates the internal DMM’s relative accuracy over its full measurement range for short time intervals and within a stable environment. Short-term accuracy is usually specified for a 24-hour period and for a ±1 °C temperature range. 90-Day and 1-Year Accuracy These long-term accuracy specifications are valid for a 23 °C ± 5 °C temperature range.
