Characterizing noise in voltage-reference ICs
A new voltage reference has the accuracy and temperature coefficient typical of high-grade, low-voltage references.
Jim Williams, EDN consulting editor and Linear Technology staff scientist -- Test & Measurement World, 10/1/2009 2:00:00 AM
Voltage-reference stability and noise frequently define the measurement limits of instrumentation systems. In particular, reference noise often sets stable resolution limits.
Reference voltages have decreased with the continuing drop in system power-supply voltages, making reference noise increasingly important. The compressed signal-processing voltage range mandates a commensurate reduction in reference noise to maintain resolution. Noise ultimately translates into quantization uncertainty in ADCs, introducing jitter in applications such as scales, inertial navigation systems, infrared thermography, digital voltmeters, and medical-imaging apparatus.
A new voltage reference, the LTC6655, has the accuracy and temperature coefficient typical of high-grade, low-voltage references. No other low-voltage electronic reference, however, can equal the new device's extremely low 0.1- to 10-Hz noise. You must use special techniques to verify the part's noise performance.
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Figure 1. A conceptual 0.1- to 10-Hz noise-testing scheme includes a low-noise preamplifier, a filter, and a peak-to-peak noise detector. A forward gain of 1 million allows the circuit to drive a conventional instrument. |
A straightforward approach appears simple. Figure 1 illustrates a 0.1- to 10-Hz noise-testing scheme that includes a low-noise preamplifier, filters, and a peak-to-peak noise detector. But the practical implementation represents a measurement with a high order of difficulty. The preamplifier's 160-nV noise floor requires special design and layout techniques. For example, you'll need to strip out the reference-under-test's DC potential with a capacitor/resistor combination, and you'll need to choose a highly specialized tantalum capacitor optimized for leakage, and you should note that the capacitor's dielectric absorption requires a 24-hr charge time to ensure meaningful measurement results.
My article (Ref. 1) in sibling publication EDN provides complete details on measuring the capacitance leakage and constructing the measurement circuit, which has a forward gain of 1 million to allow conventional instruments to provide a readout of noise values. The article also provides a table of high-sensitivity, low-noise amplifiers that you can use to implement the circuit.
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