The circuit described in this document uses the ADL5534 IF amplifier to provide a dual IF gain block for the AD9640 150MSPS dual ADC. The ADL5534 is a high linearity dual fixed 20dB gain amplifier which can be adapted for use as a driver for a high performance IF Sampling ADC. The ADL5534 provides a simple approach to interfacing the RFIN signal level from 200mVpp to the 2Vpp fullscale of the high speed ADC. The low noise and low distortion of the ADL5534 ensures that the ADC performance is not compromised.

Figure 1 shows an application circuit where the ADL5534 drives the AD9640, a 150 MSPS, 14-bit ADC.

When operated as a quasi-differential amplifier, the ADL5534's two amplifiers present 100 Ω differential input and output impedances. The input signal is easily converted to a differential signal with a 2:1 impedance ratio flux coupled transformer. Between the ADL5534 output and the AD9640 input, a 3rd order low-pass filter presenting a 100 Ω differential impedance to the source and load, is implemented.

Due to the different common mode voltage levels required by the ADL5534 and the AD9640, The ADL5534 must be ac coupled to the AD9640. Capacitors of 100 pF were chosen to reduce any low frequency noise coming from the ADL5534 and to provide dc blocking. The measured results for this filter show 0.5 dB insertion loss for a 20 MHz bandwidth centered around 92 MHz. Figure 2 shows the measured wideband response for the filter.

The single tone results shown in Figure 3 show an SNR of 69.3 dB and an SFDR of 82 dBc. The two tone results shown in Figure 4 show an IMD3 of −80.5 dBc and an SFDR of 78 dBc. The AD9640 was clocked at a sample rate of 122.8 MSPS.

The application circuit described here can be modified for any IF frequency within the operating range of the ADL5534 and AD9640. As an alternative to the ADL5534, the AD8375, a digitally programmable differential variable gain amplifier, can be used. Alternatively, the AD8352, a resistor programmable differential amplifier can be used to convert from single-ended to differential without the requirement for an external balun. AD8352, AD8375 and AD8376 (dual version of AD8375) are all true differential amplifiers which provide rejection of common mode signals at their input.