LTC2389-16_12 Datasheet, PDF (19/40 Pages) –

LTC2389-16
Applications Information
ANALOG INPUT
The analog inputs of the LTC2389-16 can be pin configured
to accept one of three input voltage ranges: fully differential
(±4.096V), pseudo-differential unipolar (0V to 4.096V), and
pseudo-differential bipolar (±2.048V). In all three ranges,
the ADC samples and digitizes the voltage difference
between the two analog input pins (IN⁺ – IN^–), and any
unwanted signal that is common to both inputs is reduced
by the common mode rejection ratio (CMRR) of the ADC.
Independent of the selected range, the analog inputs can
be modeled by the equivalent circuit shown in Figure 4.
The diodes at the input provide ESD protection. In the
acquisition phase, each input sees approximately 40pF
(C_IN) from the sampling CDAC in series with 40Ω (R_IN)
from the on-resistance of the sampling switch. The inputs
draw a small current spike while charging the C_IN capacitors
during acquisition. During conversion, the analog inputs
draw only a small leakage current.
V_DD
IN⁺
R_IN
C_IN
40pF
40Ω
Pseudo-Differential Unipolar Input Range
In the pseudo-differential unipolar input range, the ADC
digitizes the differential analog input voltage (IN⁺ – IN^–)
over a span of (0V to V_REF). In this range, a single-ended
unipolar input signal, driven on the IN⁺ pin, is measured
with respect to the signal ground reference level, driven
on the IN^– pin. The IN⁺ pin is allowed to swing from (GND
– 0.1V) to (V_REF + 0.1V), while the IN^– pin is restricted to
(GND ± 0.1V). Unwanted signals common to both inputs
are reduced by the CMRR of the ADC.
Pseudo-Differential Bipolar Input Range
In the pseudo-differential bipolar input range, the ADC
digitizes the differential analog input voltage (IN⁺ – IN^–)
over a span of (±V_REF/2). In this range, a single-ended
bipolar input signal, driven on the IN⁺ pin, is measured
with respect to the signal mid-scale reference level, driven
on the IN^– pin. The IN⁺ pin is allowed to swing from (GND
– 0.1V) to (V_REF + 0.1V), while the IN^– pin is restricted
to (V_REF/2 ± 0.1V). Unwanted signals common to both
inputs are reduced by the CMRR of the ADC.
V_DD
IN^–
C_IN
R_{IN 40pF}
40Ω
BIAS
VOLTAGE
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Figure 4. Equivalent Circuit for the Differential
Analog Input of the LTC2389-16
Fully Differential Input Range
The fully differential input range provides the widest
input signal swing, configuring the ADC to digitize the
differential analog input voltage (IN⁺ – IN^–) over a span
of (±V_REF). In this range, the IN⁺ and IN^– pins should
be driven 180 degrees out-of-phase with respect to
each other, centered around a common mode voltage
(IN⁺ + IN^–)/2 that is restricted to (V_REF/2 ± 0.1V). Both the
IN⁺ and IN^– pins are allowed to swing from (GND – 0.1V)
to (V_REF + 0.1V). Unwanted signals common to both inputs
are reduced by the CMRR of the ADC.
INPUT DRIVE CIRCUITS
A low impedance source can directly drive the high im-
pedance inputs of the LTC2389-16 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 signals 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 LTC2389-16. The ampli-
fier provides low output impedance enabling fast settling
of the analog signal during the acquisition phase. It also
provides isolation between the signal source and the current
spike drawn by the ADC inputs when entering acquisition.
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