Datasheet

LTC6430-15
14
643015f
Driving the LTC2158, 14-Bit, 310Msps ADC with
1.25GHz of Bandwidth
Boasting high linearity, low associated noise and wide
bandwidth, the LTC6430-15 is well suited to drive high
speed, high resolution ADCs with over a GHz of input
bandwidth. To demonstrate its performance, the LTC6430-
15 was used to drive an LTC2158 14-bit, 310Msps ADC
with 1.25GHz of input bandwidth in an undersampling
application. Typically, a filter is used between the ADC
driver amplifier and ADC input to minimize the noise
contribution from the amplifier. However, with the typical
SNR of higher sample rate ADCs, the LTC6430-15 can
drive them without any intervening filter, and with very
little penalty in SNR. This system approach has the added
benefit of allowing over two octaves of usable frequency
range. The LTC6430-15 driving the LTC2158, as shown
in the circuit in Figure 3, with band limiting provided
only by the 1.25GHz input BW of the ADC, still produces
64.4dB SNR, and offers IM performance that varies little
from 300MHz to 1GHz. At the lower end of this frequency
range, the IM contribution of the ADC and amplifier are
comparable, and the third-order IM products may be ad-
ditive, or may see cancelation. At 1GHz input, the ADC is
dominant in terms of IM and noise contribution, limited
by internal clock jitter and high input signal amplitude.
Table 3 shows noise and linearity performance. Example
outputs at 380MHz and 1000MHz are shown in Figure 5,
Figure 6, Figure 7, Figure 8 and Figure 9.
As a final display of the utility of this LTC6430-15/LTC2158
combination with real world signals, Figure 9 shows a
wideband code division multiple access (WCDMA) signal
was introduced to the LTC6430-15/LTC2158 combination
at 830MHz. The output indicates an ACPR near 60dB cal-
culated from the adjacent power on the upper side where
the filter stop band suppresses the contribution from
the generator. Please note that the adjacent channels on
the lower side are not suppressed as they are within the
passband of the filter.
The LTC6430-15 can directly drive the high speed ADC
inputs and settles quickly. Most feedback amplifiers require
protection from the sampling disturbances, the mixing
products that result from direct sampling. This is in part
due to the fact that unless the ADC input driving circuitry
offers settling in less than one-half clock cycle, the ADC
may not exhibit the expected linearity. If the ADC
samples
the recovery
process of an amplifier it will be seen as
distortion. If an amplifier exhibits envelope detection in
applicaTions inForMaTion
Table 3. LTC6430-15 and LTC2158 Combined Performance
Frequency
(MHz)
Sample Rate
(Msps)
IM3
(Low, Hi)
(dBFS)
HD3
(3rd Harmonic)
(dBc)
SFDR
(dB)
SNR
(dB)
380 310 (–98, –105) –80.2 68.7 61.8
533 307.2 –82.2 79.3 59.4
656 291.8 (–94, –92)
690 307.2 (–93, –92) –80.8 70.5 58.2
842 307.2 –78 66.7 57.1
1000 307.2 (–83,–83) –89.7 69.3 56.0