Datasheet

R10

15 W

25 W

100 nF

15 W

100 W

R11

100 W

L13

8.2 nH

25 W

100 nF

C18

68 pF

C17

100 nF

ADC+

ADC-

ADC V

PGA870

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SBOS436A –DECEMBER 2009–REVISED FEBRUARY 2011

Testing the PGA870 with a First-Order Bandpass Filter

Differential phase shift in a differential filter because of component mismatches can lead to severely degraded

even-order distortion performance. In applications where good SFDR performance at the expense of slight

degradation in SNR is acceptable, a first-order filter can prove to be less sensitive and provide more repeatable

results compared to higher-order filters.

Figure 51 shows a simplified schematic of the PGA870 driving Channel D of the ADS58C48 on an

ADS58C48EVM with a first-order bandpass filter designed for 50-MHz bandwidth at a center frequency of

170 MHz. As a result of board parasitics, the measured –3-dB bandwidth of the filter is 70 MHz. The

measured –1-dB bandwidth of the filter is 40 MHz. At 20 dB of gain, the output voltage noise specification of the

PGA870 is 30 nV/√Hz. With 2-V

differential output swing and 70-MHz bandwidth, the expected SNR from the

combined amplifier and filter is 68.7 dB. Added in combination with the ADS58C48 SNR, the expected SNR of

the amplifier, filter, and ADC chain is 64 dBFS.

Figure 51. First-Order Bandpass Filter Schematic

Figure 52 shows the resulting FFT plot captured using the TSW1200 software with the PGA870 and first-order

filter driving the ADS to –1 dBFS, with a single-tone input 170-MHz sine wave sampled at 200 MSPS. The results

show 81.7-dBc SFDR and 63.7-dBFS SNR; analysis of the plot is provided in Table 5. The PGA870 is set to a

maximum gain of 20 dB. Figure 53 shows the FFT plot with the PGA870 set to a gain of –4 dB, with the input

signal amplitude increased accordingly to achieve –1 dBFS at the ADC input. The results show 5 dB lower SFDR

at this gain setting, which is expected at lower gains (see the Harmonic Distortion vs Frequency graphs,

Figure 23 through Figure 30); however, the SNR remains the same at approximately 63.8 dBFS.

Product Folder Link(s): PGA870