LTC2412IGN datasheet(30/36 Pages) LINER

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LTC2412

2412f

APPLICATIO S I FOR ATIO

Input Bandwidth

The combined effect of the internal Sinc4 digital filter and

of the analog and digital autocalibration circuits deter-

mines the LTC2412 input bandwidth. When the internal

oscillator is used with the notch set at 60Hz (FO = LOW),

the 3dB input bandwidth is 3.63Hz. When the internal

oscillator is used with the notch set at 50Hz (FO = HIGH),

the 3dB input bandwidth is 3.02Hz. If an external conver-

sion clock generator of frequency fEOSC is connected to the

FO pin, the 3dB input bandwidth is 0.236 • 10–6 • fEOSC.

Due to the complex filtering and calibration algorithms

utilized, the converter input bandwidth is not modeled very

accurately by a first order filter with the pole located at the

3dB frequency. When the internal oscillator is used, the

shape of the LTC2412 input bandwidth is shown in Fig-

ure 31 for FO = LOW and FO = HIGH. When an external

oscillator of frequency fEOSC is used, the shape of the

LTC2412 input bandwidth can be derived from Figure 31,

FO = LOW curve in which the horizontal axis is scaled by

fEOSC/153600.

The conversion noise (800nVRMS typical for VREF = 5V)

can be modeled by a white noise source connected to a

noise free converter. The noise spectral density is

62.75nV/

√Hz for an infinite bandwidth source and

86.1nV/

√Hz for a single 0.5MHz pole source. From these

numbers, it is clear that particular attention must be given

to the design of external amplification circuits. Such

circuits face the simultaneous requirements of very low

bandwidth (just a few Hz) in order to reduce the output

referred noise and relatively high bandwidth (at least

500kHz) necessary to drive the input switched-capacitor

network. A possible solution is a high gain, low bandwidth

amplifier stage followed by a high bandwidth unity-gain

buffer.

When external amplifiers are driving the LTC2412, the

ADC input referred system noise calculation can be simpli-

fied by Figure 32. The noise of an amplifier driving the

LTC2412 input pin can be modeled as a band limited white

noise source. Its bandwidth can be approximated by the

bandwidth of a single pole lowpass filter with a corner

frequency fi. The amplifier noise spectral density is ni.

From Figure 32, using fi as the x-axis selector, we can find

on the y-axis the noise equivalent bandwidth freqi of the

Figure 29. Resolution (NoiseRMS ≤ 1LSB)

vs Output Data Rate and Reference Voltage

Figure 30. Resolution (INLMAX ≤ 1LSB)

vs Output Data Rate and Reference Voltage

Figure 31. Input Signal Bandwidth

Using the Internal Oscillator

OUTPUT DATA RATE (READINGS/SEC)

102030405060708090 100

2412 F29

VREF = 5V

VCC = 5V

REF – = GND

VINCM = 2.5V

VIN = 0V

FO = EXTERNAL OSCILLATOR

TA = 25°C

RESOLUTION = LOG2(VREF/NOISERMS)

VREF = 2.5V

OUTPUT DATA RATE (READINGS/SEC)

102030405060708090 100

2412 F30

TA = 25°C

VCC = 5V

REF – = GND

VINCM = 0.5 • REF

–0.5V • VREF < VIN < 0.5 • VREF

FO = EXTERNAL OSCILLATOR

VREF = 2.5V

VREF = 5V

RESOLUTION =

LOG2(VREF/INLMAX)

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0.5

1.5

2.5

3.5

4.5

2412 F31

0.0

–0.5

–1.0

–1.5

–2.0

–2.5

–3.0

–3.5

–4.0

–4.5

–5.0

–5.5

–6.0

FO = HIGH

FO = LOW

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