LTC2208IUP-14 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LTC2208IUP-14

14-Bit, 130Msps ADC

Linear Technology 

LTC2208-14

APPLICATIONS INFORMATION

Figure 4a shows transformer coupling using a transmis-

sion line balun transformer. This type of transformer has

much better high frequency response and balance than

flux coupled center tap transformers. Coupling capaci-

tors are added at the ground and input primary terminals

to allow the secondary terminals to be biased at 1.25V.

Figure 4b shows the same circuit with components suit-

able for higher input frequencies.

VCM

ANALOG

INPUT

0.1μF

T1

0.1μF 1:1

10Ω

25Ω 0.1μF

25Ω 10Ω

2.2μF

5Ω AIN+

LTC2208-14

4.7pF

4.7pF

5Ω AIN–

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2μF

4.7pF

220814 F04a

Figure 4a. Using a Transmission Line Balun Transformer.

Recommended for Input Frequencies from 100MHz to 250MHz

VCM

ANALOG

INPUT

0.1μF

T1

0.1μF 1:1

25Ω 0.1μF

25Ω

2.2μF

5Ω

2.2pF

AIN+ LTC2208-14

5Ω AIN–

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2μF

2.2pF

220814 F04b

Figure 4b. Using a Transmission Line Balun Transformer.

Recommended for Input Frequencies from 250MHz to 500MHz

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.

20

Reference Operation

Figure 6 shows the LTC2208-14 reference circuitry con-

sisting of a 2.5V bandgap reference, a programmable gain

amplifier and control circuit. The LTC2208-14 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 reference; it will not be

stable without this capacitor. The minimum value required

for stability is 2.2μF.

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER 25Ω

++

CM

––

25Ω

AMPLIFIER = LTC6600-20,

LT1993, ETC.

VCM

2.2μF

AIN+

12pF

LTC2208-14

AIN–

12pF

220814 F05

Figure 5. DC Coupled Input with Differential Amplifier

TIE TO VDD TO USE

INTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 1.25V

REFERENCE

SENSE

VCM

2.2μF

RANGE

SELECT

AND GAIN

CONTROL

INTERNAL

ADC

REFERENCE

PGA

BUFFER

1.25V

2.5V

BANDGAP

REFERENCE

Figure 6. Reference Circuit

220814 F06

220814fb

Share Link: