Single-ended input driver circuit for differential input intermediate frequency sampling ADC

Circuit Function and Advantages

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 band-pass filter for noise reduction and anti-aliasing. The single-ended IF gain stage is followed by a transformer to perform the single-ended to differential conversion. This is the optimal solution for applications that require low noise and low distortion.

The ADL5535/ADL5536 are highly linear (OIP3 = +45 dBm, third-order output intercept 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, easily boosting the signal from approximately 400 mV pp to the 2 V pp full-scale level required by the ADC. The low noise figure (3.2 dB at 190 MHz) and low distortion of the ADL5535 ensure that the ADC performance is not compromised. When a gain of 20 dB is required, the ADL5536 can be used.

Figure 1. ADL5535 driving the AD9268 16-bit ADC (simplified schematic, decoupling and all connections not shown)

Circuit Description

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

Figure 2. Normalized frequency response of the ADC interface shown in Figure 1.

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 two-tone performance is shown in Figure 4.

Figure 3. Single-tone performance of the circuit shown in Figure 1 measured at an input frequency of 170 MHz and a sampling rate of 122.88 MSPS.

Figure 4. Two-tone performance of the circuit shown in Figure 1 measured with an input tone centered at 170 MHz and a sampling rate of 122.88 MSPS.

Common Changes

The application circuit described in this article can be modified for any IF frequency within the operating range of the ADL5535/ADL5536 and AD9268. The AD9640, the AD6657, or the AD9* can replace the AD9268 ADC in this application.