LTC2203 datasheet(21/32 Pages) LINER

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LTC2203/LTC2202

22032fb

Figure 5 demonstrates the use of an LTC1994 differential

ampliﬁer to convert a single ended input signal into a

differential input signal. The advantage of this method is

that it provides low frequency input response; however,

the limited gain bandwidth of any op amp will limit the

SFDR at high input frequencies.

The 25Ω resistors and 12pF capacitor on the analog

inputs serve two purposes: isolating the drive circuitry

from the sample-and-hold charging glitches and limiting

the wideband noise at the converter input.

Reference Operation

Figure 6 shows the LTC2203/LTC2202 reference circuitry

consisting of a 2.5V bandgap reference, a programmable

gain ampliﬁer and control circuit. The LTC2203/LTC2202

has three modes of reference operation: Internal Refer-

ence, 1.25V external reference or 2.5V external reference.

To use the internal reference, tie the SENSE pin to VDD. To

use the external reference, simply apply either a 1.25V or

2.5V reference voltage to the SENSE input pin. Both 1.25V

and 2.5V applied to SENSE will result in a full scale range

of 2.5VP-P (PGA = 0). A 1.25V output, VCM, is provided

for a common mode bias for input drive circuitry. An

external bypass capacitor is required for the VCM output.

This provides a high frequency low impedance path to

ground for internal and external circuitry. This is also the

compensation capacitor for the reference; it will not be

stable without this capacitor. The minimum value required

for stability is 2.2µF.

APPLICATIO S I FOR ATIO

The internal programmable gain ampliﬁer provides the

internal reference voltage for the ADC. This ampliﬁer has

very stringent settling requirements and is not accessible

for external use.

Figure 5. DC Coupled Input with Differential Ampliﬁer

VCM

LTC2203/02

22032 F05

–

CM LT1994

499

100pF

2.2

μF

499

523

499

53.6

AIN

–

Figure 6. Reference Circuit

PGA

1.25V

SENSE

VCM

BUFFER

INTERNAL

ADC

REFERENCE

RANGE

SELECT

AND GAIN

CONTROL

2.5V

BANDGAP

REFERENCE

2.2

μF

TIE TO VDD TO USE

INTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 1.25V

REFERENCE

22032 F07

LTC2203/02

Figure 7. A 2.25V Range ADC with

an External 2.5V Reference

VCM

SENSE

1.25V

3.3V

2.2

μF

2.2

μF

22032 F08

LTC2203/02

LT1461-2.5

The SENSE pin can be driven ±5% around the nominal 2.5V

or 1.25V external reference input. This adjustment range

can be used to trim the ADC gain error or other system

gain errors. When selecting the internal reference, the

SENSE pin should be tied to VDD as close to the converter

as possible. If the sense pin is driven externally it should

be bypassed to ground as close to the device as possible

with at least a 1µF ceramic capacitor.

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