It's essential to understand the synergy between the ADC LTC2185 and the differential amplifier ADA4927, as the exceptional linearity of the LTC2185 demands high-performance amplifiers to maintain signal integrity. The ADA4927 is specifically designed to drive high-speed ADCs up to 125 MHz, making it a perfect match for the LTC2185 in applications where precision matters most.
Today, let’s dive into the key relationship between these two components—how they work together to deliver top-tier performance in demanding environments.
ADC LTC2185
The LTC2185 is a 16-bit, 125 MSPS analog-to-digital converter that offers remarkable noise performance and linearity while maintaining a low power consumption of just 185 mW per channel. This makes it ideal for applications where both high AC performance and energy efficiency are critical, such as in communication systems or medical imaging equipment.
Available in full-rate CMOS and DDR CMOS/LVDS output formats, the LTC2185 provides flexibility in system design. It supports multiple speed grades, including 25, 40, 65, 80, and 105 MSPS, with a power consumption of approximately 1.5 mW/MSPS per channel. Additional features include digital random number generators and alternating polarity (ABP) modes, which help reduce digital feedback when using parallel outputs. With a 550 MHz analog bandwidth and ultra-low jitter of 0.07 ps RMS, the LTC2185 excels at undersampling IF signals while preserving excellent noise performance.
To fully leverage the LTC2185’s capabilities, it’s important to pair it with a high-performance driver. The ADA4927-1 is an excellent choice, offering the necessary linearity while consuming only 215 mW.
ADA4927-1 Differential Amplifier
The ADA4927-1 is a high-speed differential current feedback amplifier built on Analog Devices’ silicon-germanium process. It delivers outstanding distortion performance and a low input voltage noise of just 1.3 nV/√Hz, making it well-suited for driving high-speed ADCs like the LTC2185.
The gain of the ADA4927-1 is set by an external resistor connected to the input pin. Its layout is optimized with feedback and input pins placed close together, reducing parasitic capacitance and simplifying the design. This makes it ideal for driving high-performance ADCs from DC up to 125 MHz.
This combination of ADC and driver delivers superior performance in the 62.5 MHz to 125 MHz range, outperforming other high-speed amplifiers in terms of linearity and noise.
Seamless Integration
Figure 1 shows the schematic of the ADA4927-1 driving one channel of the LTC2185. Figure 2 displays the corresponding PCB layout. The feedback pin is placed near the input pin to minimize parasitic capacitance and improve phase margin. The feedback resistor is directly connected across the two pins, avoiding unnecessary routing and simplifying the layout.
Figure 1. Schematic of the ADA4927-1 driving a channel of the LTC2185.
Figure 2. Layout of a channel for the LTC285 driven by the ADA4927-1.
A simple filter is placed between the amplifier and the ADC to suppress wideband noise and improve the system’s SNR. It also helps reduce sampling spurs generated by the ADC before they reach the amplifier, preventing potential oscillations in the output network. This filtering can be adjusted based on the required input bandwidth.
Figures 3 and 4 illustrate the SNR and SFDR performance of the LTC2185 when driven by the ADA4927-1. At 125 MHz, the SFDR remains above 67 dB, and the SNR exceeds 63 dB. The total power consumption of the combination is only 250 mW. This setup performs exceptionally well across the second Nyquist region, while other amplifiers begin to lose linearity at this frequency.
Figure 3. SNR of the LTC2185 driven by the ADA4927-1.
Figure 4. SFDR of the LTC2185 driven by the ADA4927-1.
As shown, the ADA4927-1 provides excellent linearity while keeping power consumption low. It maintains strong performance even at 125 MHz, making this ADC+amplifier combination ideal for applications that require high accuracy in the second Nyquist zone, such as in advanced communications or medical diagnostics. The compact pin configuration and filter design of the ADA4927-1 simplify the layout without compromising performance, all while staying within tight power budgets.
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