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LTC2209IUP

16-Bit, 160Msps ADC

Linear Technology 

LTC2209

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

0.1µF

ANALOG

INPUT

T1

1:1

0.1µF

10Ω

25Ω 0.1µF

25Ω 10Ω

2.2µF

5Ω AIN+

4.7pF

LTC2209

4.7pF

5Ω AIN–

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2µF

4.7pF

2209 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

LTC2209

5Ω AIN–

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

EXCEPT 2.2µF

2.2pF

2209 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

degrade the ADC SFDR at high input frequencies. Addi-

tionally, 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 LTC2209 reference circuitry consisting

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

fier and control circuit. The LTC2209 has three modes of

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER 25Ω

++

CM

––

25Ω

AMPLIFIER = LTC6600-20,

LTC1993, ETC.

VCM

2.2µF

AIN+

12pF

LTC2209

AIN–

12pF

2209 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

2209 F06

2209fb

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