LTC2485CDD datasheet(21/40 Pages) LINER

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LTC2485

2485fa

Reference Voltage Range

The LTC2485 external reference voltage range is 0.1V to

VCC. The converter output noise is determined by the

thermal noise of the front-end circuits, and as such, its

value in nanovolts is nearly constant with reference volt-

age. A reduced reference voltage will improve the con-

verter performance when operated with an external con-

version clock (external FO signal) at substantially higher

output data rates (see the Output Data Rate section). VREF

must be

≥1.1V to use the internal temperature sensor.

The reference input is differential. The differential refer-

ence input range (VREF = REF+ – REF–) is 100mV to VCC and

the common mode reference input range is 0V to VCC.

Input Voltage Range

The analog input is truly differential with an absolute/

common mode range for the IN+ and IN– input pins

extending from GND – 0.3V to VCC + 0.3V. Outside

these limits, the ESD protection devices begin to turn on

and the errors due to input leakage current increase rapidly.

Within these limits, the LTC2485 converts the bipolar

differential input signal, VIN = IN+ – IN–, from – FS to +FS

where FS = 0.5 • VREF. Beyond this range, the converter

indicates the overrange or the underrange condition using

distinct output codes. Since the differential input current

cancellation does not rely on an on-chip buffer, current

cancellation and DC performance is maintained rail-to-rail.

Input signals applied to IN+ and IN– pins may extend by

300mV below ground and above VCC. In order to limit any

fault current, resistors of up to 5k may be added in series

with the IN+ and IN– pins without affecting the perfor-

mance of the devices. The effect of the series resistance on

the converter accuracy can be evaluated from the curves

presented in the Input Current/Reference Current sec-

tions. In addition, series resistors will introduce a tem-

perature dependent offset error due to the input leakage

current. A 1nA input leakage current will develop a 1ppm

offset error on a 5k resistor if VREF = 5V. This error has a

very strong temperature dependency.

Driving the Input and Reference

The input and reference pins of the LTC2485 converter are

directly connected to a network of sampling capacitors.

Depending upon the relation between the differential input

voltage and the differential reference voltage, these capaci-

tors are switching between these four pins transferring

small amounts of charge in the process. A simplified equiva-

lent circuit is shown in Figure 12.

Figure 12. LTC2485 Equivalent Analog Input Circuit

VREF

VIN

VCC

RSW (TYP)

10k

ILEAK

VCC

ILEAK

VCC

RSW (TYP)

10k

CEQ

12pF

(TYP)

RSW (TYP)

10k

ILEAK

IIN

VIN

–

IIN

–

IREF

–

2485 F12

ILEAK

VCC

ILEAK

SWITCHING FREQUENCY

fSW = 123kHz INTERNAL OSCILLATOR

fSW = 0.4 • fEOSC EXTERNAL OSCILLATOR

VREF

–

RSW (TYP)

10k

IIN

I REF

where

AVG

IN CM

REF CM

AVG

REF

INCM

REFCM

REF

REF CM

IN CM

REF

REF+

REF–

() = () =

−

() =

−

−≅

()

–

()

.–

–

VIN

INTERNAL OSCILLATOR

Hz MODE

REFCM

INCM

−

Ω

()

−

, REF =

REF+

REF–

271

Ω

2.98M

INTERNAL OSCILLATOR 50Hz AND 60Hz MODE

0.833 10

/ f

EXTERNAL OSCILLATOR

D IS THE DENSITY OF A DIGITAL TRANSITION AT THE MODULATOR OUTPUT

EOSC

WHERE REF– IS INTERNALLY TIED TO GND

APPLICATIO S I FOR ATIO

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