LTC2378-16
applicaTions inForMaTion
OVERVIEW
TRANSFER FUNCTION
TheLTC2378-16isalownoise,lowpower,highspeed16-bit
successive approximation register (SAR) ADC. Operating
from a single 2.5V supply, the LTC2378-16 supports a
The LTC2378-16 digitizes the full-scale voltage of 2 × REF
16
into 2 levels, resulting in an LSꢀ size of 152µV with
REF = 5V. The ideal transfer function is shown in Figure 2.
The output data is in 2’s complement format.
large and flexible ±V fully differential input range with
REF
V
ranging from 2.5V to 5.1V, making it ideal for high
REF
performance applications which require a wide dynamic
range. The LTC2378-16 achieves ±±.5LSꢀ ꢁIL max, no
missing codes at 16 bits and 97dꢀ SIR.
011...111
BIPOLAR
011...110
ZERO
000...001
000...000
111...111
111...110
Fast 1Msps throughput with no cycle latency makes the
LTC2378-16 ideally suited for a wide variety of high speed
applications.Aninternaloscillatorsetstheconversiontime,
easingexternaltimingconsiderations.TheLTC2378-16dis-
sipatesonly13.5mWat1Msps,whileanautopower-down
feature is provided to further reduce power dissipation
during inactive periods.
100...001
FSR = +FS – –FS
1LSB = FSR/65536
100...000
–1 0V
LSB
INPUT VOLTAGE (V)
1
LSB
–FSR/2
FSR/2 – 1LSB
237816 F02
The LTC2378-16 features a unique digital gain compres-
sion(DGC)function,whicheliminatesthedriveramplifier’s
negative supply while preserving the full resolution of the
ADC. When enabled, the ADC performs a digital scaling
Figure 20 5TC2378-16 Transfer Function
ANA5OG INPUT
function that maps zero-scale code from 0V to 0.1 • V
The analog inputs of the LTC2378-16 are fully differential
in order to maximize the signal swing that can be digitized.
Theanaloginputscanbemodeledbytheequivalentcircuit
shown in Figure 3. The diodes at the input provide ESD
protection. ꢁn the acquisition phase, each input sees ap-
REF
and full-scale code from V
to 0.9 • V . For a typical
REF
REF
reference voltage of 5V, the full-scale input range is now
±.5V to 4.5V, which provides adequate headroom for
powering the driving amplifier from a single 5.5V supply.
proximately 45pF (C ) from the sampling CDAC in series
ꢁI
with 4±Ω (R ) from the on-resistance of the sampling
OI
CONVERTER OPERATION
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 LTC2378-16 operates in two phases. During the ac-
quisition phase, the charge redistribution capacitor D/A
+
–
the C capacitors during acquisition. During conversion,
the analog inputs draw only a small leakage current.
converter (CDAC) is connected to the ꢁI and ꢁI pins
to sample the differential analog input voltage. A rising
edge on the CIV pin initiates a conversion. During the
conversionphase,the16-bitCDACissequencedthrougha
successiveapproximationalgorithm,effectivelycomparing
the sampled input with binary-weighted fractions of the
ꢁI
REF
C
45pF
IN
R
40Ω
ON
+
IN
IN
referencevoltage(e.g.V /2,V /4…V /65536)using
REF
REF
REF
BIAS
the differential comparator. At the end of conversion, the
CDAC output approximates the sampled analog input. The
ADC control logic then prepares the 16-bit digital output
code for serial transfer.
VOLTAGE
REF
C
45pF
IN
R
40Ω
ON
–
237816 F03
Figure 30 The Equivalent Circuit for the
Differential Analog Input of the 5TC2378-16
237816f
10
LINEAR_DIMENSIONS [ Linear Dimensions ]
