LTC2379-18_15 Datasheet, PDF (11/26 Pages) Linear Technology – 18-Bit, 1.6Msps, Low Power SAR ADC with 101.2dB SNR

LTC2379-18
APPLICATIONS INFORMATION
INPUT DRIVE CIRCUITS
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.
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.
It also provides isolation between the signal source and
the current spike the ADC inputs draw.
Input Filtering
The noise and distortion of the buffer amplifier and signal
source must be considered since they add to the ADC noise
and distortion. Noisy input signals should be filtered prior
to the buffer amplifier input with an appropriate filter to
minimize noise. The simple 1-pole RC lowpass filter (LPF1)
shown in Figure 4 is sufficient for many applications.
LPF2
SINGLE-ENDED-
INPUT SIGNAL LPF1
500Ω
3300pF
20Ω
3300pF
6600pF
20Ω
SINGLE-ENDED- 3300pF
BW = 48kHz ^{TO-DIFFERENTIAL}
DRIVER
BW = 800kHz
Figure 4. Input Signal Chain
IN⁺
LTC2379-18
IN^–
237918 F04
Another filter network consisting of LPF2 should be used
between the buffer and ADC input to both minimize the
noise contribution of the buffer and to help minimize distur-
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 SNR.
High quality capacitors and resistors should be used in the
RC filters since these components can add distortion. NPO
and silver mica type dielectric capacitors have excellent
linearity. Carbon surface mount resistors can generate
distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resistors
are much less susceptible to both problems.
Single-Ended-to-Differential Conversion
For single-ended input signals, a single-ended to differential
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.
Figure 5a shows the LT6350 being used to convert a 0V
to 5V single-ended input signal. In this case, the first
amplifier is configured as a unity gain buffer and the single-
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.
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. In 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. In the inverting amplifier configuration,
the single-ended input signal source no longer directly
drives a high impedance input of the first amplifier. The
input impedance is instead set by resistor R_IN. R_IN must
be chosen carefully based on the source impedance of the
signal source. Higher values of R_IN tend to degrade both
the noise and distortion of the LT6350 and LTC2379-18
as a system.
237918fa
11