While improving performance, how does analog signal link "move green"?

A long time ago, accurate electrical measurements were carried out in the original laboratory environment, where adequate power supply and time allocation ensured extremely high accuracy. Today, people want to bring the instrument to the scene, let the meter run on battery power, and immediately achieve higher accuracy. Analog circuits, unlike digital circuits, do not benefit from the proportional effect of smaller geometric sizes. If the power consumption is less, then the noise (the exact measurement of the enemy) actually increases. With the advent of new low-pressure processes, signal-to-noise ratio (SNR) has become worse, which is understandable, because the signal amplitude is reduced. So, in improving performance at the same time, analog signal link How to "green"?

The core of many high-speed instruments is a high-speed analog-to-digital converter (ADC). For example, nondestructive testing of metallic objects uses an imaging method similar to that used in medical ultrasound, in which digital image sensors provide signals for high-speed ADC. In some cases, there are many channels, so size and power consumption are key. Portable instruments obviously need to conserve battery power, but even a stationary installation will focus on the power, whether for the "green" program or just to minimize heat in a compact gauge. The trend for ADC is to move towards smaller geometries and use 1.8V power supplies to reduce power consumption, but to achieve the same or higher performance as similar 3V devices requires smarter ADC design.

Linear has developed several pin-compatible, 125Msps-rated 1.8V ultra-low-power 12-bit/14-bit and 16-bit ADC series that provide exceptional dynamic performance at very low power. These new devices can greatly reduce power consumption without reducing functionality or increasing the requirements of front-end amplifiers. Because the single, dual, 4, and 8 channel ADC can be selected, customers can achieve very high channel density while ensuring the lowest heat in the system. However, ADC is only part of the link. The entire signal link must be well matched to enable the instrument to work smoothly.

Matching signal path design

The LTC2195 series is an ideal solution for applications that require 16-bit performance and ultra-low power consumption to extend battery life. A portable instrument is a perfect example. In many applications, signals from sensors must be recuperated before ADC sampling. For this task, it is important to choose a low noise, low power amplifier that matches the ADC performance, such as the choice of LTC6406, which is a good match for the LTC2195 series.

The LTC6406 is a fully differential amplifier with a low noise (at the input end of 1.6nv/√hz) and a high linearity (+44DBMOIP3 at 20MHz), with a small 3MMX3MMQFN package. The gain is set with an external resistor, providing the user with the greatest design flexibility. Low power consumption (59mW with 3.3V power supply) minimizes the impact on the system power budget. This amplifier's common-mode voltage range also extends to 0.5V, which means that it can be seamlessly paired with LTC2195 because the LTC2195 has 0.9V nominal common-mode voltage.

Typically, the output of a digital sensor is single-ended. This requires a single-ended to differential conversion before ADC sampling. If the DC response is also required, then the transformer cannot be used. This requires a low noise amplifier such as LTC6406, which can perform single-ended to differential conversion.

The amplifier must then follow a filter to reduce the amplifier's broadband noise and isolate the amplifier output from the ADC input, which produces the common mode interference associated with the sampled capacitor commutation. Filters help to attenuate such disturbances and thus protect the amplifier. Higher-order filters are not required because the amplifier's noise is quite low. A filter with a corner frequency of 12MHz is sufficient here to not degrade ADC performance.

The final filter should be designed to reduce the broadband noise of the amplifier, not as a selective filter with a steep transition band. The steep transition band of the filter increases the insertion loss and reduces the amplifier OIP3, which results in a distorted sensor signal. The circuit shown in Figure 1 achieves this goal.

Figure 1: Single-end to differential interface to high-speed ADC

The ADC used is LTC2195, the device is a 16-bit 125Msps, simultaneous sampling, dual-channel ADC, with a single 1.8V power supply work. The device achieves almost the same Snr performance as the 1.25W power-consuming ADC with a power of 216mW per channel. The LVDS serial interface allows the device to occupy less than half the circuit board space, and also allows for smaller FPGA due to reduced I/O numbers. This circuit, together with LTC6406, consumes only 275mW of power, which is an obvious advantage for multichannel systems. This circuit can be easily applied to the series of 14-bit and 12-bit devices, or to a converter with a much lower sampling rate, thereby saving power further.

The results show that the linearity of the amplifier does not degrade the SFDR of ADC at low input frequency. The SNR remains unchanged at 76.5dB. When used in a unit gain, the LTC6406 does not reduce the SNR or sfdr of the LTC2195.

Conclusion

The trend towards "green" instrumentation and test equipment is inevitable, both for stationary installations and for portable devices. As performance increases and power consumption requirements decrease, the components that match the entire signal link become very important. In terms of 16-bit performance, LTC2195 is best suited for high resolution and power-focused sensor applications, while LTC6406 is a well matched drive amplifier that does not discount LTC2195 performance and has low power requirements. By using a relatively low order filter to reduce the broadband noise of the amplifier, the performance of the ADC's datasheet can be easily achieved. The pairing combination of LTC2195 and LTC6406 is suitable for any portable image sensor application, while providing excellent performance and low power consumption.