LTC2480CDD datasheet(35/40 Pages) LINER

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LTC2480

2480f

APPLICATIO S I FOR ATIO

Figure 34. Input Normal Mode Rejection vs Input Frequency

with Input Perturbation of 100% Full Scale (60Hz Notch)

Figure 35. Input Normal Mode Rejection vs Input Frequency

with Input Perturbation of 100% Full Scale (50Hz Notch)

Figure 36. Input Normal Mode Rejection vs Input Frequency

with Input Perturbation of 100% Full Scale (50Hz/60Hz Mode)

INPUT FREQUENCY (Hz)

105 120 135 150 165 180 195 210 225 240

2480 F34

–20

–40

–60

–80

–100

–120

VCC = 5V

VREF = 5V

VIN(CM) = 2.5V

VIN(P-P) = 5V

TA = 25°C

MEASURED DATA

CALCULATED DATA

INPUT FREQUENCY (Hz)

12.5

37.5

62.5

87.5 100 112.5 125 137.5 150 162.5 175 187.5 200

2480 F35

–20

–40

–60

–80

–100

–120

VCC = 5V

VREF = 5V

VIN(CM) = 2.5V

VIN(P-P) = 5V

TA = 25°C

MEASURED DATA

CALCULATED DATA

INPUT FREQUENCY (Hz)

100

120

140

160

180

200

220

2483 F36

–20

–40

–60

–80

–100

–120

VCC = 5V

VREF = 5V

VIN(CM) = 2.5V

VIN(P-P) = 5V

TA = 25°C

MEASURED DATA

CALCULATED DATA

As a result of these remarkable normal mode specifica-

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

of the LTC2480. If passive RC components are placed in

front of the LTC2480, the input dynamic current should be

considered (see Input Current section). In this case, the

differential input current cancellation feature of the LTC2480

allows external RC networks without significant degrada-

tion in DC performance.

Traditional high order delta-sigma modulators, while pro-

viding very good linearity and resolution, suffer from po-

tential instabilities at large input signal levels. The

proprietary architecture used for the LTC2480 third order

modulator resolves this problem and guarantees a pre-

dictable stable behavior at input signal levels of up to 150%

of full scale. In many industrial applications, it is not un-

common to have to measure microvolt level signals super-

imposed over volt level perturbations and the LTC2480 is

eminently suited for such tasks. When the perturbation is

differential, the specification of interest is the normal mode

rejection for large input signal levels. With a reference

voltage VREF = 5V, the LTC2480 has a full-scale differen-

tial input range of 5V peak-to-peak. Figures 37 and 38

show measurement results for the LTC2480 normal mode

rejection ratio with a 7.5V peak-to-peak (150% of full scale)

input signal superimposed over the more traditional nor-

mal mode rejection ratio results obtained with a 5V peak-

to-peak (full scale) input signal. In Figure 37, the LTC2480

uses the internal oscillator with the notch set at 60Hz (FO

= LOW) and in Figure 38 it uses the internal oscillator with

the notch set at 50Hz. It is clear that the LTC2480 rejection

performance is maintained with no compromises in this

extreme situation. When operating with large input signal

levels, the user must observe that such signals do not

violate the device absolute maximum ratings.

Using the 2x speed mode of the LTC2480, the device

bypasses the digital offset calibration operation to double

the output data rate. The superior normal mode rejection

is maintained as shown in Figures 30 and 31. However, the

magnified details near DC and fS = 256fN are different, see

Figures 39 and 40. In 2x speed mode, the bandwidth is

11.4Hz for the 50Hz rejection mode, 13.6Hz for the 60Hz

rejection mode and 12.4Hz for the 50Hz/60Hz rejection

superimposed over the theoretical calculated curve. Simi-

larly, the measured normal mode rejection of the LTC2480

for the 50Hz rejection mode and 50Hz/60Hz rejection mode

are shown in Figures 35 and 36.

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