LTC2365 datasheet(14/20 Pages) LINER

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LTC2315-12

231512f

For more information www.linear.com/2315-12

applicaTions inForMaTion

problems.Whenhighamplitudeunwantedsignalsareclose

in frequency to the desired signal frequency, a multiple

pole filter is required. High external source resistance,

combined with external shunt capacitance at Pin 4 and

13pF of input capacitance on the LTC2315-12 in sample

mode, will significantly reduce the internal 130MHz input

bandwidth and may increase the required acquisition time

beyond the minimum acquisition time (tACQ-MIN) of 40ns.

ADC Reference

A low noise, low temperature drift reference is critical to

achieving the full data sheet performance of the ADC. The

LTC2315-12 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 LTC2315-12 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 the following table.

External Reference

An external reference can be used with the LTC2315-12

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 LTC2315-12.

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/4,096.

Figure 11. LTC2315-12 Transfer Function

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

INPUT VOLTAGE (V)

231512 F11

111...111

111...110

000...000

000...001

FS – 1LSB

0 1LSB

Thereferencevoltagealsodeterminesthefull-scaleanalog

input range of the LTC2315-12. For example, a 2.048V

reference voltage will accommodate an analog input range

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.

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

thetimedomain.TheLTC2315-12excelsinboth.Thenoisein

the time domain histogram is the transition noise associated

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