LTC2378-16
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 LTC2378-16 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 LTC2378-16. 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 LTC2378-16. The LT635± ADC
driver is recommended for performing single-ended-to-
differential conversions. The LT635± is flexible and may
be configured to convert single-ended signals of various
amplitudes to the ±5V differential input range of the
LTC2378-16. The LT635± is also available in H-grade to
complement the extended temperature operation of the
LTC2378-16 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 LT635± being used to convert a ±V
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 LT635± drives the LTC2378-16 to near full data sheet
performance.
LPF2
6800pF
SINGLE-ENDED-
20Ω
LPF1
INPUT SIGNAL
+
–
IN
500Ω
3300pF
The LT635± can also be used to buffer and convert large
true bipolar signals which swing below ground to the
±5V differential input range of the LTC2378-16 in order
to maximize the signal swing that can be digitized. Fig-
ure 6a shows the LT635± being used to convert a ±1±V
true bipolar signal for use by the LTC2378-16. ꢁn this
case, the first amplifier in the LT635± is configured as
an inverting amplifier stage, which acts to attenuate and
level shift the input signal to the ±V to 5V input range of
the LTC2378-16. ꢁn the inverting amplifier configuration,
the single-ended input signal source no longer directly
drives a high impedance input of the first amplifier. The
LTC2378-16
6600pF
IN
20Ω
237816 F04
SINGLE-ENDED- 6800pF
TO-DIFFERENTIAL
DRIVER
BW = 48kHz
BW = 600kHz
Figure 40 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 LT635± and LTC2378-16
as a system.
237816f
11
LINEAR_DIMENSIONS [ Linear Dimensions ]
