LTC2224I 查看數據表（PDF） - Linear Technology

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LTC2224I

12-Bit, 135Msps ADC

Linear Technology 

LTC2224

APPLICATIO S I FOR ATIO

signal at its optimum DC level. Terminating on the trans-

former secondary is desirable, as this provides a common

mode path for charging glitches caused by the sample and

hold. Figure 3 shows a 1:1 turns ratio transformer. Other

turns ratios can be used if the source impedance seen by

the ADC does not exceed 100Ω for each ADC input. A

disadvantage of using a transformer is the loss of low

frequency response. Most small RF transformers have

poor performance at frequencies below 1MHz.

Figure 4 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 band-

width of most op amps will limit the SFDR at high input

frequencies.

Figure 5 shows a single-ended input circuit. The imped-

ance seen by the analog inputs should be matched. This

circuit is not recommended if low distortion is required.

The 25Ω resistors and 12pF capacitor on the analog inputs

serve two purposes: isolating the drive circuitry from the

sample-and-hold charging glitches and limiting the

wideband noise at the converter input. For input frequen-

cies higher than 100MHz, the capacitor may need to be

decreased to prevent excessive signal loss.

For input frequencies above 100MHz the input circuits of

Figure 6, 7 and 8 are recommended. The balun trans-

former gives better high frequency response than a flux

coupled center tapped transformer. The coupling capaci-

tors allow the analog inputs to be DC biased at 1.6V. In

Figure 8 the series inductors are impedance matching

elements that maximize the ADC bandwidth.

Reference Operation

Figure 9 shows the LTC2224 reference circuitry consisting

of a 1.6V bandgap reference, a difference amplifier and

switching and control circuit. The internal voltage refer-

ence can be configured for two pin selectable input ranges

of 2V (±1V differential) or 1V (±0.5V differential). Tying the

SENSE pin to VDD selects the 2V range; typing the SENSE

pin to VCM selects the 1V range.

The 1.6V bandgap reference serves two functions: its

output provides a DC bias point for setting the common

mode voltage of any external input circuitry; additionally,

the reference is used with a difference amplifier to gener-

ate the differential reference levels needed by the internal

ADC circuitry. An external bypass capacitor is required for

the 1.6V reference output, VCM. This provides a high

frequency low impedance path to ground for internal and

external circuitry.

HIGH SPEED

DIFFERENTIAL

AMPLIFIER 25Ω

ANALOG

INPUT

+ + 3pF

CM

–

–

25Ω

LTC6600-20

OR LT1993

VCM

2.2µF

AIN+

LTC2224

12pF

AIN–

3pF

2224 F04

Figure 4. Differential Drive with an Amplifier

1k 1k

0.1µF

ANALOG

25Ω

INPUT

VCM

2.2µF

AIN+ LTC2224

12pF

25Ω

0.1µF

AIN–

2224 F05

Figure 5. Single-Ended Drive

2224fa

15

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