Evaluation Engineering - December 2008 - (Page 29)

VOUT CURRENT SOURCE R2 R1 R3 Figure 1. Transimpedance Current-to-Voltage Circuit Courtesy of Maxim Integrated Products/Dallas Semiconductor www. ev alua t ion e n gin e e rin g.com December 2008 • EE • 29 ≥ ≥ pliﬁers use tracking preﬁlters to reduce noise levels away from the reference frequency. Up to 20 dB greater noise can be handled in this way, but such ﬁlters add their own noise and contribute to phase error. Increased gain following the detector also is problematic because of detector DC offsets and drifts. An important aspect of dynamic reserve is its behavior near the reference frequency. DSP-based filtering provides a totally stable low-pass characteristic with very steep skirts, which increases the dynamic reserve close to the reference. In effect, the detector bandwidth is narrower. A digital lock-in ampliﬁer can have as much as 100 dB of dynamic reserve, where analog systems may be limited to about 60 dB. In addition to basic performance beneﬁts, digital designs typically include analysis functions such as curve ﬁtting, statistics and calculations, and data smoothing algorithms. Multiple auxiliary channels may be available and often the input can be switched between a high-impedance for voltages or a low impedance for currents. Several companies make lock-in ampliﬁers and the speciﬁcations vary widely. A 1-V maximum full-scale range is common, but the most sensitive range varies from 2 nV on the SR850 and Signal Recovery Model 7124 to 1 µV on the Scitec Model 450S. The Signal Recovery instrument is unusual because it has a separate remote input chassis controlled by a ﬁber-optic link to avoid introducing digital noise into sensitive experiments. It’s also possible to assemble lock-in ampliﬁers from off-the-shelf building blocks. As described in a National Instruments (NI) case study, a Harvard University research team at The Department of Chemistry needed to make 128 simultaneous low-current measurements on nanowire FET sensors. They used NI’s Model PXI-4472 Dynamic Signal Acquisition modules to sample all 128 channels and a software-based lock-in amplifier technique to extract the desired signals from the noise. Picoammeters Most commonly used ammeters and DMMs indicate current but actually measure the voltage drop, called the burden voltage, across a small shunt resistance. This method is low cost and sufﬁciently accurate for many appliContinued on page 30 cations. However, accuracy degrades when the shunt re15 V sistance approaches the signal source resistance and the I I = ID R1 burden voltage is no ID longer negligible. In the extreme, a very large shunt is required for nanoRL I L 0.2 pA amp currents, leadR2 1 GΩ ing to long settling times and slow measurement rates. Instead, the feedback ammeter approach 15 V RG = 1 GΩ IG = = 15 nA shown in Figure 1 is 1 GΩ used. The input impedance of the operational amplifier must be very high, but with a suitable Figure 2. Guarded Diode Current Measurement device, the equiva- Courtesy of Keithley Instruments lent burden voltage can be as low as a few tens of microvolts. Because an additional gain factor (R2 + R3)/R3 is provided by the circuit, the values of the resistors can be relatively small even for nanoamp sensitivity, leading to faster measurements. A Keithley Instruments Application Note discusses several noise and error sources encountered in low-current measurements. 1 In addition to the burden voltage, several other error sources become important at lower current levels. For example, leakage currents across or through insulation, noise current generated by triboelectric or piezoelectric effects, current associated with dielectric absorption, and electrochemical effects due to surface contamination can all contribute to measurement errors. Some suggested solutions are obvious: secure connections so that they cannot move and create triboelectric or piezoelectric currents. Clean the PCB or material being measured so that electrochemical effects are minimized. Beyond these precautions, guarding is a very common means of reducing errors caused by leakage currents. Figure 2 shows a typical measurement set up with guarding. The basic idea is to divert the leakage current http://www.evaluationengineering.com

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Evaluation Engineering - December 2008 Contents Editorial Product Briefing Test Software C-V Measurements Nanoelectronics Test Product Guide Company Guide Machine Vision EMC Test Index of Advertisers

Evaluation Engineering - December 2008

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