THS770006_10 Datasheet, PDF (24/36 Pages) Texas Instruments – Broadband, Fully-Differential, 14-/16-Bit ADC DRIVER AMPLIFIER

THS770006
SBOS520 – JULY 2010
www.ti.com
TESTING THE ADS5493 WITH AN AC-COUPLED BANDPASS FILTER
For testing purposes, a 30MHz, third-order Butterworth bandpass filter with center frequency at 100MHz is
designed and built on the EVM. The design target for the source impedance is 40Ω differential, and for load
impedance is 400Ω differential. Therefore, approximately 1dB insertion loss is expected in the pass-band,
requiring the amplifier output amplitude to be 2.5V_PP to drive the ADC to –1dBFS.
The output noise voltage specification for the THS770006 is 3.4 nV/√Hz. With 2.5V_PP amplifier output voltage
swing and 30MHz bandwidth, the expected SNR from the amplifier + antialias filter is 93.5dB. When added in
combination with the ADS5493, the expected total SNR is 75.1dBFS for the typical case.
Figure 38 shows the resulting FFT plot when driving the ADC to –1dBFS with a single-tone 100MHz sine wave,
and sampling at 125MSPS. Test results show 98dBc SFDR from the second-order harmonic and 75.6 dBFS
SNR; analysis of the plot is shown in Table 3 versus typical ADC specifications. The test results from circuit
board to circuit board shows over 10dB of variation in the second order harmonic and a balun is inserted
between the filter and ADC inputs to get repeatable performance. With balun, the minimum expected results
should be better than 90dBc SFDR and 75dBFS SNR.
Figure 38 shows the same circuit with a two-tone input at 96MHz and 100MHz. The near-in 3^rd order
intermodulation terms are about -100dBc.
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
-120
0 5 10 15 20 25 30 35 40 45 50 55 62.5
Frequency (MHz)
Figure 38. FFT Plot of THS770006 + 30MHz BPF + ADS5493 with Single-Tone at 100MHz
Table 3. Analysis of FFT for THS770006 + BPF + ADS5493 at 100MHz vs Typical ADC Specifications
CONFIGURATION
THS770006 + BPF + ADS5493
ADS5493 Only (typ)
ADC INPUT
–1dBFS
–1dBFS
SNR
75.6dBFS
75.2dBFS
HD2
–98dBc
–100dBc
HD3
–107dBc
–100dBc
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
-120
130
140
0 5 10 15 20 25 30 35 40 45 50 55 61
Frequency (MHz)
Figure 39. FFT Plot of THS770006 + 30MHz BPF + ADS5493 with Two-Tone Input at 96MHz and 100MHz
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