SLYT853 May   2024 ADC12DJ5200RF

 

  1.   1
  2. 1Introduction
  3. 2AAF design approach
  4. 3AAF design process and parameters
  5. 4AAF design trade-offs
  6. 5AAF design example
  7. 6AAF design conclusion
  8. 7Additional resources

5 AAF design example

The design example circuit shown in Figure 4 is a wideband low-pass receiver front end based on the Texas Instruments (TI) TRF1208, a 10MHz to 11GHz, 3dB-bandwidth single-ended to differential amplifier and the TI ADC12DJ5200RF, a radio frequency (RF) sampling 12-bit dual-channel 5.2GSPS ADC. I optimized the third-order Butterworth AAF based on the performance and interface requirements of the amplifier and ADC; the total insertion loss caused by the filter network and other components was less than 6dB. In this AC-coupled design, the 0.1µF capacitors block the common-mode voltages between the amplifier, its termination resistors, and the ADC inputs.

The 10MHz to 11GHz TRF1208 differential amplifier accepts a single-ended input and converts it to a differential signal operating at a gain of 16dB to compensate for the insertion loss of the filter network, providing an overall signal gain of +7.8dB.

An input signal of –6.8dBm produces a full-scale 800mV peak-to-peak differential signal at the ADC input.

The overall circuit has a bandwidth of 2.34GHz with a pass-band flatness of <3dB. The SNR and SFDR measured with a 534MHz analog input frequency are 52.5dBFS and 71.4dBFS, respectively. The sampling frequency is 5.2GSPS, thereby creating a wideband low-pass filter covering the entire first Nyquist zone between 10MHz and 2.5GHz. Figure 4 shows the values chosen for the final filter passive components (after adjusting for actual circuit parasitics).

The AAF was designed as a third-order Butterworth filter using a standard filter design program with a differential source impedance (ZAAFS) of 39Ω (2 ´ 18Ω + 3Ω), a differential load impedance of 103Ω (ZAAFL) and a cutoff frequency of 2.4GHz. Because of the higher values of series inductance required in simulation, I decreased these inductors to 3nH in order to account for the inherent trace inductances in the layout and proportionally increased the initial 1.8pF capacitors to ground in the simulations to 2.2pF, thereby helping maintain appropriate rolloff around the 2.4GHz requirement.

The TRF1208 was not back-terminated in this case in order to achieve net performance, and the net differential impedance load was 139Ω (ZAL). Implementing the 18Ω series resistors isolated the filter capacitance from the amplifier outputs. For further insight on the FDA’s impedances, you candownload the S-parameters.

Installing the 15Ω resistors in series with the ADC inputs isolated the internal switching transients from the filter and the amplifier, as well as providing the necessary characterized loading to the FDA.

I used the ADC’s 100Ω input impedance per the data sheet. For further insight on the ADC’s impedances, download the S-parameters.

FDA, AAF, ADC wideband receiver front-end design (simplified schematic). Figure 4 FDA, AAF, ADC wideband receiver front-end design (simplified schematic).

Table 2 summarizes the measured performance of the system, where the total insertion loss of the network is approximately 5.8dB.

Table 2 Measured performance of the circuit.
Performance specs at –1dBFS (FS = 0.8V peak to peak), sample rate = 5.2GSPS, JMODE 3 Final results
Cutoff frequency 2,340MHz
Pass-band flatness (10MHz-2.2GHz) <3.0dB
SNR full scale at 534MHz 52.5dBFS
SFDR at 534MHz 71.4dBFS
H2/H3 at 534MHz –71.4dBFS/–73.0dBFS
Overall gain at 534MHz +7.8dB
Input drive at 534MHz -12.8dBm (−6dBFS)

Figure 5 shows the resulting combined FDA, AAF and ADC signal chain’s frequency response.

Pass-band flatness performance vs. frequency. Figure 5 Pass-band flatness performance vs. frequency.

Figure 6 shows the SNR and SFDR performance versus frequency, respectively.

SNR/SFDR performance vs. frequency, sample rate = 5.2GSPS. Figure 6 SNR/SFDR performance vs. frequency, sample rate = 5.2GSPS.