LTC2255 datasheet(15/24 Pages) LINER

15 / 24 page

LTC2201

2201f

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 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

2201 F05

–

CM LT1994

499Ω

100pF

2.2 μF

499Ω

523Ω

25Ω

499Ω

53.6Ω

AIN

–

LTC2201

Figure 6. Reference Circuit

PGA

1.25V

SENSE

VCM

BUFFER

INTERNAL

ADC

REFERENCE

RANGE

SELECT

AND GAIN

CONTROL

2.5V

BANDGAP

REFERENCE

2.2μF

TIE TO VDD TO USE

INTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 1.25V

REFERENCE

2201 F06

LTC2201

Figure 7. A 2.5V Range ADC

with an External 2.5V Reference

VCM

SENSE

1.25V

3.3V

2.2μF

1μF

2201 F07

LT1461-2.5

LTC2201

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.

APPLICATIONS INFORMATION

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 LTC2201 reference circuitry consisting

of a 2.5V bandgap reference, a programmable gain ampli-

ﬁer and control circuit. The LTC2201 has three modes of

reference operation: Internal Reference, 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.

LTC2255 Datasheet(PDF) 15 Page - Linear Technology

LTC2255 Datasheet(HTML) 15 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL