LTC1865 datasheet(12/18 Pages) LINER

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LTC2356-12/LTC2356-14

2356fb

GND

LTC2356-12/

LTC2356-14

LT1790-3

VREF

10µF

3.5V TO 18V

2356 F03

AIN+

10µF

2356 F02

1.6k

1µF

C1, C2: FILM TYPE

C3: COG TYPE

C4: CERAMIC BYPASS

1.6k

LTC2356-12/

LTC2356-14

AIN–

VREF

56pF

VIN

10µF

VCM

1.5V DC

AIN–

LTC2356-12/

LTC2356-14

AIN+

47pF

GND

VREF

2356 F01

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 LTC2356-12/LTC2356-14 noise and distortion. The

small-signal bandwidth of the sample-and-hold circuit is

50MHz. Any noise or distortion products that are pres-

ent at the analog inputs will be summed over this entire

bandwidth. Noisy input circuitry should be filtered prior

to the analog inputs to minimize noise. A simple 1-pole

RC filter is sufficient for many applications. For example,

Figure 1 shows a 47pF capacitor from AIN+togroundanda

51Ωsourceresistortolimittheinputbandwidthto47MHz.

The 47pF capacitor also acts as a charge reservoir for the

input sample-and-hold and isolates the ADC input from

sampling-glitchsensitivecircuitry.Highqualitycapacitors

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.Whenhighamplitudeunwantedsignalsareclose

in frequency to the desired signal frequency, a multiple

pole filter is required. High external source resistance,

combined with the 13pF of input capacitance, will reduce

the rated 50MHz bandwidth and increase acquisition time

beyond 39ns.

Figure 1. RC Input Filter

inverting input. The ±1.25V range is also ideally suited for

AC-coupled signals in single supply applications. Figure 2

shows how to AC couple signals in a single supply system

withoutneedingamid-supply1.5Vexternalreference.The

DC common mode level is supplied by the previous stage

that is already bounded by the single supply voltage of the

system. The common mode range of the inputs extend

from ground to the supply voltage VDD. If the difference

between the AIN+ and AIN– inputs exceeds 1.25V, the

output code will stay fixed at zero and all ones and if this

difference goes below –1.25V, the output code will stay

fixed at one and all zeros.

Figure 2. AC Coupling of AC Signals with 1kHz

Low Cutoff Frequency

INPUT RANGE

The analog inputs of the LTC2356-12/LTC2356-14 may be

driven fully differentially with a single supply. Each input

may swing up to 2.5VP-P individually. When using the

internal reference, the non-inverting input should never be

more than 1.25V more positive or more negative than the

INTERNAL REFERENCE

TheLTC2356-12/LTC2356-14hasanon-chip,temperature

compensated, bandgap reference that is factory trimmed

to 2.5V to obtain a bipolar ±1.25V input span. The refer-

ence amplifier output VREF, (Pin 3) must be bypassed

with a capacitor to ground. The reference amplifier is

stable with capacitors of 1µF or greater. For the best noise

performance, a 10µF ceramic or a 10µF tantalum in paral-

lel with a 0.1µF ceramic is recommended. The VREF pin

can be overdriven with an external reference as shown in

Figure 3. Overdriving VREF Pin with an External Reference

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