Datasheet

LTC2378-18
10
237818f
OVERVIEW
The LTC2378-18 is a low noise, low power, high speed 18-bit
successive approximation register (SAR) ADC. Operating
from a single 2.5V supply, the LTC2378-18 supports a
large and flexible ±V
REF
fully differential input range with
V
REF
ranging from 2.5V to 5.1V, making it ideal for high
performance applications which require a wide dynamic
range. The LTC2378-18 achieves ±1.75LSB INL max, no
missing codes at 18 bits and 102dB SNR.
Fast 1Msps throughput with no cycle latency makes the
LTC2378-18 ideally suited for a wide variety of high speed
applications. An internal oscillator sets the conversion time,
easing external timing considerations. The LTC2378-18 dis-
sipates only 13.5mW at 1Msps, while an auto power-down
feature is provided to further reduce power dissipation
during inactive periods.
The LTC2378-18 features a unique digital gain compres-
sion (DGC) function, which eliminates the driver amplifier’s
negative supply while preserving the full resolution of the
ADC. When enabled, the ADC performs a digital scaling
function that maps zero-scale code from 0V to 0.1 • V
REF
and full-scale code from V
REF
to 0.9 • V
REF
. For a typical
reference voltage of 5V, the full-scale input range is now
0.5V to 4.5V, which provides adequate headroom for
powering the driving amplifier from a single 5.5V supply.
CONVERTER OPERATION
The LTC2378-18 operates in two phases. During the ac-
quisition phase, the charge redistribution capacitor D/A
converter (CDAC) is connected to the IN
+
and IN
–
pins to
sample the differential analog input voltage. A rising edge
on the CNV pin initiates a conversion. During the conversion
phase, the 18-bit CDAC is sequenced through a succes-
sive approximation algorithm, effectively comparing the
sampled input with binary-weighted fractions of the refer-
ence voltage (e.g. V
REF
/2, V
REF
/4 … V
REF
/262144) using
the differential comparator. At the end of conversion, the
CDAC output approximates the sampled analog input. The
ADC control logic then prepares the 18-bit digital output
code for serial transfer.
APPLICATIONS INFORMATION
Figure 2. LTC2378-18 Transfer Function
INPUT VOLTAGE (V)
0V
OUTPUT CODE (TWO’S COMPLEMENT)
–1
LSB
237818 F02
011...111
011...110
000...001
000...000
100...000
100...001
111...110
1
LSB
BIPOLAR
ZERO
111...111
FSR/2 – 1LSB–FSR/2
FSR = +FS – –FS
1LSB = FSR/262144
TRANSFER FUNCTION
The LTC2378-18 digitizes the full-scale voltage of 2 × REF
into 2
18
levels, resulting in an LSB size of 38µV with
REF = 5V. The ideal transfer function is shown in Figure 2.
The output data is in 2’s complement format.
R
ON
40
C
IN
45pF
R
ON
40
REF
REF
C
IN
45pF
IN
+
IN
–
BIAS
VOLTAGE
237818 F03
Figure 3. The Equivalent Circuit for the
Differential Analog Input of the LTC2378-18
ANALOG INPUT
The analog inputs of the LTC2378-18 are fully differential
in order to maximize the signal swing that can be digitized.
The analog inputs can be modeled by the equivalent circuit
shown in Figure 3. The diodes at the input provide ESD
protection. In the acquisition phase, each input sees ap-
proximately 45pF (C
IN
) from the sampling CDAC in series
with 40Ω (R
ON
) from the on-resistance of the sampling
switch. Any unwanted signal that is common to both
inputs will be reduced by the common mode rejection of
the ADC. The inputs draw a current spike while charging
the C
IN
capacitors during acquisition. During conversion,
the analog inputs draw only a small leakage current.