029-030-031_EETE-NEW-VF

EETE JANUARY 2013

TEST & MEASUREMENT “Green” pathways for test, measurement and instrumentation By Todd Nelson and Clarence Mayott LoNg ago, preCisioN electrical measurements were per- formed in pristine laboratory environments where ample elec- tricity was available and time was allotted to assure extreme accuracy. Today, the instrument is expected to be ported to the field, running on battery power and achieving even greater accuracy instantly. Analogue circuitry does not benefit from the scaling effects of smaller geometries in the same manner as digital circuitry. Noise, the enemy of precision measure- ments, actually increases if less power is consumed. and the signal-to-noise ratio (SNR) understandably gets worse with new low voltage processes as the signal amplitude is de- creased. Then how does the analog signal chain “go green” while increasing performance? Fig. 1: Single-ended to differential interface to high speed ADC. The core of many fast instruments is a high speed analogue to digital converter (ADC). For example, non-destructive testing 20MHz) in a small 3x3mm QFN package. External resistors set of metal objects uses an imaging technique similar to medical the gain, giving the user maximum design flexibility. Low power ultrasound, where a digital image sensor feeds a high speed consumption (59mW with a 3.3V supply) minimizes the effect ADC. In some cases there are a great many channels, so the on the system power budget. This amplifier also has a common size and power consumption are key. portable instruments mode voltage range that extends down to 0.5V, meaning it can obviously need to conserve battery power, but even fixed instal- be paired seamlessly with the LTC2195, which has a nominal lations are power conscious – whether for “green” initiatives or common mode voltage of 0.9V. simply to minimize heat dissipation in compact form factors. Typically the output of a digital sensor is single-ended. This The trend in ADCs is to move to smaller process geometries requires a single-ended to differential translation before being and use 1.8V supplies to reduce the power, but clever ADC sampled by the ADC. If response to DC is also required, a trans- design is required to achieve the same or better performance as former cannot be used. This situation mandates a low noise similar 3V devices. amplifier that is capable of doing single-ended to differential Linear Technology has developed several pin-compatible translation, like the LTC6406. families of 1.8V ultralow power 12-/14- and 16-bit ADCs at The amplifier must be followed by a filter to reduce the sample rates up to 125Msps that provide excellent dynamic wideband noise of the amplifier and to isolate the output of performance at very low power levels. Without eliminating func- the amplifier from the ADC inputs—the ADC inputs produce tions or increasing the front-end amplifier requirements, the new common mode glitches associated with the commutation of the devices dramatically reduce power consumption. By providing sample caps. A filter helps attenuate these glitches, protecting a choice of single, dual, quad and octal ADCs, customers can the amplifier. A high order filter is not required, since the noise achieve high channel density while ensuring the lowest heat of the amplifier is fairly low. With a corner frequency of 12MHz, dissipation in their system. However, the ADC is only part of the filter used here is adequate—it does not degrade the perfor- the chain. The entire signal chain must be well matched for the mance of the ADC. The final filter should be designed to reduce instrument to be successful. Matched signal path design The LTC2195 family is an ideal solution for applications that require 16-bit performance and ultralow power consumption to extend battery life. portable instrumentation is a perfect example. In many applications the signal from the sensor must be conditioned before being sampled by the ADC. For this task, it is important to choose a low noise, low power amplifier that matches the performance of the ADC, such as the LTC6406, which makes a good match for the LTC2195 family. The LTC6406 is a fully differential amplifier with low noise (1.6nV/√Hz at the input) and high linearity (+44dBm OIP3 at Todd Nelson is Signal Chain Module Development Manager at Linear Technology Fig. 2: FFT results of the circuit from figure 1 with FS = Clarence Mayott is Applications Engineer at Linear Technology – 125Msps and FIN = 1MHz www.linear.com www.electronics-eetimes.com Electronic Engineering Times Europe January 2013 29


EETE JANUARY 2013
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