LT6230 datasheet(14/22 Pages) LINER

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LTC2312-12

231212fa

For more information www.linear.com/LTC2312-12

applicaTions inForMaTion

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 LTC2312-12 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 Table 2.

Table 2. Reference Voltage vs Supply Range

SUPPLY VOLTAGE (VDD)

REF VOLTAGE (VREF)

2.7V < VDD < 3.6V

2.048V

4.75V < VDD < 5.25V

4.096V

Thereferencevoltagealsodeterminesthefull-scaleanalog

input range of the LTC2312-12. 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.

External Reference

An external reference can be used with the LTC2312-12

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

Figure 11. LTC2312-12 Transfer Function

Figure 12. Histogram for 16384 Conversions

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 LTC2312-12.

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/4096.

DC Performance

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

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

gram in the time domain. The LTC2312-12 excels in both.

The noise in the time domain histogram is the transition

noise associated with a 12-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

largenumberofconversions.Theshapeofthedistribution

of codes will give an indication of the magnitude of the

transition noise. In Figure 12, the distribution of output

codes is shown for a DC input that has been digitized

INPUT VOLTAGE (V)

231212 F11

111...111

111...110

000...000

000...001

FS – 1LSB

0 1LSB

CODE

2047

2048

231212 F12

2046

2049

σ = 0.33

4000

8000

16000

12000

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