Circuit functions and advantages
The circuit described here uses the AD8376 dual, digitally programmable, ultra-low distortion, high output linearity, variable gain amplifier (VGA), and high speed ADC to provide high performance, high frequency sampling. The AD8376 is optimized for driving high frequency IF sampling ADCs. When used with high-speed ADCs such as Analog Devices’ AD9445 or AD9246, it provides excellent SFDR (spurious free dynamic range) performance above 100 MSPS at maximum gain.
　
circuit description
This circuit uses the AD8376 VGA, which provides variable gain, isolation, and source impedance matching for high-speed ADCs such as the AD9445. Using this circuit, when the AD8376 has a gain of 20 dB (maximum gain), the SFDR performance at 100 MHz can reach 86 dBc, as shown in Figure 2.
　
The AD8376 VGA should be driven differentially (for best performance) through a broadband 1:1 transmission line balun (or impedance transformer) followed by two 37.4 Ω resistors in parallel with the AD8376’s 150 Ω input impedance. This enables broadband matching to the 50 Ω source impedance shown in Figure 1. Open set of AD8376
　
The electrode outputs are biased through two 1 μH inductors and ac-coupled to two 82 Ω load resistors. These 82 Ω load resistors are placed in parallel with the series-terminated ADC impedance, resulting in a differential load impedance of 150 Ω, which is the recommended value for the AD8376 to achieve specified gain accuracy. The load resistors are ac-coupled through the AD9445 to eliminate common-mode dc loads. The 33 Ω series resistor improves the isolation between the AD8376 and any switching currents present in the analog-to-digital track-and-hold input circuit.
　
The output IP3 (third-order intercept point) and noise floor of the AD8376 remain essentially stable over the available 24 dB gain range, which is useful for variable gain receivers where instantaneous dynamic range is expected to remain the same as the receiver gain changes. is an important advantage. The typical value of the output noise density is about 20 nV/√Hz, which is comparable to the 14-bit to 16-bit sensitivity limit. The two-tone IP3 performance of the AD8376 is typically about +50 dBm. As a result, the SFDR performance is better than 86 dBc at input frequencies up to 140 MHz when driving the AD9445, a 14-bit, 105 MSPS/125 MSPS analog-to-digital converter. When using the AD8376, the designer has a variety of configuration options to choose from. The open collector output is capable of driving many different loads. Figure 1 shows a simplified wideband interface where the AD8376 drives the AD9445.

Figure 1. Example of a Wideband ADC Interface Using the AD8376 and AD9445

The AD9445 is a 14-bit, 125 MSPS analog-to-digital converter with a buffered wideband input, resulting in a 2 kΩ||3 pF differential load impedance requiring a 2 V peak-to-peak differential input swing to reach full scale. In Figure 1, adding a series Inductor L (series) extends the bandwidth of the system with response flatness. When L (series) is a 100 nH inductor, the broadband system response shown in Figure 3 is obtained. Wideband frequency response is also an advantage in wideband applications such as predistortion receiver design and instrumentation. However, when designing for a wide analog input frequency range, the cascaded SNR (signal-to-noise ratio) performance is degraded due to the aliasing of high frequency noise into the target Nyquist frequency region.

Figure 2. Measured single-tone performance of the circuit shown in Figure 1 with a 100 MHz input signal, 105 MSPS sampling rate

Figure 3. Frequency Response Measurement Results of the Wideband Circuit of Figure 1

common changes
Figure 4 provides another narrowband approach. By designing a narrow bandpass antialiasing filter between the AD8376 and the target ADC, the AD8376 output noise outside the target Nyquist frequency region is attenuated, helping to maintain the ADC’s usable SNR performance.
　
In general, SNR performance improves by several dB when an antialiasing filter of the appropriate order is used. This example uses a low loss 1:3 (impedance ratio) input transformer to match the AD8376’s 150 Ω balanced input to a 50 Ω unbalanced source impedance to minimize insertion loss at the input.
　
The narrowband circuit shown in Figure 4 is optimized for driving some of Analog Devices’ popular unbuffered input ADCs, such as the AD9246, AD9640, and AD6655. Table 1 lists recommended values ​​for the associated antialiasing filter components for common IF sampling center frequencies. The inductor L5 is connected in parallel with the on-chip ADC input capacitor and a part of the capacitor provided by C4 to form a resonant circuit. This resonant circuit helps ensure that the ADC input behaves like a true resistor at the target center frequency.

Figure 4. Narrowband IF Sampling Solution for Unbuffered Switched Capacitor ADC Inputs

Additionally, inductor L5 will short the ADC input at dc, introducing a zero into the transfer function. The 1 nF AC-coupling capacitor and 1 μH bias choke introduce more zeros into the transfer function. The resulting overall frequency response exhibits a bandpass characteristic that helps suppress noise outside the target Nyquist frequency region. Table 1 provides some preliminary suggested values ​​for prototyping. Some empirical optimization methods may also need to be considered to help compensate for actual PCB parasitics. For details on interstage filter design, refer to the application note in the “Learn More” section.
　
In the circuit shown in Figure 1, both 37.4 Ω resistors require an accuracy of 1% (1/10 of a watt). Other resistors can be accurate to 10% (1/10 of a watt). Capacitors should be 10% ceramic chips. In the circuit shown in Figure 2, both 165 Ω resistors require an accuracy of 1% (1/10 of a watt). Other resistors, capacitors and inductors can be accurate to 10%.
　
To achieve the desired performance of the circuits discussed in this article, excellent routing, grounding, and decoupling techniques are required. At least four layers of PCB should be used: one is the ground plane, one is the power plane, and the other two are the signal layers.
　
All IC power pins must be decoupled from the ground plane with 0.01 μF to 0.1 μF low-inductance multilayer ceramic capacitors (MLCCs) (not shown for simplicity). The relevant recommendations of the IC data sheet in the “Learn More” section should also be followed.
　
Product evaluation boards should be consulted for routing and critical component placement recommendations. Evaluation boards can be found on the device’s product home page (see the “Learn More” section).
　
To prevent damage to the AD8376’s internal ESD protection diodes, the digital inputs “A” and “B”, as well as ENBA, ENBB, should not exceed 0.6 V above the AD8376’s positive supply voltage, or below 0.6 V above ground. This does not happen if the logic power to drive the AD8376 is derived from the AD8376’s power supply. The AD8376 is fabricated on a bipolar process and is not prone to latch-up.
　
Even if the AD8376 and AD9445 (or other ADCs) are powered by different power supplies, since the input signal to the ADC is AC-coupled, timing control is not an issue.
　
For the correct timing of the AVDD and DVDD supplies (if separate supplies are used), the appropriate ADC data sheet should be consulted.

Table 1: Recommended Interface Filter Values ​​for Different IF Sampling Frequencies

Further reading
Kester, Walt. High Speed ​​System Applications. Chapter 2 (Optimizing Data Converter Interfaces). Analog Devices. 2006.
Kester, Walt. The Data Conversion Handbook. Chapters 6, 7. Analog Devices. 2005.
Kester, Walt, James Bryant, and Mike Byrne. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of AGND and DGND. Analog Devices.
MT-036 Tutorial, Op Amp Output Phase Reversal and Input Overvoltage Protection. Analog Devices.
MT-073 Tutorial, High Speed ​​Variable Gain Amplifiers. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Newman, Eric and Rob Reeder. AN-827 Application Note, A Resonant Approach to Interfacing Amplifiers to Switched-Capacitor ADCs. Analog Devices.
Reeder, Rob. AN-742 Application Note, Frequency Domain Response of Switched Capacitor ADCs. Analog Devices.
　　
Datasheets and Evaluation Boards
AD8376 data sheet.
AD8376 evaluation board.
AD9246 data sheet.
AD9246 evaluation board.
AD9445 data sheet.
AD9445 evaluation board.
　　
revise history
4/09—Rev. 0 to Rev. A
Updated Format ………………………………………………………….Universal
10/08—Revision 0: Initial Version

The Links:   DMF682ANY-EB   PM50CLS120