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LTC2208CUP-PBF

16-Bit, 130Msps ADC

Linear Technology 

LTC2208

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 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.

VCM

0.1μF

ANALOG

INPUT

T1

1:1

0.1μF

10Ω

25Ω 0.1μF

25Ω 10Ω

2.2μF

5Ω AIN+

4.7pF

LTC2208

4.7pF

5Ω AIN–

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2μF

4.7pF

2208 F04a

Figure 4a. Using a Transmission Line Balun Transformer.

Recommended for Input Frequencies from 100MHz to 250MHz

VCM

0.1μF

ANALOG

INPUT

T1

1:1

0.1μF

25Ω 0.1μF

25Ω

2.2μF

5Ω AIN+

2.2pF

LTC2208

5Ω AIN–

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2μF

2.2pF

2208 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 reference circuitry consisting

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

fier and control circuit. The LTC2208 has three modes of

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER 25Ω

++

CM

––

25Ω

AMPLIFIER = LTC6600-20,

LTC1993, ETC.

VCM

2.2μF

AIN+

12pF

LTC2208

AIN–

12pF

2208 F05

Figure 5. DC-Coupled Input with Differential Amplifier

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.

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

2208 F06

2208fc

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