LTC2302 datasheet(15/20 Pages) LINER

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LTC2302/LTC2306

23026fa

APPLICATIONS INFORMATION

Figure 3a. Optional RC Input Filtering for Single-Ended Input

Figure 3b. Optional RC Input Filtering for Differential Inputs

Figure 4. 1kHz Sine Wave 8192 Point FFT Plot (LTC2306)

Figures 3a and 3b show respective examples of input

ﬁltering for single-ended and differential inputs. For the

single-ended case in Figure 3a, a 50Ω source resistor

and a 2000pF capacitor to ground on the input will limit

the input bandwidth to 1.6MHz. High quality capacitors

and resistors should be used in the RC ﬁlter since these

components can add distortion. NPO and silver mica type

dielectric capacitors have excellent linearity. Carbon surface

mount resistors can 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.

Dynamic Performance

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 algorithm, the

ADC’s spectral content can be examined for frequencies

outside the fundamental.

Signal-to-Noise and Distortion Ratio (SINAD)

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 4 shows a typical SINAD of 72.8dB with

a 500kHz sampling rate and a 1kHz input. A SNR of 73.2dB

can be achieved with the LTC2302/LTC2306.

23026 F03a

CH0, CH1

LTC2306

VREF

2000pF

10μF

0.1μF

50Ω

ANALOG

INPUT

LT1790A-4.096

VOUT

VIN

1000pF

23026 F03b

CH0, IN+

CH1, IN–

LTC2302

LTC2306

VREF

1000pF

10μF

0.1μF

50Ω

DIFFERENTIAL

ANALOG

INPUTS

0.1μF

LT1790A-4.096

VOUT

VIN

Total Harmonic Distortion (THD)

Total Harmonic Distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamental

itself. The out-of-band harmonics alias into the frequency

band between DC and half the sampling frequency(fSMPL/2).

THD is expressed as:

THD

VVV

log

...

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN are the amplitudes of the second

through Nth harmonics.

FREQUENCY (kHz)

–40

–20

200

23026 F04

–60

–80

100

150

250

–100

–120

–50

–30

–10

–70

–90

–110

–130

–140

SNR = 73.2dB

SINAD = 72.8dB

THD = –88.7dB

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