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to the input of the load. Two sense lines from the power supply are connected to the power inputs. These sense leads are voltage measuring lines that connect to a high impedance voltage measuring circuit in the power supply. Since the voltage measuring circuit is a high input impedance circuit, the voltage drop in the sense leads is negligible. The sense lead voltage measurement circuit becomes the feedback control loop for the power supply. The voltage at the load is fed back to the power supply by the sense leads. The power supply raises its output, VOut to overcome the voltage drop in the source leads so that VLoad =VProgrammed. Thus, only with remote sensing can the accuracy of the power supply be applied to the load. When using multiple channels to output a higher voltage or current than a single channel alone could output, it is important to ensure that the remote sense connections are properly configured. Each channel’s sense leads must be monitoring only the voltage of its channel. For a series connection of channels to increase voltage, connect the sense lines between the DUT and the common point of the two channels (see Figure 2). This will ensure that each channel’s sense lines monitor only the output of their channel, and the total voltage at the load is the sum of the programmed voltage without any loss due to voltage drops in the output leads. Connect the common sense leads to the same point at the common output connection, to avoid any small differential voltage between the sense leads. This could create a small error in the voltage at the load.


circuit by selecting power supplies with high read-back accuracy and resolution, and use those supplies to measure load currents.


Measuring load currents You could measure load current by putting a sense resistor in the line and use a DMM to measure the voltage across the sense resistor. Alternatively, you could insert the DMM in series with the power supply and the load to directly measure load current. Using either method, you are adding another instrument into your circuit. You have also added another voltage drop in your circuit due either to the sense resistor when the DMM is measuring voltage or the internal shunt resistor when the DMM is in the current mode. Select a power supply that has good read-back accuracy and high resolution, then the power supply can measure the load current directly without the complexity of the sense resistor and the DMM. Some power supplies have current measurement accuracies as excellent as 0.05%. Eliminate the extra complexity and instrument cost of a adding another instrument in the test


Noise sources If you are powering a circuit that measures very low signals such as a transducer that must pick-up millivolts or microamp current signals, then noise sources may cause problems. The power supply, itself, is one noise source, and this noise can be broken into two components: normal mode noise, and common mode noise. Normal mode noise is noise generated across the power supply’s output terminals due to the power supply’s internal circuitry. Common mode noise is earth-referenced noise originating from the power line and stray capacitance across the main transformer. For sensitive circuits, linear power supplies provide much lower normal mode output noise than supplies designed using switching technology. A linear supply can have five- to-10 times lower noise than a switching supply. Any time that noise is an important concern, use a linear supply. Common mode current can create performance problems and load current measurement errors. Good quality, transformer construction with sufficient shielding between primary and secondary can minimise the stray capacitance. When developing or testing low power and sensitive circuits, use a power supply with low common mode performance. A good, low common mode current specification is under 10µA. Keep environmental noise out of a low level signal detection circuit by using


shielded sense leads. The sense lines should always be shielded and separate from all other wiring since they are transmitting a signal to a high impedance input, measurement circuit. Even small amounts of current can create a large error voltage in a high impedance circuit. For DUTs that measure small signals, the source lines should be shielded as well to eliminate all potential sources of interfering signals. Finally, ensure the circuit is grounded at only one point to eliminate ground currents which can create error voltages in conducting pathways.


Conclusion While you are waiting for the next generation of power supplies, ensure that the supplies you need today will meet all your requirements. Review the supply’s power envelope to ensure it has all the capacity you will need. For maximum accuracy, include remote sensing and make sure the measurement accuracy is sufficient. Know the power supply’s design topology (linear vs switching) and its noise performance. Then take advantage of the performance you have selected by using good measurement techniques. A good review upfront will ensure that your power supply is of the same calibre as the rest of your instrumentation.


Keithley Instruments | www.keithley.com


Robert Green is Senior Market Development Manager, Keithley Instruments, a Tektronix Company


www.cieonline.co.uk


Components in Electronics


December 2012/January 2013 37


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