Datasheet

LTC2378-20
10
237820f
For more information www.linear.com/LTC2378-20
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.
INPUT DRIVE CIRCUITS
A low impedance source can directly drive the high imped-
ance inputs of the LTC2378-20 without gain error. A high
impedance source should be buffered to minimize settling
time during acquisition and to optimize ADC linearity. For
best performance, a buffer amplifier should be used to
drive the analog inputs of the LTC2378-20. The amplifier
provides low output impedance, which produces fast set-
applicaTions inForMaTion
Figure 2. LTC2378-20 Transfer Function
INPUT VOLTAGE (V)
0V
OUTPUT CODE (TWO’S COMPLEMENT)
–1
LSB
237820 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/1048576 ≈ 1ppm
tling of the analog signal during the acquisition phase. It
also provides isolation between the signal source and the
ADC input currents.
Noise and Distortion
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.
R
ON
40Ω
C
IN
45pF
R
ON
40Ω
REF
REF
C
IN
45pF
IN
+
IN
–
BIAS
VOLTAGE
237820 F03
Figure 3. The Equivalent Circuit for the
Differential Analog Input of the LTC2378-20
A coupling filter network (LPF2) should be used between
the buffer and ADC input to minimize disturbances 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 typically requires a
wider bandwidth than LPF1. This filter also helps minimize
the noise contribution from the buffer. 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.
Input Currents
One of the biggest challenges in coupling an amplifier to
the LTC2378-20 is in dealing with current spikes drawn
by the ADC inputs at the start of each acquisition phase.
10Ω
3300pF
6600pF
10Ω
500Ω
LPF2
LPF1
BW = 1.2MHz
BW = 48kHz
SINGLE-ENDED-
TO-DIFFERENTIAL
DRIVER
SINGLE-ENDED-
INPUT SIGNAL
LTC2378-20
IN
+
IN
–
237820 F04
6800pF
6800pF
Figure 4. Input Signal Chain