Issues to consider when selecting test & measurement equipment for your test system
by Bill Watry, Complete Test, INC.

     After you have evaluated the requirements document for a proposed test system, the next step is to determine your approach to testing. How will you run the tests? What measurement equipment will you need to purchase? For example: Can you use a digital IO approach to testing? Do you need to make precise Ohms/Volts measurements? Can you do this with a Scope? Can you do this with a voltmeter? Do you need to multiplex signals? The requirements document will spell out the specifications of the device or product for which you are writing the test proposal. For example, if the product you are testing is a source used to drive another device, then you will probably have specifications for: Frequency, Voltage, Drive current, and Timing requirements. All these specified parameters will typically have limits of performance and will indicate to you how accurate and how fast your measurements need to be.

     Watch out for the banner specs often quoted by equipment manufacturers. Banner specs will highlight the best possible performance of a measurement product, but how will it measure up at the frequency or speed you will need in your test system. Banner Specs will not always apply to all modes of instrument operation.

     The following list of issues discuss some of the more important decisions you will have to make.

Price performance
     This is always a trade off and you will want to balance price performance such that you keep the price as low as possible and still insure that what you purchase will do the job and give you some margin of performance for future growth.

     Many manufacturers will offer a product that will perform the required function, but you need to consider more than that.

Resolution and Accuracy
     Remember just the fact that you have 6 digits of resolution in a voltmeter does not insure accuracy to the 6^th digit on all ranges, at all speeds, and in all modes of operation. Consider a 10 bit A/D converter! Ten bits of resolution does not necessarily insure that the last 2 bits are not bouncing around in noise. More importantly you need to look at the effective bits spec of the A/D. If you do have resolution, accuracy can often be improved if you are willing to trade off measurement time.

Measurement Speed
     Often accuracy can be improved by varying measurement parameters. You can improve the effective bits of an A/D by injecting white noise and averaging, but averaging takes time. An integrating voltmeter will cancel out noise if you use at least one power line cycle (1 NPLC). Remember if you use 10 power line cycles(10 NPLC), one measurement will take 170 msec. That seems OK until you realize that you have 1000 measurements to make.

Switching speed
     Are you multiplexing signals into a voltmeter through switches? Then you need to consider the type of switches you will need.

      FET switches are fastest (<0.16 ms), but have high through resistance (up to 3K) and typically a 16V range limit. If you need to make accurate low resistance measurements, FETs work well. In the four-wire ohms mode, through resistance is eliminated by using 2 wires as sense lines. It is worth noting that the life of FET switches is not limited by the number of executions.

     Reed relays are slower than FETS (>3 ms), and can handle higher voltages. While reeds are fast, they can not carry much current and closed switch resistance can be up to 100 ohms.

     Armature relays are slowest(> 5 ms) but can carry higher currents and higher voltages. You will want to select the right relays depending on what type of measurement you are making.

     For all the classes of switches, you need to be aware of the thermal offset voltage of the switch if you plan to use the switches in thermocouple measurements. If you need to conserve power, you may need to use latching armature relays.

Range
     Does the product you are looking at have enough range to make the measurement. If you are measuring temperature with a T type thermocouple, then you need a voltmeter that can make accurate measurements on the microvolt range. (10 microvolts/deg C for T type thermocouples). For that matter, Will a T type thermocouple have enough range for the temperature measurement?

Functionality
     Can you purchase a multifunction instrument that will make a variety of measurements. For example; there are voltmeters which can also measure frequency, there are Scopes can make both timing and voltage measurements!

     Complete Test has engineers who have been in the test and measurement business for years and their expertise on these matters can save your company money on equipment purchases as well as optimizing test throughput.