LTC2351-14_15 Datasheet, PDF (14/20 Pages) Linear Technology – 6-Channel, 14-Bit, 1.5Msps Simultaneous Sampling ADC with Shutdown

LTC2351-14
APPLICATIONS INFORMATION
INPUT FILTERING AND SOURCE IMPEDANCE
The noise and the distortion of the input amplifier and
other circuitry must be considered since they will add to
the LTC2351-14 noise and distortion. The small-signal
bandwidth of the sample-and-hold circuit is 50MHz. Any
noise or distortion products that are present at the analog
inputs will be summed over this entire bandwidth. Noisy
input circuitry should be filtered prior to the analog inputs.
A simple 1-pole RC filter is sufficient for many applications.
For example, Figure 1 shows a 47pF capacitor from CH0⁺
to ground and a 51Ω source resistor to limit the net input
bandwidth to 30MHz. The 47pF capacitor also acts as a
charge reservoir for the input sample-and-hold and isolates
the ADC input from sampling-glitch sensitive circuitry. High
quality capacitors and resistors should be used since these
components can add distortion. NPO and silvermica 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. When high amplitude unwanted signals
are close in frequency to the desired signal frequency a
multiple pole filter is required.
High external source resistance, combined with 13pF
of input capacitance, will reduce the rated 50MHz input
bandwidth and increase acquisition time beyond 39ns.
ANALOG ^51Ω*
INPUT
ANALOG ^51Ω*
INPUT
1 _CH0+
47pF*
2 _CH0–
3
10μF
11
LTC2351-14
V_REF
GND
4 _CH1+
47pF*
5 _CH1–
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*TIGHT TOLERANCE REQUIRED TO AVOID
APERTURE SKEW DEGRADATION
Figure 1. RC Input Filter
INPUT RANGE
The analog inputs of the LTC2351-14 may be driven fully
differentially with a single supply. Either input may swing
up to V_CC, provided the differential swing is no greater
than 2.5V with BIP (Pin 29) LOW, or ±1.25V with (BIP Pin
29) HIGH. The 0V to 2.5V range is also ideally suited for
single-ended input use with single supply applications. The
common mode range of the inputs extend from ground
to the supply voltage V_CC. If the difference between the
CH⁺ and CH^– at any input pair exceeds 2.5V (unipolar) or
1.25V (bipolar), the output code will stay fixed at positive
full-scale, and if this difference goes below 0V (unipolar) or
–1.25V (bipolar), the output code will stay fixed at nega-
tive full-scale.
INTERNAL REFERENCE
The LTC2351-14 has an on-chip, temperature compen-
sated, bandgap reference that is factory trimmed to 2.5V
to obtain a precise 2.5V input span. The reference amplifier
output V_REF, (Pin 23) must be bypassed with a capacitor
to ground. The reference amplifier is stable with capaci-
tors of 1μF, or greater. For the best noise performance, a
10μF ceramic or a 10μF tantalum in parallel with a 0.1μF
ceramic is recommended. The V_REF pin can be overdriven
with an external reference as shown in Figure 2. The
voltage of the external reference must be higher than the
2.5V of the open-drain P-channel output of the internal
reference. The recommended range for an external refer-
ence is 2.55V to V_DD. An external reference at 2.55V will
see a DC quiescent load of 0.75mA and as much as 3mA
during conversion.
3.5V to 18V
LT1790-3
23
V_REF
10μF
LTC2351-14
22 _GND
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Figure 2. External Reference
14
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