The "core" story between the ADC LTC2185 + differential amplifier ADA4927

It's a good idea to describe the ADC LTC2185 + differential amplifier ADA4927, because the excellent linearity of the LTC2185 requires high-performance amplifiers to ensure that the ADA4927 is designed to drive high-performance ADCs from DC to 125 MHz. Health.

Today, let’s talk about the “core” story between them~

ADC LTC2185

The LTC2185 is a 16-bit, 125 MSPS ADC with excellent noise performance and linearity while consuming only 185 mW per channel, making it ideal for demanding, low-power applications requiring excellent AC performance.

The LTC2185 is available in full-rate CMOS and double data rate (DDR) CMOS/LVDS digital outputs. Pin-compatible speed grade options include 25 MSPS, 40 MSPS, 65 MSPS, 80 MSPS, and 105 MSPS with an approximate power consumption of only 1.5 mW/MSPS per channel. It features digital output random number generators and alternating polarity (ABP) modes to minimize digital feedback when using parallel CMOS outputs. The 550 MHz analog full-power bandwidth and ultra-low jitter of 0.07 ps rms allow for undersampling of the IF frequency while achieving excellent noise performance.

To maintain the outstanding performance levels of the LTC2185, we need to drive with an appropriate, high performance amplifier. The ADA4927-1 meets the linearity requirements of the LTC2185 while consuming only 215mW.

Drive ADA4927

The ADA4927 is a high speed differential current feedback amplifier fabricated on Analog Devices' silicon-germanium process with excellent distortion performance and input voltage noise of only 1.3 nV Hz. Therefore, it can drive high speed ADCs such as the LTC2185. The gain of the ADA4927-1 is set by an external feedback resistor next to the input pin. The feedback pins and input pins of the ADA4927-1 are placed close to the package for a simple layout and minimize parasitic capacitance in the feedback network. Therefore, the ADA4927-1 is ideal for driving high performance ADCs from DC to 125 MHz, such as the LTC2185.

This combination of "ADC+ drivers" provides excellent performance in the 62.5 MHz to 125 MHz range that other high speed amplifiers cannot.

ADC+ driver, seamless!

Figure 1 shows the schematic of the ADA4927-1 driving LTC2185. See Figure 2 for the corresponding layout. The feedback pin of the ADA4927-1 is next to the input pin to minimize the parasitic capacitance of the feedback node while increasing the phase margin of the amplifier. The feedback resistor is directly connected across the two pins and avoids additional routing in the feedback path and simplifies 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.

There is a simple filter between the amplifier and the ADC to reduce the amplifier's wideband noise while improving the SNR of the system. The filter also attenuates the sampling spurs caused by the ADC before it reaches the amplifier. This helps prevent the ADA4927's output network from oscillating in response to these spurs. The filtering network can be modified according to various input bandwidth requirements.

Figures 3 and 4 show the SNR and SFDR for the LTC2185 and ADA4927-1 combinations. At 125 MHz, the SFDR remains above 67 dB and the SNR is better than 63 dB. The combination consumes only 250 mW. At a sampling rate of 125 MSPS, this combination provides good performance throughout the second Nyquist frequency region, while the linearity of other amplifiers begins to degrade.

Figure 3. SNR of the LTC2185 driven by the ADA4927-1

Figure 4. SFDR driving the LTC2185 with ADA4927-1

We can see that using the ADA4927-1 to drive the LTC2185 provides excellent linearity while maintaining low power consumption. The ADA4927-1 maintains excellent linearity at 125 MHz, making this ADC+Amplifier combination for demanding communication and medical applications requiring the LTC2185 second Nyquist frequency region. The pin and filter design of the ADA4927-1 greatly simplifies layout while ensuring excellent performance while meeting low power budgets.

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