LTC2412IGN datasheet(28/36 Pages) LINER

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LTC2412

2412f

APPLICATIO S I FOR ATIO

(50ppm/

°C) are used for the external source impedance

seen by REF+ and REF–, the expected drift of the dynamic

current gain error will be insignificant (about 1% of its

value over the entire temperature and voltage range). Even

for the most stringent applications a one-time calibration

operation may be sufficient.

In addition to the reference sampling charge, the reference

pins ESD protection diodes have a temperature dependent

leakage current. This leakage current, nominally 1nA

(

±10nA max), results in a small gain error. A 100Ω source

resistance will create a 0.05

µV typical and 0.5µV maxi-

mum full-scale error.

Output Data Rate

When using its internal oscillator, the LTC2412 can pro-

duce up to 7.5 readings per second with a notch frequency

of 60Hz (FO = LOW) and 6.25 readings per second with a

notch frequency of 50Hz (FO = HIGH). The actual output

data rate will depend upon the length of the sleep and data

output phases which are controlled by the user and which

can be made insignificantly short. When operated with an

external conversion clock (FO connected to an external

oscillator), the LTC2412 output data rate can be increased

as desired. The duration of the conversion phase is 20510/

fEOSC. If fEOSC = 153600Hz, the converter behaves as if the

internal oscillator is used and the notch is set at 60Hz.

There is no significant difference in the LTC2412 perfor-

mance between these two operation modes.

An increase in fEOSC over the nominal 153600Hz will

translate into a proportional increase in the maximum

output data rate. This substantial advantage is neverthe-

less accompanied by three potential effects, which must

be carefully considered.

First, a change in fEOSC will result in a proportional change

in the internal notch position and in a reduction of the

converter differential mode rejection at the power line

frequency. In many applications, the subsequent perfor-

mance degradation can be substantially reduced by rely-

ing upon the LTC2412’s exceptional common mode

rejection and by carefully eliminating common mode to

differential mode conversion sources in the input circuit.

The user should avoid single-ended input filters and

should maintain a very high degree of matching and

symmetry in the circuits driving the IN+ and IN– pins.

Second, the increase in clock frequency will increase

proportionally the amount of sampling charge transferred

through the input and the reference pins. If large external

input and/or reference capacitors (CIN, CREF) are used, the

previous section provides formulae for evaluating the

effect of the source resistance upon the converter perfor-

mance for any value of fEOSC. If small external input and/

or reference capacitors (CIN, CREF) are used, the effect of

the external source resistance upon the LTC2412 typical

performance can be inferred from Figures 13, 14, 18 and

19 in which the horizontal axis is scaled by 153600/fEOSC.

Third, an increase in the frequency of the external oscilla-

tor above 460800Hz (a more than 3

× increaseintheoutput

data rate) will start to decrease the effectiveness of the

internal autocalibration circuits. This will result in a pro-

gressive degradation in the converter accuracy and linear-

ity. Typical measured performance curves for output data

rates up to 100 readings per second are shown in Fig-

ures 23, 24, 25, 26, 27, 28, 29 and 30. In order to obtain

the highest possible level of accuracy from this converter

at output data rates above 20 readings per second, the

user is advised to maximize the power supply voltage used

and to limit the maximum ambient operating temperature.

In certain circumstances, a reduction of the differential

reference voltage may be beneficial.

Figure 22. INL vs Differential Input Voltage (VIN = IN

+ – IN–)

and Reference Source Resistance (RSOURCE at REF+ and REF–

for Large CREF Values (CREF ≥ 1µF)

VINDIF/VREFDIF

–0.5 –0.4–0.3–0.2–0.1 0

0.1 0.2 0.3 0.4 0.5

–3

–6

–9

–12

–15

VCC = 5V

REF+ = 5V

REF– = GND

VINCM = 0.5 • (IN

+ + IN–) = 2.5V

FO = GND

CREF = 10µF

TA = 25°C

RSOURCE = 1000Ω

RSOURCE = 500Ω

RSOURCE = 100Ω

2412 F22

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