THS770006 Datasheet, PDF (24/39 Pages) National Semiconductor (TI) – Broadband, Fully-Differential, 14-/16-Bit ADC DRIVER AMPLIFIER

THS770006
SBOS520B – JULY 2010 – REVISED JANUARY 2012
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The calculated values for the ADS5485 give the lowest attenuation. As a result of the high V_FS of 3V_PP, 3.66V_PP
is required from the amplifier to drive to –1dBFS. Performance of the THS770006 is still very good up to 130MHz
at this level, but for best performance, back off further from full-scale and consider trading reduced SNR
performance for better SFDR performance.
The values calculated for the ADS6149 show reasonable design targets and should work with good performance.
Note the ADS6149 does not have buffered inputs, and the inputs have equivalent resistive impedance that varies
with sampling frequency. The calculation shown in Table 2 uses a value of 70MHz for R_IN, taken from the
ADS6149 data sheet.
The values calculated for the low input common-mode voltage of the ADS4149 result in a large attenuation of the
amplifier signal leading to 5.31V_PP being required for full-scale ADC drive. This amplitude is greater than the
maximum capability of the device. With a single +5V supply, the THS770006 is not suitable to drive this ADC in
dc-coupled applications unless the ADC input is backed off towards –6dBFS.
Another option is to operate the THS770006 with a split ±2.5V supply. The R_P and gains are shown in the last
two rows of Table 2 for pull-up voltages of 2.5V and 5V. For this situation, if the +2.5V is used as the pull-up
reference voltage, only 2.93V_PP is required for the –1dBFS input to the ADS4149. If a 5V reference is used, only
2.26VPP is required to reach the –1dBFS input to the ADS4149. See the Operation with Split Supply ±2.5V
section for more detail. Note that, similar to the ADS6149, the ADS4149 does not have buffered inputs and the
inputs have equivalent resistive impedance that varies with sampling frequency. The R_IN value at 70MHz taken
from the ADS4149 data sheet was used in the calculation.
As with any design, testing is recommended to validate whether it meets the specific design goals.
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