LTC1867LACGN 데이터 시트보기 (PDF) - Linear Technology

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LTC1867LACGN

Micropower, 3V, 12-/16-Bit, 8-Channel 175ksps ADCs

Linear Technology 

LTC1863L/LTC1867L

APPLICATIONS INFORMATION

many applications. For instance, Figure 1 shows a 50Ω

source resistor and a 2000pF capacitor to ground on the

input will limit the input bandwidth to 1.6MHz. The source

impedance has to be kept low to avoid gain error and

degradation in the AC performance. The capacitor also

acts as a charge reservoir for the input sample-and-hold

and isolates the ADC input from sampling glitch sensitive

circuitry. High quality capacitors and resistors should be

used since these components can add distortion. NPO

and silver mica type dielectric capacitors have excellent

linearity. Carbon surface mount resistors can also generate

distortion from self heating and from damage that may

occur during soldering. Metal film surface mount resistors

are much less susceptible to both problems.

algorithm, the ADC’s spectral content can be examined

for frequencies outside the fundamental.

Signal-to-Noise Ratio

The Signal-to-Noise and Distortion Ratio (SINAD) is the

ratio between the RMS amplitude of the fundamental input

frequency to the RMS amplitude of all other frequency

components at the A/D output. The output is band limited

to frequencies from above DC and below half the sampling

frequency. Figure 3a shows a typical SINAD of 81.4dB with

a 175kHz sampling rate and a 1kHz input. Higher SINAD

can be obtained with a 3V supply. For example, when an

external 3V is applied to REFCOMP (tie VREF to GND), a

SINAD of 83.5dB can be achieved as shown in Figure 3b.

DC Performance

One way of measuring the transition noise associated

with a high resolution ADC is to use a technique where

a DC signal is applied to the input of the ADC and the

resulting output codes are collected over a large number

of conversions. For example, in Figure 2 the distribution

of output codes is shown for a DC input that had been

digitized 4096 times. The distribution is Gaussian and the

RMS code transition noise is about 1.6LSB.

1200

1000

800

1044

895

830

VDD = 2.7V

INTERNAL REF

600

465

400

200

170

333

261

58

7

0

23 9 1

20 21 22 23 24 25 26 27 28 29 30 31

CODE

1863L7L G12

Figure 2. LTC1867L Histogram for 4096 Conversions

0

–20

–40

–60

–80

–100

fSAMPLE = 175ksps

fIN = 1kHz

SNR = 82.9dB

SINAD = 81.4dB

THD = 86.8dB

–120

–140

0

21.875 43.75 65.625 87.5

FREQUENCY (kHz)

1863L7L G03

Figure 3a. LTC1867L Nonaveraged 4096 Point

FFT Plot with 2.7V Supply

0

–20

–40

–60

–80

–100

fSAMPLE = 175ksps

fIN = 1kHz

SNR = 84.7dB

SINAD = 83.5dB

THD = 90dB

REFCOMP = EXT 3V

Dynamic Performance

–120

FFT (Fast Fourier Transform) test techniques are used to

test the ADC’s frequency response, distortion and noise

at the rated throughput. By applying a low distortion

sine wave and analyzing the digital output using an FFT

–140

0

21.875 43.75 65.625 87.5

FREQUENCY (kHz)

1863L7L F03b

Figure 3b. LTC1867L Nonaveraged 4096 Point

FFT Plot with 3V Supply

1863l7lfd

For more information www.linear.com/LTC1863L

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