LTC2185CUPPBF 查看數據表（PDF） - Linear Technology

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LTC2185CUPPBF

16-Bit, 125/105/80Msps Low Power Dual ADCs

Linear Technology 

LTC2185/LTC2184/LTC2183

Applications Information

Converter Operation

The LTC2185/LTC2184/LTC2183 are low power, two-chan-

nel, 16-bit, 125Msps/105Msps/80Msps A/D converters

that are powered by a single 1.8V supply. The analog

inputs should be driven differentially. The encode input

can be driven differentially, or single ended for lower

power consumption. The digital outputs can be CMOS,

double data rate CMOS (to halve the number of output

lines), or double data rate LVDS (to reduce digital noise

in the system.) Many additional features can be chosen

by programming the mode control registers through a

serial SPI port.

The two channels are simultaneously sampled by a shared

encode circuit (Figure 2).

Single-Ended Input

For applications less sensitive to harmonic distortion, the

AIN+ input can be driven single-ended with a 1VP-P signal

centered around VCM. The AIN– input should be connected

to VCM. With a single-ended input the harmonic distortion

and INL will degrade, but the noise and DNL will remain

unchanged.

Input Drive Circuits

Analog Input

The analog inputs are differential CMOS sample-and-hold

circuits (Figure 2). The inputs should be driven differen-

tially around a common mode voltage set by the VCM1 or

VCM2 output pins, which are nominally VDD/2. For the 2V

input range, the inputs should swing from VCM – 0.5V

to VCM + 0.5V. There should be 180° phase difference

between the inputs.

LTC2185

VDD

10Ω

AIN+

VDD

10Ω

AIN–

VDD

CPARASITIC

1.8pF

CPARASITIC

1.8pF

RON

CSAMPLE

5pF

15Ω

CSAMPLE

RON 5pF

15Ω

ENC+

1.2V

10k

ENC–

10k

1.2V

218543 F02

Figure 2. Equivalent Input Circuit. Only One of the Two

Analog Channels Is Shown

Input filtering

If possible, there should be an RC lowpass filter right at

the analog inputs. This lowpass filter isolates the drive

circuitry from the A/D sample-and-hold switching, and

also limits wideband noise from the drive circuitry. Figure  3

shows an example of an input RC filter. The RC component

values should be chosen based on the application’s input

frequency.

Transformer Coupled Circuits

Figure 3 shows the analog input being driven by an RF

transformer with a center-tapped secondary. The center

tap is biased with VCM, setting the A/D input at its optimal

DC level. At higher input frequencies a transmission line

balun transformer (Figure 4 to Figure 6) has better balance,

resulting in lower A/D distortion.

50Ω

VCM

ANALOG

INPUT

0.1µF T1

1:1

25Ω

25Ω 0.1µF

25Ω 25Ω

0.1µF

AIN+

12pF

AIN–

LTC2185

T1: MA/COM MABAES0060

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

218543 F03

Figure 3. Analog Input Circuit Using a Transformer.

Recommended for Input Frequencies from 5MHz to 70MHz

218543f

19

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