1000VRMS HP-4195A P6021 LT1223 LT1004 HP3403 000VRMS DN101 HP4195A LT1010

A Precision Wideband Current Probe for LCD Backlight Measurement Design Note 101 Jim Williams Evaluation and optimization

advertisement A Precision Wideband Current Probe for LCD Backlight Measurement Design Note 101 Jim Williams Evaluation and optimization of Cold Cathode Fluorescent Lamp (CCFL) performance requires highly accurate AC current measurement. CCFLs, used to backlight LCD displays, typically operate at 30kHz to 70kHz with measurable harmonic content into the low MHz region1. Accurate determination of RMS operating current is important for electrical and emissivity efficiency computations and to ensure long lamp life. Additionally, it is desirable to be able to perform current measurements in the presence of high common-mode voltage (> 1000VRMS 1000VRMS). This capability allows investigation and quantification of display and wiring induced losses, regardless of their origins in the lamp drive circuitry. common voltages noted. The current probe biases A1, operating at a gain of about 3.75. No impedance matching is required due to the probe's low output impedance termination. Additional amplifiers provide distributed gain, maintaining wide bandwidth with an overall gain of about 200. The individual amplifiers avoid any possible crosstalkbased error that could be introduced by a monolithic quad amplifier. D1 and Rx are selected for polarity and value to trim overall amplifier offset. The 100 trimmer sets gain, fixing the scale factor. The output drives a thermallybased, wideband RMS voltmeter. In practice, the circuit is built into a 2.25" × 1" × 1" enclosure which is directly connected, via BNC hardware, to the voltmeter. No cable is used. The result is a "clip-on" current probe with 1% accuracy over a 20kHz to 10MHz bandwidth. Figure 2 shows response for the probe-amplifier as measured on a Hewlett-Packard HP-4195A HP-4195A network analyzer. Current Probe Circuitry Figure 1's circuitry meets the discussed requirements. It signal conditions a commercially available "clip-on" current probe with a precision amplifier to provide 1% measurement accuracy to 10MHz. The "clip-on" probe provides convenience, even in the presence of the high , LTC and LT are registered trademarks of Linear Technology Corporation. 1 Williams, Jim, "Techniques for 92% Efficient LCD Illumination." Linear Technology Corporation AN55, August 1993. + A1 LT ®1223 TEKTRONIX P6021 P6021 CURRENT PROBE 2mA/mV + 1k A2 LT1223 LT1223 SELECT RX VALUE AND D1 POLARITY. ±15V SEE TEXT 3k 5% + A3 LT1223 LT1223 1k RX TYP 1.1M 5% + 1k 365 D1 LT1004 LT1004 1.2V A4 LT1223 LT1223 365 365 ALL RESISTORS ARE 1% FILM UNLESS NOTED USE RF LAYOUT TECHNIQUES SUPPLIES = ±15V Figure 1. Precision "Clip-On" Current Probe for CCFL Measurements Maintains 1% Accuracy Over 20kHz to 10MHz Bandwith 04/95/101 1k OUTPUT TO THERMALLY-BASED RMS VOLMETER, e.g., HP3403 HP3403, 3400, FLUKE 8920A. SCALE FACTOR IS 10.00mARMS = 1.000VRMS 000VRMS 20kHz TO 10MHz 301 100 CALIBRATE DN101 DN101 · F01 AMPLITUDE 1dB/DIV Current Calibrator Figure 3's circuit, a current calibrator, permits calibration of the probe-amplifier and can be used to periodically check probe accuracy. A1 and A2 form a Wein bridge oscillator. Oscillator output is rectified by A4 and A5 and compared to a DC reference at A3. A3's output controls Q1, closing an amplitude stabilization loop. The stabilized amplitude is terminated into a 100, 0.1% resistor to provide a precise 10.00mA, 60kHz current through the series current loop. Trimming is performed by altering the nominal 15k resistor for exactly 1.000VRMS 000VRMS across the 100 unit. 100kHz 1MHz 10MHz FREQUENCY DN101 DN101 · F02 Figure 2. Amplitude vs Frequency Output of HP4195A HP4195A Network Analyzer. Current Probe-Amplifier Maintains 1% (0.1dB) Error Bandwidth from 20kHz to 10MHz. Small Aberrations Between 10MHz and 20MHz Are Test Fixture Related In use, this current probe has shown 0.2% baseline stability with 1% absolute accuracy over one year's time. The sole maintenance requirement for preserving accuracy is to keep the current probe jaws clean and avoid rough or abrupt handling of the probe2. 2 0.012µF Private Communication. Tektronix, Inc. 0.012µF 240 10k A2 LT1010 LT1010 A1 LT1122 LT1122 10k A4 LT1122 LT1122 10k 5% 0.03µF 1µF 100 0.1% 10k A3 LT1097 LT1097 + 100k 5% 1N4148 1N4148 Q1 2N4338 2N4338 10k 5% 10.00mA 60kHz CURRENT LOOP OUTPUT A5 LT1122 LT1122 + 5.6k 500 1N4148 1N4148 10k + 2.4k + 240 619 5k 10k 15V 15k NOMINAL. TRIM FOR 1.000V ACROSS 100 RESISTOR. SEE TEXT 4.7k 5% LT1029 LT1029 5V ALL RESISTORS ARE 1% FILM UNLESS NOTED SUPPLIES = ±15V DN101 DN101 · F03 Figure 3. Current Calibrator for Probe Trimming and Accuracy Checks. Stabilized Oscillator Forces 10.00mA Through Output Current Loop at 60kHz For literature on our High Speed Amplifiers, call 1-800-4-LINEAR 1-800-4-LINEAR. For applications help, call (408) 432-1900, Ext. 456 Linear Technology Corporation LT/GP 0495 190K · PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7487 (408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977 © LINEAR TECHNOLOGY CORPORATION 1995