LTC2365 datasheet(14/20 Pages) LINER

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LTC2314-14

231414f

For more information www.linear.com/2314-14

applicaTions inForMaTion

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 external

shuntcapacitanceatPin4and13pFofinputcapacitanceon

the LTC2314-14 in sample mode, will significantly reduce

theinternal130MHzinputbandwidthandmayincreasethe

requiredacquisitiontimebeyondtheminimumacquisition

time (tACQ-MIN) of 40ns.

Table 2: Reference Voltage vs Supply Range

SUPPLY VOLTAGE (VDD)

REF VOLTAGE (VREF)

2.7V –> 3.6V

2.048V

4.75V –> 5.25V

4.096V

47pF

231414 F10

AIN

LTC2314-14

LT1818

GND

ANALOG IN

–

Figure 10. RC Input Filter

ADC Reference

A low noise, low temperature drift reference is critical to

achieving the full data sheet performance of the ADC. The

LTC2314-14 provides an excellent internal reference with

aguaranteed20ppm/°Cmaximumtemperaturecoefficient.

Foraddedflexibility,anexternalreferencemayalsobeused.

The high speed, low noise internal reference buffer is used

only in the internal reference configuration. The reference

buffer must be overdriven in the external reference con-

figuration with a voltage 50mV higher than the nominal

reference output voltage in the internal configuration.

Using the Internal Reference

The internal bandgap and reference buffer are active by

default when the LTC2314-14 is not in sleep mode. The

reference voltage at the REF pin scales automatically with

the supply voltage at the VDD pin. The scaling of the refer-

ence voltage with supply is shown in table 2.

from0Vto2.048V.Ananaloginputvoltagethatgoesbelow

0V will be coded as all zeros and an analog input voltage

that exceeds 2.048V will be coded as all ones.

It is recommended that the REF pin be bypassed to ground

with a low ESR, 2.2µF ceramic chip capacitor for optimum

performance.

External Reference

An external reference can be used with the LTC2314-14

if better performance is required or to accommodate a

larger input voltage span. The only constraints are that

the external reference voltage must be 50mV higher than

the internal reference voltage (see Table 2) and must be

less than or equal to the supply voltage (or 4.3V for the 5V

supply range). For example, a 3.3V external reference may

be used with a 3.3V VDD supply voltage to provide a 3.3V

analog input voltage span (i.e. 3.3V > 2.048V + 50mV).

Or alternatively, a 2.5V reference may be used with a 3V

supply voltage to provide a 2.5V input voltage range (i.e.

2.5V > 2.048V + 50mV). The LTC6655-3.3, LTC6655-2.5,

available from Linear Technology, may be suitable for

many applications requiring a high performance external

reference for either 3.3V or 2.5V input spans respectively.

Transfer Function

Figure 11 depicts the transfer function of the LTC2314-14.

The code transitions occur midway between successive

integer LSB values (i.e. 0.5LSB, 1.5LSB, 2.5LSB… FS-

0.5LSB). The output code is straight binary with 1LSB =

VREF/16,384.

DC Performance

The noise of an ADC can be evaluated in two ways:

signal-to-noise ratio (SNR) in the frequency domain and

histogram in the time domain. The LTC2314-14 excels

in both. The noise in the time domain histogram is the

transition noise associated with a 14-bit resolution ADC

which can be measured with a fixed DC signal applied

to the input of the ADC. The resulting output codes are

collected over a large number of conversions. The shape

of the distribution of codes will give an indication of the

magnitude of the transition noise. In Figure 12, the distri-

bution of output codes is shown for a DC input that has

Thereferencevoltagealsodeterminesthefull-scaleanalog

input range of the LTC2314-14. For example, a 2.048V

reference voltage will accommodate an analog input range

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