SUPPLEMENT TEST & MEASUREMENT
There is a growing trend towards ‘green’ instruments and test equipment, however as performance levels increase and power consumption requirements decrease, it is important to match the components of the entire signal chain. Todd Nelson, signal chain module development manager, and Clarence Mayott, applications engineer, Linear Technology, explain
T
imes have changed – we’ve moved on from the days where electrical
measurements were carried out in laboratories, to using high accuracy portable battery-powered instruments in the field. However, analogue circuitry does not
benefit from the scaling effects of smaller geometries in the same manner as digital circuitry. Noise, the enemy of precision measurements, actually increases if less power is consumed. Furthermore, the signal-to-noise ratio (SNR) understandably gets worse with new low voltage processes as the signal amplitude is decreased. So does the analogue signal chain ‘go green’ while increasing performance? A high speed analogue to digital
converter (ADC) features at the core of many fast instruments. As an example, non-destructive testing of metal objects uses an imaging technique similar to medical ultrasound, where a digital image sensor feeds a high speed ADC. In some cases there are many channels, so the size and power consumption are key. Portable instruments obviously need to conserve battery power, but even fixed installations are power conscious. The trend in ADCs is to move to smaller process geometries and use 1.8V supplies to reduce the power, but clever ADC design is required to achieve the same or better performance as similar 3V devices. Linear Technology has developed
several pin-compatible families of 1.8V ultralow power 12-/14- and 16-bit ADCs at sample rates up to 125Msps that provide excellent dynamic performance at very low power levels. Without eliminating functions or increasing the front-end amplifier requirements, the
‘GO GREEN’ WITH portable instruments
new devices dramatically reduce power consumption. By providing a choice of single, dual, quad and octal ADCs, customers can achieve high channel density while ensuring the lowest heat dissipation in their system. However, the ADC is only part of the chain. The entire signal chain must be well matched for the instrument to be successful.
MEETING THE DEMANDS For applications, such as portable instrumentation, that require 16-bit performance and ultra-low power consumption in order to extend battery life, the LTC2195 family is available. 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
Single-ended to differential interface to high speed ADC
amplifier with low noise (1.6nV/√Hz at the input) and high linearity (+44dBm OIP3 at 20MHz) in a small 3x3mm QFN package. External resistors set the gain, providing maximum design flexibility; while low power consumption (59mW with a 3.3V supply) minimises the effect on the system power budget. This amplifier also has a common mode voltage range that extends down to 0.5V, meaning it can be paired seamlessly with the LTC2195, which has a nominal
common mode voltage of 0.9V. Typically the output of a digital sensor
is single-ended. This requires a single- ended to differential translation before being sampled by the ADC. If response to DC is also required, a transformer cannot be used. This situation mandates a low noise amplifier that is capable of doing single-ended to differential translation, like the LTC6406. The amplifier must be followed by a
filter to reduce the wideband noise of the amplifier and to isolate the output of the amplifier from the ADC inputs – the ADC inputs produce common mode glitches associated with the commutation of the sample caps. A filter helps attenuate these glitches, protecting the amplifier. A high order filter is not required, since the noise of the amplifier is fairly low. With a corner frequency of 12MHz, the filter used here is adequate – it does not degrade the performance of the ADC. The final filter should be designed to
reduce only the wideband noise of the amplifier, not as a selectivity filter with a steep transition band. A steep transition band in the filter increases insertion loss and degrades the OIP3 of the amplifier, which leads to distortion of the signal from the sensor. The circuit shown in the image accomplishes this goal. The ADC used is the LTC2195, a 16-bit
125Msps, simultaneous sampling, dual ADC operating from a single 1.8V supply. At 216mW per channel, this achieves nearly identical SNR performance as ADCs drawing 1.25W. The LVDS serial interface allows the part to consume less than half the board space of preceding ADCs and also allows the use of smaller FPGAs due to the reduced number of I/O. Combined with the LTC6406, this circuit consumes only 275mW – an obvious advantage for multichannel systems. This circuit can be easily applied to the 14- or 12-bit members of the family or to converters that sample at much lower sample rates, further saving power.
Linear Technology S10 OCTOBER 2016 | INSTRUMENTATION: TEST & MEASUREMENT SUPPLEMENT
www.linear.com
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