It's important to understand the synergy between the ADC LTC2185 and the differential amplifier ADA4927, as the LTC2185’s exceptional linearity demands a high-performance driver to fully realize its potential. The ADA4927 is specifically designed to drive high-speed ADCs across a wide frequency range, from DC up to 125 MHz, ensuring optimal signal integrity and performance.
Today, let’s dive into the “core†relationship between these two components and explore why they work so well together.
LTC2185 – A High-Performance ADC
The LTC2185 is a 16-bit, 125 MSPS analog-to-digital converter (ADC) that delivers outstanding noise performance and linearity while operating at just 185 mW per channel. This makes it ideal for low-power applications where high AC performance is critical.
Available in full-rate CMOS and DDR CMOS/LVDS output formats, the LTC2185 offers multiple speed grades, including 25, 40, 65, 80, and 105 MSPS, with power consumption as low as 1.5 mW/MSPS. It also features digital output random number generators and alternating polarity (ABP) modes to reduce digital feedback when using parallel CMOS outputs. With a 550 MHz analog bandwidth and ultra-low jitter of 0.07 ps rms, the LTC2185 excels in undersampling applications and maintains excellent noise performance even at high frequencies.
To maintain the LTC2185’s top-tier performance, a high-quality amplifier is essential. That’s where the ADA4927-1 comes in.
ADA4927-1 – A High-Speed Differential Amplifier
The ADA4927-1 is a high-speed differential current feedback amplifier built on Analog Devices’ silicon-germanium process. It offers excellent distortion performance and an input voltage noise of just 1.3 nV/√Hz, making it a perfect match for high-speed ADCs like the LTC2185.
The gain of the ADA4927-1 is set by an external feedback resistor connected near the input pin. Its feedback and input pins are placed close to the package, simplifying layout and reducing parasitic capacitance in the feedback network. This design makes the ADA4927-1 ideal for driving high-performance ADCs from DC up to 125 MHz, such as the LTC2185.
This combination of ADC and driver provides superior performance in the 62.5 MHz to 125 MHz range—something many other high-speed amplifiers struggle to achieve.
ADC + Driver = Seamless Performance
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 of the ADA4927-1 is placed close to the input pin to minimize parasitic capacitance and improve phase margin. The feedback resistor is directly connected between the two pins, eliminating 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 LTC2185 driven by the ADA4927-1.
A simple filter is placed between the amplifier and the ADC to suppress the amplifier’s wideband noise and enhance the system’s SNR. It also helps attenuate sampling spurs generated by the ADC before they reach the amplifier, preventing potential oscillation in the output network. This filtering can be adjusted based on specific input bandwidth requirements.
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 system is only 250 mW. At a sampling rate of 125 MSPS, this combination delivers excellent performance across the second Nyquist band, whereas other amplifiers often begin to degrade in 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, using the ADA4927-1 to drive the LTC2185 delivers excellent linearity while maintaining low power consumption. The ADA4927-1 retains strong linearity even at 125 MHz, making this ADC+amplifier combination ideal for demanding applications in communications and medical imaging that require performance in the second Nyquist zone. The ADA4927-1’s pin configuration and filter design simplify layout and ensure consistent performance without exceeding power budgets.
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