Datasheet

R10
15 W
L16
56 nH
L17
56 nH
R2
25 W
C21
15 nF
R9
15 W
R1
100 W
R11
100 W
R13
49.9 W
R12
49.9 W
L15
8.2 nH
R3
25 W
C22
15 nF
C23
100 pF
C20
3.3 pF
C17
100 nF
ADC+
ADC-
ADC V
CM
PGA870
SBOS436A –DECEMBER 2009–REVISED FEBRUARY 2011 www.ti.com
Testing the PGA870 with a Second-Order Bandpass Filter
For better combined SNR performance, narrower bandwidth and/or higher-order filters are required between the
PGA870 and ADC. However, narrow filter bandwidth and highe-order filters cause the signal chain performance
to depend more strongly on component tolerance and mismatch. Component values that are 5% off from nominal
can detune a narrowband filter to the point that the desired signals do not fall within the useful passband and
become attenuated. Mismatch between corresponding series components on the positive and negative sides of
the differential filter can result in a differential phase shift that degrades even-order distortion performance. As
mentioned in the SFDR Considerations section, a transformer or balun is recommended at the ADC input in
these applications to restore the phase balance in the input signal to the ADC. The results shown in this
discussion interface the PGA870 and filter directly to the ADS58C48 input, and other builds of the same filter on
the same EVM showed over 10 dB of variation in distortion performance.
Figure 56 shows a simplified schematic of the PGA870 driving Channel D of the ADS58C48 on an
ADS58C48EVM with a second-order bandpass filter designed for 50-MHz bandwidth at a center frequency of
170 MHz. The measured –3-dB bandwidth of the filter is 57 MHz. The measured –1-dB bandwidth of the filter is
34 MHz. At a gain of 20 dB, the output voltage noise specification of the PGA870 is 30 nV/√Hz. With 2-V
PP
differential output swing and 57-MHz bandwidth, the expected SNR from the combined amplifier and filter is
70 dB. Added in combination with the typical ADS58C48 SNR, the expected SNR of the amplifier, filter, and ADC
chain is 64.5 dBFS.
Figure 56. Second-Order Bandpass Filter Schematic
Figure 57 shows the resulting FFT plot captured using the TSW1200 software with the PGA870 and a
second-order filter driving the ADS to –1 dBFS, with a single-tone input 170-MHz sine wave sampled at
200 MSPS. The results show 87.76-dBc SFDR and 65-dBFS SNR; analysis of the plot is shown in Table 6. The
PGA870 is set to a maximum gain of 20 dB. Figure 58 shows the FFT plot with the PGA870 set instead to a gain
of –4 dB, with the input signal amplitude increased accordingly to obtain –1 dBFS at the ADC input. The results
show about 1 dB lower SFDR at this gain setting and no change in the SNR.
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Product Folder Link(s): PGA870