LTC2453_1 datasheet(13/18 Pages) LINER

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LTC2453

2453fb

INPUT SIGNAL FREQUENCY (MHz)

–40

1.00

1.25

1.50

2453 F12

–60

–80

–20

–100

2.5

5.0

7.5

INPUT SIGNAL FREQUENCY (Hz)

–20

–10

480

2453 F13

–30

–40

–25

–15

–5

–35

–45

–50

120

240

180

360 420

540

300

600

Figure 12. LTC2453 Input Signal Attentuation vs Frequency

Figure 13. LTC2453 Input Signal Attenuation

vs Frequency (Low Frequencies)

These considerations need to be balanced out by the input

signal bandwidth. The 3dB bandwidth ≈ 1/(2

πRSCIN).

Finally, if the recommended choice for CIN is unacceptable

for the user’s speciﬁc application, an alternate strategy is to

eliminate CIN and minimize CPAR and RS. In practical terms,

this conﬁguration corresponds to a low impedance sensor

directly connected to the ADC through minimum length

traces. Actual applications include current measurements

through low value sense resistors, temperature meas-

urements, low impedance voltage source monitoring,

and so on. The resultant INL vs VIN is shown in Figure 11.

The measurements of Figure 11 include a capacitor CPAR

corresponding to a minimum sized layout pad and a

minimum width input trace of about 1 inch length.

Signal Bandwidth, Transition Noise and Noise

Equivalent Input Bandwidth

The LTC2453 includes a sinc1 type digital ﬁlter with the ﬁrst

notch located at f0 = 60Hz. As such, the 3dB input signal

bandwidth is 26.54Hz. The calculated LTC2453 input signal

attenuation vs frequency over a wide frequency range is

shown in Figure 12. The calculated LTC2453 input signal

attenuation vs frequency at low frequencies is shown in

Figure 13. The converter noise level is about 1.4μVRMS

and can be modeled by a white noise source connected

at the input of a noise-free converter.

On a related note, the LTC2453 uses two separate A/D

converters to digitize the positive and negative inputs.

Each of these A/D converters has 1.4μVRMS transition

noise. If one of the input voltages is within this small

transition noise band, then the output will ﬂuctuate one

bit, regardless of the value of the other input voltage. If

both of the input voltages are within their transition noise

bands, the output can ﬂuctuate 2 bits.

For a simple system noise analysis, the VIN drive circuit can

be modeled as a single-pole equivalent circuit characterized

by a pole location fi and a noise spectral density ni. If

the converter has an unlimited bandwidth, or at least a

bandwidth substantially larger than fi, then the total noise

contribution of the external drive circuit would be:

=π /•

Then, the total system noise level can be estimated as

the square root of the sum of (Vn2) and the square of the

LTC2453 noise ﬂoor (~1.4μV2).

APPLICATIONS INFORMATION

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