LTC2274I(RevB) View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC2274I
(Rev.:RevB)

16-Bit, 105Msps Serial Output ADC

Linear Technology 

LTC2274

APPLICATIONS INFORMATION

Direct Coupled Circuits

Figure 5 demonstrates the use of a 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 or closed-loop amplifier will de-

grade the ADC SFDR at high input frequencies. Additionally,

wideband op amps or differential amplifiers tend to have

high noise. As a result, the SNR will be degraded unless

the noise bandwidth is limited prior to the ADC input.

Reference Operation

Figure 6 shows the LTC2274 reference circuitry consisting

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

fier and control circuit. The LTC2274 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 an 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 2.25VP-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

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER 25Ω

++

CM

––

25Ω

AMPLIFIER = LTC6600-20,

LTC1993, ETC.

VCM

2.2μF

AIN+

12pF

LTC2274

AIN–

12pF

2274 F05

Figure 5. DC Coupled Input with Differential Amplifier

reference; it will not be stable without this capacitor. The

minimum value required for stability is 2.2μF.

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.

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

or 1.25V external reference inputs. 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 1μF (or larger) ceramic capacitor.

PGA Pin

The PGA pin selects between two gain settings for

the ADC front-end. PGA = 0 selects an input range of

2.25VP-P; PGA = 1 selects an input range of 1.5VP-P. The

2.25V input range has the best SNR; however, the distor-

tion will be higher for input frequencies above 100MHz.

For applications with high input frequencies, the low

input range will have improved distortion; however, the

SNR will be 2.4dB worse. See the Typical Performance

Characteristics section of this datasheet.

LTC2274

TIE TO VDD TO USE

INTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 1.25V

REFERENCE

SENSE

RANGE

SELECT

AND GAIN

CONTROL

INTERNAL

ADC

REFERENCE

1x OR 2x

2.5V

BANDGAP

REFERENCE

VCM

2.2μF

BUFFER

1.25V

2274 F06

Figure 6. Reference Circuit

2274fb

20

Share Link: