THS770006 Datasheet, PDF (25/39 Pages) National Semiconductor (TI) – Broadband, Fully-Differential, 14-/16-Bit ADC DRIVER AMPLIFIER

THS770006
www.ti.com
SBOS520B – JULY 2010 – REVISED JANUARY 2012
APPLICATION INFORMATION
THS770006 DRIVING 16-BIT ADC
To illustrate the performance of the THS770006 as an ADC driver, the device is tested with a 16-bit ADC.
TESTING WITH AN AC-COUPLED BANDPASS FILTER
For testing purposes, a 30MHz, third-order Butterworth bandpass filter with center frequency at 100MHz is
designed. 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 16-bit ADC, the expected total SNR is 75.1dBFS for the typical case.
Figure 39 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 39 shows the same circuit with a two-tone input at 96MHz and 100MHz. The near-in 3^rd order
intermodulation terms are about -100dBc.
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