LTC1419CSW datasheet(12/20 Pages) LINER

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LTC1419

APPLICATIONS INFORMATION

Differential Inputs

The LTC1419 has a unique differential sample-and-hold

circuit that allows rail-to-rail inputs. The ADC will always

convert the difference of + AIN – (–AIN) independent of the

common mode voltage (see Figure 11a). The common

mode rejection holds up to extremely high frequencies,

see Figure 10a. The only requirement is that both inputs

can not exceed the AVDD or AVSS power supply voltages.

Integral nonlinearity errors (INL) and differential

nonlinearity errors (DNL) are independent of the common

mode voltage, however, the bipolar zero error (BZE) will

vary. The change in BZE is typically less than 0.1% of the

common mode voltage. Dynamic performance is also

affected by the common mode voltage. THD will degrade

as the inputs approach either power supply rail, from 86dB

with a common mode of 0V to 76dB with a common mode

of 2.5V or – 2.5V.

Figure 10b. Selectable 0V to 5V or

±2.5V Input Range

Differential inputs allow greater flexibility for accepting

different input ranges. Figure 10b shows a circuit that

converts a 0V to 5V analog input signal with only an

additional buffer that is not in the signal path.

Full-Scale and Offset Adjustment

Figure 11a shows the ideal input/output characteristics

for the LTC1419. The code transitions occur midway

between successive integer LSB values (i.e., – FS +

0.5LSB, – FS + 1.5LSB, – FS + 2.5LSB,... FS – 1.5LSB,

FS – 0.5LSB). The output is two’s complement binary with

1LSB = FS – (– FS)/16384 = 5V/16384 = 305.2

µV.

In applications where absolute accuracy is important,

offset and full-scale errors can be adjusted to zero. Offset

error must be adjusted before full-scale error. Figure 11b

shows the extra components required for full-scale error

adjustment. Zero offset is achieved by adjusting the offset

ANALOG

INPUT

1419 F11b

100

Ω

50k

24k

50k

47k

0.1

µF

+AIN

–AIN

VREF

REFCOMP

AGND

LTC1419

Figure 11b. Offset and Full-Scale Adjust Circuit

Figure 10a. CMRR vs Input Frequency

Figure 11a. LTC1419 Transfer Characteristics

1419 F11a

011...111

011...110

000...001

000...000

111...111

111...110

100...001

100...000

FS – 1LSB

– (FS – 1LSB)

INPUT VOLTAGE [+AIN – (–AIN)]

INPUT FREQUENCY (Hz)

100

1419 G09

1000

10000

LTC1419

+AIN

ANALOG INPUT

–AIN

VREF

0V TO

REFCOMP

AGND

1419 F10

±2.5V

µF

–

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