The circuit shown in Figure 1 uses the ADL5535 / ADL5536 single-ended intermediate frequency (IF) low-noise 50 Ω gain block to drive the AD9268 16-bit differential input analog-to-digital converter (ADC) . The circuit includes an interstage bandpass filter for noise reduction and anti-aliasing. The single-ended IF gain stage is followed by a transformer that performs single-ended to differential conversion. This is the optimal solution for applications requiring low noise and distortion.

The ADL5535/ADL5536 are high linearity (third-order output intercept point OIP3 = +45 dBm at 190 MHz), single-ended, fixed-gain amplifiers that can be used as drivers for high-performance IF sampling ADCs. The ADL5535 provides 16 dB of gain and can easily boost signals from approximately 400 mV pp to the 2 V pp full-scale level required by the ADC. The ADL5535's low noise figure (3.2 dB at 190 MHz) and low distortion ensure that ADC performance is not compromised. When 20 dB of gain is required, the ADL5536 can be used.

Figure 1 shows a schematic diagram of the ADL5535/ADL5536 driving the AD9268 16-bit ADC with a sampling rate of 122.88 MSPS. The ADL5535 has single-ended input and output impedances of 50 Ω. A 1:1 impedance converter (M/A-COM BA-007159-000000, 4.5 MHz to 3000 MHz) is used with termination resistors and series beads to provide a 50 Ω load to the anti-aliasing filter interface. The filter interface between the ADL5535 and AD9268 is a sixth-order Butterworth low-pass filter designed using a standard filter program. It provides 50 MHz, 1 dB bandwidth centered at 175 MHz. The sixth-order filter is followed by a shunt LC (72 nH, 8.2 pF) oscillation circuit to further reduce the low-frequency response of the filter and make the filter have more band-pass response. The normalized broadband response is shown in Figure 2.

For an input frequency of 170 MHz and a sampling rate of 122.88 MSPS, the single-tone performance is shown in Figure 3 and the dual-tone performance is shown in Figure 4.