LTC2205-14(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC2205-14
(Rev.:RevA)

14-Bit, 65Msps ADC

Linear Technology 

LTC2205-14

APPLICATIONS INFORMATION

Center-tapped transformers provide a convenient means

of DC biasing the secondary; however, they often show

poor balance at high input frequencies, resulting in large

2nd order harmonics.

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 capacitors

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 suitable for

higher input frequencies.

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.

VCM

5Ω

10Ω

T1

0.1µF

10Ω

2.2µF

5Ω AIN+

8.2pF

LTC2205-14

35Ω

8.2pF

35Ω

5Ω AIN–

T1 = MA/COM ETC1-1T

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2µF

8.2pF

220514 F03

Figure 3. Single-Ended to Differential Conversion Using a Transformer.

Recommended for Input Frequencies from 5MHz to 150MHz

0.1mF

ANALOG

INPUT

T1

1:1

0.1mF

10Ω

25Ω 0.1µF

25Ω 10Ω

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2mF

VCM

2.2µF

5Ω AIN+

LTC2205-14

4.7pF

4.7pF

5Ω AIN–

4.7pF

220514 F04a

Figure 4a. Using a Transmission Line Balun Transformer.

Recommended for Input Frequencies from 150MHz to 250MHz

VCM

ANALOG

INPUT

0.1µF

T1

0.1µF 1:1

25Ω 0.1µF

25Ω

2.2µF

5Ω

2.2pF

AIN+

LTC2205-14

5Ω AIN–

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2µF

2.2pF

22054 F04b

Figure 4b. Using a Transmission Line Balun Transformer.

Recommended for Input Frequencies from 250MHz to 500MHz

220514fa

17

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