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LTC1407/LTC1407A

1407f

APPLICATIO S I FOR ATIO

INPUT RANGE

The analog inputs of the LTC1407/LTC1407A may be

driven fully differentially with a single supply. Either input

may swing up to 3V, provided the differential swing is no

greater than 2.5V. In the valid input range, the noninvert-

ing input of each channel should always be more positive

than the inverting input of each channel. The 0V to 2.5V

range is also ideally suited for single-ended input use with

single supply applications. The common mode range of

the inputs extend from ground to the supply voltage VDD.

If the difference between the CH0+ and CH0– inputs or the

CH1+ and CH1– inputs exceeds 2.5V, the output code will

stay fixed at all ones, and if this difference goes below 0V,

the ouput code will stay fixed at all zeros.

INTERNAL REFERENCE

The LTC1407/LTC1407A have an on-chip, temperature

compensated, bandgap reference that is factory trimmed

near 2.5V to obtain a precise 2.5V input span. The refer-

ence amplifier output VREF, (Pin 3) must be bypassed with

a capacitor to ground. The reference amplifier is stable with

capacitors of 1

µF or greater. For the best noise perfor-

mance, a 10

µF ceramic or a 10µF tantalum in parallel with

a 0.1

µF ceramic is recommended. The VREF pin can be

overdriven with an external reference as shown in Figure 2.

The voltage of the external reference must be higher than

the 2.5V of the open-drain P-channel output of the internal

reference. The recommended range for an external refer-

ence is 2.55V to VDD. An external reference at 2.55V will

see a DC quiescent load of 0.75mA and as much as 3mA

during conversion.

INPUT SPAN VERSUS REFERENCE VOLTAGE

The differential input range has a unipolar voltage span

that equals the difference between the voltage at the

reference buffer output VREF (Pin 3) and the voltage at the

Exposed Pad ground. The differential input range of ADC

is 0V to 2.5V when using the internal reference. The

internal ADC is referenced to these two nodes. This

relationship also holds true with an external reference.

DIFFERENTIAL INPUTS

The ADC will always convert the unipolar difference of

CH0+ minus CH0– or the unipolar difference of CH1+

minus CH1–, independent of the common mode voltage at

either set of inputs. The common mode rejection holds up

at high frequencies (see Figure 3.) The only requirement is

that both inputs not go below ground or exceed VDD.

Figure 2

LTC1407/

LTC1407A

VREF

GND

1407 F02

µF

3V REF

FREQUENCY (Hz)

–80

–40

–100

–60

–20

100

1407 G08

–120

10k

100k

10M

100M

CH0

CH1

Figure 3. CMRR vs Frequency

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