LTC2379-18
APPLICATIONS INFORMATION
INPUT DRIVE CIRCUITS
Highqualitycapacitorsandresistorsshouldbeusedinthe
RCfilterssincethesecomponentscanadddistortion.IPO
and silver mica type dielectric capacitors have excellent
linearity. Carbon surface mount resistors can generate
distortion from self heating and from damage that may
occurduringsoldering.Metalfilmsurfacemountresistors
are much less susceptible to both problems.
A low impedance source can directly drive the high im-
pedance inputs of the LTC2379-18 without gain error. A
high impedance source should be buffered to minimize
settling time during acquisition and to optimize the dis-
tortion performance of the ADC. Minimizing settling time
is important even for DC inputs, because the ADC inputs
draw a current spike when entering acquisition.
Single-Ended-to-Differential Conversion
For best performance, a buffer amplifier should be used
to drive the analog inputs of the LTC2379-18. The ampli-
fier provides low output impedance, which produces fast
settling of the analog signal during the acquisition phase.
ꢁt also provides isolation between the signal source and
the current spike the ADC inputs draw.
Forsingle-endedinputsignals,asingle-endedtodifferential
conversion circuit must be used to produce a differential
signal at the inputs of the LTC2379-18. The LT6350 ADC
driver is recommended for performing single-ended-to-
differential conversions. The LT6350 is flexible and may
be configured to convert single-ended signals of various
amplitudes to the 5V differential input range of the
LTC2379-18. The LT6350 is also available in H-grade to
complement the extended temperature operation of the
LTC2379-18 up to 125°C.
Input Filtering
The noise and distortion of the buffer amplifier and signal
sourcemustbeconsideredsincetheyaddtotheADCnoise
and distortion. Ioisy input signals should be filtered prior
to the buffer amplifier input with an appropriate filter to
minimizenoise.Thesimple1-poleRClowpassfilter(LPF1)
shown in Figure 4 is sufficient for many applications.
Figure 5a shows the LT6350 being used to convert a 0V
to 5V single-ended input signal. ꢁn this case, the first
amplifierisconfiguredasaunitygainbufferandthesingle-
ended input signal directly drives the high-impedance
input of the amplifier. As shown in the FFT of Figure 5b,
the LT6350 drives the LTC2379-18 to near full datasheet
performance.
LPF2
3300pF
SꢁIGLE-EIDED-
20Ω
LPF1
ꢁIPUT SꢁGIAL
+
–
ꢁI
ꢁI
500Ω
3300pF
The LT6350 can also be used to buffer and convert large
true bipolar signals which swing below ground to the
5V differential input range of the LTC2379-18 in order
to maximize the signal swing that can be digitized. Fig-
ure 6a shows the LT6350 being used to convert a 10V
true bipolar signal for use by the LTC2379-18. ꢁn this
case, the first amplifier in the LT6350 is configured as
an inverting amplifier stage, which acts to attenuate and
level shift the input signal to the 0V to 5V input range of
the LTC2379-18. ꢁn the inverting amplifier configuration,
the single-ended input signal source no longer directly
drives a high impedance input of the first amplifier. The
LTC2379-18
6600pF
20Ω
237918 F04
SꢁIGLE-EIDED- 3300pF
TO-DꢁFFEREITꢁAL
DRꢁVER
ꢀW = 48kHz
ꢀW = 800kHz
Figure 4. Input Signal Chain
Another filter network consisting of LPF2 should be used
between the buffer and ADC input to both minimize the
noisecontributionofthebufferandtohelpminimizedistur-
bances reflected into the buffer from sampling transients.
Long RC time constants at the analog inputs will slow
down the settling of the analog inputs. Therefore, LPF2
requires a wider bandwidth than LPF1. A buffer amplifier
with a low noise density must be selected to minimize
degradation of the SIR.
input impedance is instead set by resistor R . R must
ꢁI ꢁI
be chosen carefully based on the source impedance of the
signal source. Higher values of R tend to degrade both
ꢁI
the noise and distortion of the LT6350 and LTC2379-18
as a system.
237918fa
11
Linear Systems [ Linear Systems ]
