LTC2420 datasheet(26/36 Pages) LINER

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LTC2420

APPLICATIO S I FOR ATIO

As a result of the oversampling ratio (256) and the digital

filter, minimal (if any) antialias filtering is required in front

of the LTC2420. If passive RC components are placed in

front of the LTC2420 the input dynamic current should be

considered (see Input Current section). In cases where

large effective RC time constants are used, an external

buffer amplifier may be required to minimize the effects of

input dynamic current.

The modulator contained within the LTC2420 can handle

large-signal level perturbations without saturating. Signal

levels up to 40% of VREF do not saturate the analog modu-

lator. These signals are limited by the input ESD protection

to 300mV below ground and 300mV above VCC.

Operation at Higher Data Output Rates

The LTC2420 typically operates with an internal oscillator

of 153.6kHz. This corresponds to a notch frequency of

60Hz and an output rate of 7.5 samples/second. The

internal oscillator is enabled if the FO pin is logic LOW

(logic HIGH for a 50Hz notch). It is possible to drive the FO

pin with an external oscillator for higher data output rates.

As shown in Figure 28, an external clock of 2.048MHz

applied to the FO pin results in a notch frequency of 800Hz

with a data output rate of 100 samples/second.

Figure 29 shows the total unadjusted error (Offset Error +

Full-Scale Error + INL + DNL) as a function of the output

data rate with a 5V reference. The relationship between the

output data rate (ODR) and the frequency applied to the FO

pin (FO) is:

ODR = FO/20480

For output data rates up to 50 samples/second, the total

unadjusted error (TUE) is better than 16 bits, and better

than 12 bits at 100 samples/second. As shown in Figure

30, for output data rates of 100 samples/second, the TUE

is better than 15 bits for VREF below 2.5V. Figure 31 shows

an unaveraged total unadjusted error for the LTC2420 op-

erating at 100 samples/second with VREF = 2.5V. Figure 32

shows the same device operating with a 5V reference and

an output data rate of 7.5 samples/second.

At 100 samples/second, the LTC2420 can be used to

capture transient data. This is useful for monitoring set-

tling or auto gain ranging in a system. The LTC2420 can

monitor signals at an output rate of 100 samples/second.

INPUT FREQUENCY

–60

–40

2420 F27

–80

–100

fS/2

–120

–140

–20

Figure 27. Sinc4 Filter Rejection

SCK

SDO

VCC

VREF

VIN

GND

LTC2420

800Hz NOTCH (100 SAMPLES/SECOND)

60Hz NOTCH (7.5 SAMPLES/SECOND)

EXTERNAL 2.048MHz CLOCK SOURCE

INTERNAL 153.6kHz OSCILLATOR

2420 F28

Figure 28. Selectable 100 Samples/Second Turbo Mode

OUTPUT RATE (SAMPLES/SEC)

128

160

12 BITS

13 BITS

14 BITS

16 BITS

2420 F29

100

192

224

256

VREF = 5V

150

Figure 29. Total Error vs Output Rate (VREF = 5V)

LTC2420 Datasheet(PDF) 26 Page - Linear Technology

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